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Evolutionarily speaking, why do humans have 46 chromosomes


In humans, each cell normally contains 23 pairs of chromosomes, for a total of 46. Monkeys, chimpanzees, and Apes have 24 pairs (twenty-four pairs), for a total of 48.

What caused humans to have 46?

EDIT: @TomD is right, I was asking why we have one less chromosome pair than chimpanzees (for example) [23 pairs instead of 24].


I think the OP is asking why we have one less chromosome pair than chimpanzees (for example) [23 pairs instead of 24].

The is an abundance of evidence, as alluded to above by shigeta, that human chromosome 2 is the result of a telomere-to-telomere fusion of two ancestral chromosomes (IJdo et al., 1991). This event did not occur in our closest ancestors, hence we have one less chromosome pair. In fact the sequence of human chromosome 2 contains the relic of an ancestral telomere-telomere fusion (IJdo et al., 1991).

The pdf of this key reference is freely available to all from PNAS

Reference

IJdo, J.W, Baldini, A, Ward, D.C, Reeders, S.T, Wells, R.A. (1991) Origin of human chromosome 2: an ancestral telomere-telomere fusion Proc Natl Acad Sci U S A., 88 9051-9055.[pdf]

Actually it has now been shown that Neanderthals and Denisovans also exhibit the same chromosomal fusion as humans - http://m.motherjones.com/politics/2014/02/evolution-creationism-bonobos-neanderthals-denisovans-chromosome-two


@nico is right. the number of chromosomes is the result of an evolutionary timeline, puncutated by sometimes spontaneous events which shape the DNA.

These events occur in the course of evolution:

1) Chromosomal rearrangements. Large sections of the genome can flip around or become integrated in other chromosomes. By homologous recombination, regions of the genome can clip themselves out or duplicate themselves as well. If you look at the alignment of human to say chimp, there are many segments that move relative to each other.

2) chromosomal breaking or combination. Two smaller chromosomes may combine to form a larger one, or a larger one may break into two smaller chromosomes. An example of this is human chromosome 2, which is found as two smaller chromosomes in the great apes (see figure in wikipedia). We infer that this is a combination event exclusive to humans by comparing the other apes on the evolutionary tree. Birds and reptiles tend to have lots of chromosomal breakage, even to the point where the number of microchromosomes (less than 20 million bases). Mammals tend to be more conservative and not allow viable chromosomal breaks - chickens have 78 chromosomes to our 23…

3) idiomatic chromosomal behavior. Sex determining chromosomes are examples of chromosomes where a pair becomes distinctly different in size and composition.
Another example is in trypanosome which has many tiny DNA segments which code for variant surface coat proteins.

4) @rwst points, what I clean forgot, that occasionally (like maybe just a few times) there have been whole genome duplications. This can be identified by chromosomal alignments within a single genome and has not happened very often since we became eukaryotic metazoans. Not sure how many times, but perhaps just once or twice in our lineage. If anyone knows about animals/humans that would be great. As you can see the link shows whole genome duplications in plants, which don't seem care how many chromosomes there are. Plants have polyploidy, you see, so such duplication events are much better tolerated. On the other hand plants can't play video games.

P. Dehal, J. L. Boore: Two rounds of whole genome duplication in the ancestral vertebrate. In: PLoS biology. 3, 10, Oct 2005, e314, doi:10.1371/journal.pbio.0030314. PMID 16128622. PMC 1197285.

You can see that these events happen at particular moments and help shape the species and the composition of the chromosomes. It is not a priori possible to predict the number or type of chromosomes just by looking at an animal, but only by looking at the related animals.

Fungi and Plants have even more variations in chromosomal composition than animals.


Here is a paper you might want to take a look at:

Phylogenetic Origin of Human Chromosomes 7, 16, and 19 and their Homologs in Placental Mammals

From the abstract:

From their origin, these chromosomes underwent the following rearrangements to give rise to current human chromosomes: centromeric fission of the two submetacentrics in ancestors of all primates (∼80 million years ago); fusion of the HSA19p and HSA19q sequences, originating the current HSA19, in ancestors of all simians (∼55 million years ago); fusions of the HSA16p and HSA16q sequences, originating the current HSA16 and the two components of HSA7 before the separation of Cercopithecoids and Hominoids (∼35 million years ago); and finally, pericentric and paracentric inversions of the homologs to HSA7 after the divergence of orangutan and gorilla, respectively. Thus, compared with HSA16 and HSA19, HSA7 is a fairly recent chromosome shared by man and chimpanzee only.


Evolutionarily speaking, why do humans have 46 chromosomes

This is a question of why human have 10 fingers. There is nothing magical about 10 fingers and nothing magical about having 46 chromosomes. In fact there are people who have 44 chromosomes. The are normal people. They have normal fertility. However if the children with people of 46 chr, the fertility of their children (ie people with 45 chr) is reduce. Normal fertility can only be maintained by marrying other people with 44chr… who probably are their cousins.

http://genetics.thetech.org/original_news/news124

So the reason we have 46 chromosome is because we are descended from a homonid species which had 46. No reason really. Not everything produced by evolution has a purpose.


How Many Chromosomes Do Humans Have?

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The chances are good that you stumbled onto this page after looking at our Ancestry DNA test or one of our other reviews. We spent hours reading online articles and examining those tests for ourselves to find out which ones were the best for our readers. These tests look at your DNA samples and can determine where your family line started and how members of your family moved throughout history. The odds are also good that you don’t understand how those tests work and what chromosomes are or where they are in your body.

Scientists chose the name chromosome because it’s a derivative of a Greek word that means both body and color. Those two terms let you know that a chromosome has some type of color and that it has a small body. You cannot view a chromosome, even if you look at a blood sample under a microscope. Though it is quite small, it contains the genetic and DNA material that makes up who you are. Though you may want to know more about chromosomes, you’ll also want to find out how many they are in the human body. We’ll go over numbers and other information in this guide to human chromosomes.


Introducing Chromosomes and Genes

Chromosomes are coiled structures made of DNA and proteins . They are encoded with genetic instructions for making RNA and proteins . These instructions are organized into units called genes . There may be hundreds (or even thousands!) of genes on a single chromosome. Genes are segments of DNA that code for particular pieces of RNA. Once formed, some RNA molecules go on to act as blueprints for building proteins, while other RNA molecules help regulate various processes inside the cell. Some regions of DNA do not code for RNA and serve a regulatory function, or have no known function.


ELI5: Why do ferns have so many chromosomes?

If humans only have 46 and are arguably pretty complex life forms, why does something as simple as a fern need so many? What is it doing with all of those chromosomes?

[Edit] forgot to note that ferns have 630 pairs of chromosomes

[Edit 2] humans have 46 chromosomes, not 26

As a side note, researchers are beginning to find selective advantages to smaller genomes. For example, wild teosinte has less “junk DNA” at higher elevations than does the same species at lower ones. People hypothesize there is a small, but present selection pressure against extra DNA in environments that have less nutrients or resources.

As an added point, the number of chromosomes means nothing. It has no bearing on the complexity of the organism and many simpler organism have a number higher than youɽ expect

Why isn't having tons of extra junk DNA a disadvantage? The cell needs to expend extra energy to replicate it. If that cost is really negligible, then why haven't humans kept around DNA for, I dunno, leucine synthesis or sulfur respiration?

Wouldn't having multiple identical copies of a gene radically affect the rate of evolution and mutagen tolerance of a species? Humans have only two unique copies of most genes: hit both copies with some radiation, and bam, no more working hemoglobin proteins for me. But if a fern's got 100 different copies, completely knocking out a gene is impossible, and the fern might have dozens of different versions of its proteins from mutations throughout history, some functional, some not. How does evolution work in that situation?

If there's a lot of junk DNA wouldn't it require more resources to reproduce? More DNA requires more phosphorous. Oxygen, carbon, and nitrogen are all fairly readily available. For aquatic organisms like algae, blooms are caused by fertilizer runoff and a lot of extra phosphorous in the water. I wonder if algae or other organisms have ever had evolutionary pressure to reduce the length of their DNA to be more efficient with less resources. For most animals though it seems like we have an abundance of phosphorous.

Also someone correct me if I’m wrong but I believe we combine certain sequences with others in order to express different traits.

Now every time I see a fern im going to think of a crazy old man in a smelly house full of (oversized novelty) collected DNA strands.

Follow up question: Have scientists identified where this junk DNA came from? For example, if it got mixed in from unrelated plants, have we identified them?

Thanks - one follow up question. I don’t understand how being an older species means more junk builds up. It’s true that our species appeared later, but it was descended from older species, so shouldn’t they have passed on some junk to us? Unless the adaptation of a new species somehow resets everything, I don’t see how this would work. Can you explain?

Well, not really an ancient species. Their divergence from seed plants is quite ancient, but they’re just as modern as any other extant life in my book.

This is also very common in plants, somewhat due to their being less complex. If a human has one extra chromosome, it causes major problems because we are very complex and tiny changes can really mess things up. Plants being less complex are able to handle changes that come with extra chromosomes, and sometimes can even benefit from it.

Plants are able to hybridize and have extra ploidys for the same reason. Two similar plant species mating together can often have viable offspring while in the animal world, hybrids are rarer and often not viable like mules.

Extra ploidys happen by mistake in reproduction. A ploidy (n) is a set of chromosomes. Humans and animals are diploid (2n) meaning we have two sets one from each parent. A mistake in cell division in plant reproduction can cause plants to have different ploidys. Like triploid (3n) or more. Rice for example has twelve sets of chromosomes (12n). Since plants are less complicated and mobile, the extra sets of chromosomes are less harmful and can randomly lead to extra benefits

I hadn't considered that, because they're less complex there's more tolerance for mistakes. That's interesting as fuck.

Triploid and tetraploid commonly happens with marijuana and mint plants.

Does the ploidy-ness (ploidity?) need to be even?

When I studied these kind of things I was taught there was a frog that hade multi-polidy-blabla. Maybe it was just as a genetic disease, but I think it was basically an entire species, that was just a ploidy-multiple of another species. (please feel free to correct if I remember incorrectly!)

Anyway, the only impact this seemed to have is that they got a lot bigger, because the cells became bigger. I imagine as a fern, this is an advantage as you can reach higher and catch more sun light. So perhaps that's why they evolved to have so many chromosomes.

Humans have *46 chromosomes.

And as many commenters have already pointed out: junk DNA. The sheer size of an organism's genome is not and indication of the organism's complexity. Polyploidy, or having more than 2 sets of chromosomes, is especially common in plants. Sometimes an aberration occurs causing the number of sets to change, most commonly a duplication, and BAM! - the genome is twice as large, without the organism's complexity being terribly affected.

We dont know concisely but genetics can explain a lot.

A fern isnt some ancient species, a relic kept through time. its a lifeform alive at the same time as you, and underwent the same genetic experiences as the same organisms that descended from /it/. The consensus is its "junk DNA", however that doesnt at all encapsulate why this is and what those many chromosomes do, and anyone telling you just that is immensly googling and trying to sound high and mighty.

Ferns in particular were some of the early land plants, and these types of plants had weird things. Particularly in the difference of gamete synthesis and generational life stages. back in this time of the evolutionary tale plants would have a diploid stage and a haploid stage - imagine if sperm or eggs lived for a while as humans, and then got together made a diploid human (closest analagy i can think of bear with me).

Nobody knows exactly how it happened, but the genome of the fern has an issue during meiosis, and this happened throughout the past. For some reason the normal process keeps another set of chromosomes, and there needs to be more research as to how.

Where other organisms have selective pressure and undergo constant recombination and mutation to forward evolution under selection, the fern has not underwent much morphological change, but its genome has underwent the same rigor as the other genomes of life right now (albeit instead of descent, the fern kept it all).

Though DNA replication has a high fidelity and high retention, the process is very automatic. and when it comes to the generational setup of the fern in particular, the replication phase to create a gamete insures that any genetic information that did not bear fruitful during natural selection is still conserved its mind blowing when you realize it, but a genome isnt everything that will be used to make everything, a lot of it is many repititve sequences and structural partitions, the most important part of the genome is arguably what is actually transcribed.

Ferns have a few genes they transcribe, and thats stuck. Because they started so early in the evolutionary timeframe their genetics is pretty stuck to transcribing pretty much what it needs and has needed for a while. This leads to an organism that has only a few genes, that will be transcribed to read /other/ genes, also few in number, and if you dont change the code on that first part of the transcription pathway, then its stuck on what actually gets read from the genome, no matter the size of it.

Everytime any sexual reproduction happens, the genome undergos small copies, flips, transformations and reversals. Because the fern never really changed what it transcribed from the genome, not only do you have an organism that looks damn near what it was, it never killed the fern over generations to keep so many of its copies and changes that expanded its genome. Its still kickin, and so you get an organism with a huge genetic library with such an "archaeic" morphology.

some organisms branched off from this, well actually MOST plants have some fern in them, but you can see now how this is a case of goldilocks finding it just right, genetically speaking.


A chromosome is a long chain of DNA packaged together with a collection of proteins. The human genome (all the genetic information in your cells) is contained in 23 pairs of chromosomes. Each cell has its own complete compete of your genome. The reason eye-ball cells are different from liver cells is that certain liver genes are turned off in eye-ball cells and vice versa.

When a sexually reproducing plant or animal reproduces, half of its chromosomes are shuffled together with half the chromosomes of its sexual partner. The result is a child with half its DNA from its mother and half from its father.

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Transcript

A chromosome is a single strand of DNA along with a group of proteins that process and package that DNA.

The purpose of a chromosome, is to carry the genes of a living thing. Genes are special segments of DNA that tell a creature’s body how to grow and function.

Most bacteria just have one chromosome, wolves and dogs have 78, pea plants have 14, and us Humans have 46 chromosomes inside each one of our cells.

Together, your 46 chromosomes contain all the genes or, units of instruction, needed to make you who you are.

For most of a cell’s life, its chromosomes are loosely mixed together sort of like a bowl of spaghetti. This loose packaging of DNA and protein, allow the cell to find and use all of the individual genes it needs.

Cells reproduce through a special process called mitosis. They duplicate each Chromosome, or strand of DNA inside their guts, separate the two copies to either side of their body, and then split in two, right down the middle.

The process of sorting out individual DNA strands and safely moving these delicate structures to either side of the cell is virtually impossible to do when chromosomes are loosely mixed and tangled up in spaghetti form.

For this reason, after DNA is duplicated but before it’s moved to either side of the cell, special packaging proteins called Histones, attach to and gently coil each strand of DNA. They start by forming beads which bunch up to make tubes which clump into loops that eventually condense into a durable, flexible chromosome capsule.

Condensed chromosomes can be easily identified and sorted by the cell. They can be moved and manipulated without breaking.

The 4 legged chromosome shown here, which can be thought of as two spools of yarn, joined together at the hip, Is what we call a duplicated Chromosome. The left side contains the original strand of DNA, all wound up tightly, the right side contains the new copy of DNA that was made before the chromosome condensed.

Before a cell splits in two, protein fibers inside the cell grab the duplicated Chromosome by the hips, and rip it apart, dragging one half to the left, and the other half to the right.

Different Chromosomes come in different shapes and sizes. Scientists discovered that if you organize human chromosomes from longest to shortest, they sort into 23 pairs. One member of each pair came from your Dad, one member of each pair came from your Mom.

Paired Chromosomes are almost identical to each other. They contain the same genes in the same locations. The DNA code in the genes from your mother however, might be a few letters different than the DNA code that came from your father. In other words, chromosome pairs (even though they contain the same genes) may contain slightly different versions of those genes.

For example: Your mom might have given you genes for a long pointy nose, while your dad gave you genes for a wide stubby nose. Depending on who’s nose genes end up being more powerful, you might grow a pointy nose, you might grow a wide nose, or if you’re lucky, your nose might come out wide and pointy.

If you decide to have children of your own, you ‘ll pass on one copy of half of your chromosomes (some of which you originally got from you mom, some from you originally got from your dad). You’ll combine these chromosomes with half of the chromosomes from your partner, and together you’ll create a genetically unique new life.

So just to sum things up a bit, what exactly is a chromosome?

A Chromosome is a single complete strand of DNA, along with an associated group of packaging proteins.

Humans have 46 chromosomes, 23 from mom, 23 from dad. Together these 46 chromosomes contain all the genes needed to make you who you are.


Can a Quirky Chromosome Create a Second Human Species?

In this age of genome sequencing, we can lose sight of the importance of how our genomes are distributed over 23 pairs of chromosomes. Rearrangements of the pairs are invisible to sequencing if the correct amount of genetic material is present.

A recent genetic counseling session reminded me of a chromosomal quirk that flies completely under the radar of genome sequencing, yet if it were to turn up in two copies in a bunch of people who have children together, could theoretically seed a second human species, one characterized by a chromosome number of 44, not 46.

CHROMOSOME PIGGYBACKS
The young couple I counseled had suffered several early pregnancy losses, and tests had revealed extra material from chromosome 22. Although it&rsquos a tiny chromosome, it is gene-dense and the extra genetic material ends development just as an embryo is becoming a fetus.

The lab report profiled single nucleotide polymorphism (SNP) landmarks, and detected overrepresentation of pieces of chromosome 22. But this was on a genomewide basis using microarray technology &mdash not the cut-and-paste size-ordered chromosome chart that is an old-fashioned karyotype. SNPs may be newer, but the distinction between an extra chromosome 22, which the lab report inferred, and the actuality of an extra tiny chromosome glommed onto one of the other chromosomes, is critical for predicting recurrence in a family &mdash because such a piggybacked chromosome would explain the repeated losses. It is a case of not seeing the genetic forest for the trees. Chromosomes still count.

One type of chromosome adhered to another is a Robertsonian translocation, named after William Rees Brebner Robertson, Ph.D., who first described it in 1916 in grasshoppers. The person, or grasshopper, with one &ldquoRob&rdquo chromosome is fine, because the correct two gene sets are there &ndash just rearranged. But making gametes (sperm or egg) is problematical, because the Rob chromosome has a dual identity from its two parts, and won&rsquot separate as chromosome pairs typically do during meiosis. It&rsquos a little like a couple in a square dance who won&rsquot pull apart when everyone else does.

Meiosis is the form of cell division that halves the chromosome sets as sperm and eggs form.

Let&rsquos say a chromosome #22 has glommed onto a chromosome #13 in a man. His sperm can get a normal 13 and a normal 22 just the piggybacked 1322 like the man himself or 4 &ldquounbalanced&rdquo possibilities that contribute too much or too little of the implicated chromosomes. For a family, that means a 2/3 risk of unbalanced chromosomes for each pregnancy &mdash and loss or a congenital syndrome.

One in 1,000 people has one Rob chromosome, and is a carrier (heterozygote) for it. Robs happen only in chromosomes termed acrocentrics that have one long arm and one very tiny arm, or in telocentrics, which have only a long arm (but we don&rsquot have them). Our acrocentrics are chromosomes 13, 14, 15, 21, and 22. A Rob chromosome 21 accounts for the rare cases of Down syndrome that are not due to a full trisomy (extra chromosome) and are much more likely to recur in families.

OF MICE AND MUNTJACS
Reports of Robertsonian translocations in the animal kingdom are sparse, but mice are curiously adept at shuffling their chromosomes. Their acrocentrics and telocentrics glom into larger metacentrics in many ways.

The first departure from the common house mouse (Mus musculus) was the &ldquotobacco mouse&rdquo (Mus poschiavinus), described in 1869 from specimens trapped in a tobacco factory in Valle di Poschiavo, Switzerland. They had big heads and small, dark bodies. Tobacco mice were later found in the Italian Alps and their distinctive set of 22 chromosomes discovered, their 9 pairs of equal-armed metacentrics coalesced from the ancestral 40 telocentrics of the house mouse.

The dynamic chromosome count of mice likely reflects attractions of TTAGAGAG repeats at their tips, which are echoed, but in reverse, at their main constrictions, the centromeres. Such reverse repeats in chromosomes are like Velcro, mixing and matching their parts, creating at least 100 different &ldquoraces&rdquo of mice that are, genetically speaking, distinct species because they can only reproduce with each other. The resulting types of chromosomally-defined mice are geographically fixed, because mice don&rsquot travel much (unless they sneak onto ships).

Muntjacs are also prone to Robs. The ancestral Muntiacus reevesi has 46 chromosomes in both male and female, whereas the derived Muntiacus muntjac has just 6 chromosomes in the female and 7 in the male. Chromosome banding reveals that the two Asian deer species have the same genes, but they&rsquore splayed out differently among the chromosomes. Rams and cotton rats have Robs too.

Big chromosomes shattering into smaller ones, although that seems energetically more favorable than formation of a Rob, is actually rarer. Such chromosomal fission has been reported only in cultured cells, a zebra family, and in the black rat of Mauritius. Chromosome fusion at the tip rather than the exposed centromeres is also rare, but distinguishes Asian river buffaloes from Malaysian swamp water buffaloes.

A carrier of a Rob has one unusual, large chromosome.

A NEW BREED OF HUMAN?
Far far rarer than human Rob heterozygotes are homozygotes with two Robs, because these individuals can only arise from inheriting one copy of the unusual chromosome from each parent &ndash which typically means the parents are related and inherited the Rob from a shared ancestor, like a common great-grandparent. Cases of these Rob homozygotes, who have 44 chromosomes rather than the normal 46, are exceedingly rare:

&bull A 1984 report describes a family with 3 adult siblings who had 44 chromosomes, #s 13 and 14 combined.
&bull A 1988 report tells of 3 distantly-related families in Finland, also involving #s 13 and 14, whose Rob chromosome passed in carriers through at least 9 generations, appearing in at least one homozygote.
&bull A 1989 paper describes a Rob between #s 14 and 21 in a homozygote whose carrier parents were related.

Trickling into the headlines was a case report from 2013 of a 25-year-old healthy Chinese man who has 44 chromosomes because each 14 joins a 15 &ndash a combo not seen before. His parents, both translocation carriers, were first cousins. The Chinese man&rsquos sperm carry 21 autosomes and an X or Y, and he should be fertile &ndash but only with a woman who is similarly chromosomally endowed. Chances are he&rsquoll never find her. But if he does &hellip

FROM SCIENCE TO SCI-FI
The report on the Chinese man with 44 chromosomes ends with: &ldquoThe aberration can provide material for evolution. &hellip Long term isolation of a group of individuals who are homozygous for a particular Robertsonian translocation chromosome could theoretically lead to the establishment of a new human subspecies having a full genetic complement in 44 chromosomes.&rdquo

It might have happened before. Could the 48 chromosomes of a shared ancestor of humans and chimps have branched to yield our 46 chromosomes? Fusion of chimp chromosomes 12 and 13, according to banding patterns, might have generated our larger chromosome 2.

The idea of inheriting a double dose of a Robertsonian chromosome fueling human speciation isn&rsquot new. I wrote about it in 2002 in The Scientist, wherein Lisa Schaffer, PhD, of &ldquoPaw Print Genomics&rdquo at Washington State University, Spokane, who at that time studied Robs, speculated, &ldquoWith 1 in 1,000 individuals carrying a Robertsonian translocation, the likelihood of two carriers getting together and both transmitting their translocation is 1 in 4 million&ndashso they are out there, just phenotypically normal.&rdquo

(I caught up with Dr. Schaffer this week, and she said that not too many people these days work with Robs. She and I fear that cytogenetics &mdash the study of chromosomes and traits &mdash is a dying art. It was my favorite part of graduate school. I learned how to glean meaning from chromosomes from one of the masters, corn geneticist Marcus Rhoades.)

The possibility of a new human species with 44 chromosomes can flesh out science fiction plots, if different sets of mutations accumulate in the two populations derived from the ancestral one. It may explain the origin of the bluish ghoulish subterranean Morlocks who eat the sun-loving, peaceful aboveground Eloi in H.G. Wells&rsquo future world of The Time Machine, written in 1898. Or Robs may underlie the cannibalistic screeching humanoid &ldquoaberrations,&rdquo aka &ldquoAbbies,&rdquo our descendants, who will take over the future world depicted in last summer&rsquos Wayward Pines.

Neither H. G. Wells nor Wayward Pines&rsquo creator Blake Crouch evoked Robertsonian translocation as a plausible route to rapid human evolution (or devolution) &mdash but they could have. Sci-fi author Greg Bear came very close in his marvelous 1999 and 2003 novels Darwin&rsquos Radio and Darwin&rsquos Children. He imagines that a latent retrovirus awakened in the genomes of pregnant women in 1999 shuffled the genomes of a new generation in ways that created cells with 52 chromosomes instead of 46, thereby instantly establishing a group that can successfully mate only among themselves. You&rsquoll have to read the books to learn how and why the &ldquovirus children&rdquo are superior. Forced into camps by the fearful majority, they establish their own culture, further separating the two types of people, a little bit reminiscent of a presidential election in the US. Bear&rsquos alternate reality is a compelling depiction of reproductive isolation leading, presumably, to speciation, with an initial chromosomal upheaval as the impetus.

I spoke to Bear back in 2004, again for The Scientist (a mechanism for the rapid extinction of a dedicated freelance writer is a change in editor-in-chief). Greg Bear is a self-taught scientist with a soaring imagination. Said he, &ldquoMy secrets are few. I love biology. I have been researching it in constant reading since the early 1980s. I saw very clearly that DNA must be computational, a self-organizing, self-repairing system. In the early 90s, it became clear to me that modern evolutionary theory was incomplete. I set out to find all the out-of-the-way papers that I could to prove that nature was a network, from top to bottom.&rdquo The Darwin series arose from those thoughts &mdash and he clearly knew about Robs.

I&rsquom not very good at writing fiction and am in awe of people like Bear who can, but if I could, I&rsquod follow up on this theme of 44-chromosome people arising by chromosomal happenstance and staying hidden in plain sight among those smugly having their genomes sequenced. Might social media catalyze such an event, which, after all, wasn&rsquot around when the Time Machine or even Darwin&rsquos Radio were written? Alas, Facebook&rsquos Robertsonian Translocation Support Group, founded in 2011, only has 2 posts, the first of which evokes the Lord, probably turning off those seeking scientific information.

When I&rsquom through with my next round of textbook revisions, maybe I&rsquoll give fiction a shot &hellip and honor what may be the dying biological art of cytogenetics.


The Institute for Creation Research

One of the more popular arguments used for humans supposedly evolving from apes is known as the chromosome fusion. The impetus for this concept is the evolutionary problem that apes have an extra pair of chromosomes&mdashhumans have 46 while apes have 48. If humans evolved from an ape-like creature only three to six million years ago, a mere blip in the grand scheme of the evolutionary story, why do humans and apes have this discrepancy?

The evolutionary solution proposes that an end-to-end fusion of two small ape-like chromosomes (named 2A and 2B) produced human chromosome 2 (Figure 1). The concept of a fusion first came about in 1982 when scientists examined the similarities of human and ape chromosomes under a microscope. While the technique was somewhat crude, it was enough to get the idea going. 1

The So-Called Fusion Site

The first actual DNA signature of a possible fusion event was discovered in 1991 on human chromosome number 2. 2 Researchers found a small, muddled cluster of telomere-like end sequences that vaguely resembled a possible fusion. Telomeres are a six-base sequence of the DNA letters TTAGGG repeated over and over again at the ends of chromosomes.

However, the fusion signature was somewhat of an enigma based on the real fusions that occasionally occur in nature. All documented fusions in living animals involve a specific type of sequence called satellite DNA (satDNA) located in chromosomes and found in breakages and fusions. 3-5 The fusion signature on human chromosome 2 was missing this telltale satDNA. 6

Another problem is the small size of the fusion site, which is only 798 DNA letters long. Telomere sequences at the ends of chromosomes are 5,000 to 15,000 bases long. If two chromosomes had fused, you should see a fused telomere signature of 10,000 to 30,000 bases long&mdashnot 798.

Not only is the small size a problem for the fusion story, the signature doesn&rsquot really represent a clear-cut fusion of telomeres. Figure 2 shows the DNA letters of the 798-base fusion site with the six-base (DNA letter) intact telomere sequences emphasized in bold print. When the fusion sequence is compared to that of a pristine fusion signature of the same size, it is only 70% identical overall.

Secular researchers have pointed out this discrepancy and have labeled the fusion site as significantly &ldquodegenerate.&rdquo 7 Given the standard theoretical model of human evolution, it should be about 98 to 99% identical, not 70%. The researchers describing this discovery commented, &ldquoHead-to-head arrays of repeats at the fusion site have degenerated significantly (14%) from the near perfect arrays of (TTAGGG)n found at telomeres&rdquo and asked the pertinent question &ldquoIf the fusion occurred within the telomeric repeat arrays less than

6 Mya, why are the arrays at the fusion site so degenerate?&rdquo 7 It should be noted that the 14% degeneration cited by the authors refers to the corruption of just the six-base sequences themselves, not the whole 798 bases.

The Fusion Site Inside a Gene?

The most remarkable anti-evolutionary finding about the fusion site turned out to be its location and what it actually does. This discovery came about while I was reading the research paper that reported a detailed analysis of 614,000 bases of DNA sequence surrounding the alleged fusion site. I noticed in one of the figures that the fusion site was located inside a gene, and quite remarkably this oddity wasn&rsquot even acknowledged in the text of the paper. 8

A finding like this is highly noteworthy. Perhaps this piece of information would&rsquove been the nail in the evolutionary coffin, so to speak, which is why the researchers declined to discuss it. This major anomaly inspired me to give the fusion site a much closer examination. This paper was published in 2002, and I took notice of it in 2013. A huge amount of data on the structure and function of the human genome had been published in the meantime, and there was likely much more to the story that needed to be uncovered.

When I performed further research, I verified that the fusion site was positioned inside an RNA helicase gene now called DDX11L2. Most genes in plants and animals have their coding segments in pieces called exons so they can be alternatively spliced. Based on the addition or exclusion of exons, genes can produce a variety of products. The intervening regions between exons are called introns, which often contain a variety of signals and switches that control gene function. The alleged fusion site is positioned inside the first intron of the DDX11L2 gene (Figure 3). 9

The DNA molecule is double-stranded, with a plus strand and a minus strand. It was engineered this way to maximize information density while also increasing efficiency and function. As a result, there are genes running in different directions on the opposing strands. As it turns out, the DDX11L2 gene is encoded on the minus strand. Because genes in humans are like Swiss army knives and can produce a variety of RNAs, in the case of the DDX11L2 gene it produces short variants consisting of two exons and long variants with three (Figure 3). 9

The Fusion Site Is a Gene Promoter

What might this DDX11L2 gene be doing? My research showed it&rsquos expressed in at least 255 different cell or tissue types. 9 It&rsquos also co-expressed (turned on at the same time) with a variety of other genes and is connected to processes associated with cell signaling in the extracellular matrix and blood cell production. The location of the so-called fusion sequence inside a functional gene associated with the genetics of a variety of cellular processes strongly refutes the idea that it&rsquos the accidental byproduct of a head-to-head telomeric fusion. Genes are not formed by catastrophic chromosomal fusions!

Even more amazing is that the fusion site is itself functional and serves an important engineered purpose. The site actually acts as a switch for controlling gene activity. In this respect, a wealth of biochemical data showed that 12 different proteins called transcription factors regulate this segment of the gene. One of these is none other than RNA polymerase II, the main enzyme that copies RNA molecules from DNA in a process called transcription. Further supporting this discovery is the fact that the actual process of transcription initiates inside the region of the so-called fusion site.

Technically, we would call the activity in the alleged fusion site a promoter region. Promoters are the main switches at the beginning of genes that turn them on and are also where the RNA polymerase starts to create an RNA. Many genes have alternative promoters like the DDX11L2 gene.

There are actually two areas of transcription factor binding in the DDX11L2 gene. The first is in the promoter directly in front of the first exon, and the second is in the first intron corresponding to the fusion site sequence. Not only is the DDX11L2 gene itself complexly controlled, with the alleged fusion sequence playing a key role, but even the RNA transcripts produced are very intricate. The RNAs themselves contain a wide variety of binding and control sites for a class of small regulatory molecules called microRNAs. 9

Functional Internal Telomere Sequences Are All Over the Genome

The presence of internally located telomere sequence is found all over the human genome. These seemingly out-of-place telomere repeats have been dubbed interstitial telomeres. The presence of these sequences presents another challenge for the fusion site idea. It&rsquos a fact that very few of the telomere repeats in the fusion site occur in tandem. As noted in Figure 2, the sequence of the 798-base fusion site contains only a few instances where two repeats are actually in tandem and none that have three repeats or more. However, there are many other interstitial telomere sites all over the human genome where the repeats occur in perfect tandem three to ten times or more. 10-11

Even besides their role at the ends of chromosomes, it appears interstitial telomeric repeats may serve an important function in the genome related to gene expression. In a recent research project, I identified telomere repeats all over the human genome and then intersected their genomic locations with a diversity of data sets containing functional biochemical information for gene activity. 12 Literally thousands of internal telomeric repeats across the genome were directly associated with the hallmarks of gene expression. The same type of transcription factor binding and gene activity occurring at the alleged fusion site was also occurring genome-wide at numerous other interstitial telomeric repeats. Clearly, these DNA features are not accidents of evolution but purposefully and intelligently designed functional code.

Bogus Cryptic Centromere Inside a Gene

Another key problem with the fusion model is the lack of viable evidence for a signature of an extra centromere region. Centromeres are sections of chromosomes, often in central locations, that play key roles during cell division. As depicted in Figure 1, the newly formed chimeric chromosome would&rsquove had two centromere sites immediately following the alleged head-to-head fusion of the two chromosomes. In such a case, one of the centromeres would be functional while the other would be disabled. The presence of two active centromeres is bad news for chromosomes and would lead to dysfunction and cell destruction.

Interestingly, the evidence for a cryptic (disabled) centromere on human chromosome 2 is even weaker than that for a telomere-rich fusion site. Evolutionists explain the lack of a clearly distinguishable nonfunctional secondary centromere by arguing that a second centromere would&rsquove been rapidly selected against. After that, the disabled centromere would&rsquove deteriorated over time since there were no functional restraints placed on it anymore by its doing something useful in the genome.

However, the evidence for a second remnant centromere at any stage of sequence degeneration is problematic for the evolutionary paradigm. Functional centromere sequences are composed of a repetitive type of DNA called alphoid sequences, with each alphoid repeat being about 171 bases long. Some types of alphoid repeats are found all over the genome, while others are specific to centromeres. The structure of the sequences found at the cryptic centromere site on human chromosome 2 doesn&rsquot match those associated with functional human centromeres. 13 Even worse for the evolutionary model is that they have no highly similar counterparts in the chimp genome&mdashthey are human-specific. 13

The alleged fossil centromere is also exceptionally tiny compared to a real one. The size of a normal human centromere ranges in length between 250,000 and 5,000,000 bases. 14 The alleged cryptic centromere is only 41,608 bases long, but it&rsquos also important to note that there are three different regions of it that aren&rsquot even alphoid repeats. 15 Two of these are called retroelements, with one being a LPA3/LINE repeat 5,957 bases long and the other an SVA-E element with 2,571 bases. When we subtract the insertions of these non-alphoid sequences, it gives a length of only 33,080 bases, which is a fraction of the length of a real centromere.

The most serious evolutionary problem with the idea of a fossil centromere, though, is that like the alleged fusion site, it&rsquos positioned inside a gene. The alleged cryptic centromere is located inside the ANKRD30BL gene, and its sequence spans both intron and exon regions of the gene. 12,15

In fact, the part of the alleged fossil centromere sequence that lands inside an exon actually codes for amino acids in the resulting gene&rsquos protein. The gene produces a protein that&rsquos believed to be involved in the interaction of the structural network of proteins inside the cell called the cytoskeleton in connection with receptor proteins embedded in the cell membrane. 16 The fact that the so-called fossil or cryptic centromere is a functional region inside an important protein-coding gene completely refutes the idea that it&rsquos a defunct centromere.

Conclusion: No Fusion

Due to the muddled signatures and small sizes of the alleged fusion and fossil centromere sites, it&rsquos highly questionable that their sequence was evolutionarily derived from an ancient chromosome fusion. Not only that, they represent functional sequence inside genes. The alleged fusion site is an important genetic switch called a promoter inside the DDX11L2 long noncoding RNA gene, and the so-called fossil centromere contains both coding and noncoding sequence inside a large ankyrin repeat protein-coding gene.

This is an undeniable double whammy against the whole mythical fusion idea, utterly destroying its validity. The overwhelming scientific conclusion is that the fusion never happened.


Human Genes

Humans have an estimated 25,000 genes. This may sound like a lot, but it really isn&rsquot. Far simpler species have almost as many genes as humans. However, human cells use splicing and other processes to make multiple proteins from the instructions encoded in a single gene. Only about 25 percent of the nitrogen base pairs of DNA in human chromosomes make up genes and their regulatory elements. Out of this 25 percent, only two percent code for genes. The functions of many of the other base pairs are still unclear.

The majority of human genes have two or more possible versions, called alleles. Differences in alleles account for the considerable genetic variation among people. In fact, most human genetic variation is the result of differences in individual DNA base pairs within alleles.


Chromosomal Abnormalities and Genetic Testing

Chromosomal Abnormalities

Figure 5. The three major single-chromosome mutations: deletion (1), duplication (2) and inversion (3).

A chromosomal abnormality occurs when a child inherits too many or too few chromosomes. The most common cause of chromosomal abnormalities is the age of the mother. A 20-year-old woman has a 1 in 800 chance of having a child with a common chromosomal abnormality. A woman of 44, however, has a one in 16 chance. It is believed that the problem occurs when the ovum is ripening prior to ovulation each month. As the mother ages, the ovum is more likely to suffer abnormalities at this time.

Another common cause of chromosomal abnormalities occurs because the gametes do not divide evenly when they are forming. Therefore, some cells have more than 46 chromosomes. In fact, it is believed that close to half of all zygotes have an odd number of chromosomes. Most of these zygotes fail to develop and are spontaneously aborted by the body. If the abnormal number occurs on pair # 21 or # 23, however, the individual may have certain physical or other abnormalities.

An altered chromosome structure may take several different forms, and result in various disorders or malignancies:

Figure 6. The two major two-chromosome mutations: insertion (1) and Translocation (2).

  • Reciprocal translocation: Segments from two different chromosomes have been exchanged.
  • Robertsonian translocation: An entire chromosome has attached to another at the centromere – in humans, these only occur with chromosomes 13, 14, 15, 21, and 22.

One of the most common chromosomal abnormalities is on pair # 21. Trisomy 21 occurs when there are three rather than two chromosomes on #21. A person with Down syndrome has distinct facial features, intellectual disability, and oftentimes heart and gastrointestinal disorders. Symptoms vary from person to person and can range from mild to severe. With early intervention, the life expectancy of persons with Down syndrome has increased in recent years. Keep in mind that there is as much variation in people with Down Syndrome as in most populations and those differences need to be recognized and appreciated.

Watch It

Watch the following video clip about Down Syndrome from the National Down Syndrome Society:

When the chromosomal abnormality is on pair #23, the result is a sex-linked chromosomal abnormality. A person might have XXY, XYY, XXX, XO, or 45 or 47 chromosomes as a result. Two of the more common sex-linked chromosomal disorders are Turner syndrome and Klinefelter syndrome. Turner’s syndrome occurs in 1 of every 2,500 live female births (Carroll, 2007) when an ovum which lacks a chromosome is fertilized by a sperm with an X chromosome. The resulting zygote has an XO composition. Fertilization by a Y sperm is not viable. Turner syndrome affects cognitive functioning and sexual maturation. The external genitalia appear normal, but breasts and ovaries do not develop fully and the woman does not menstruate. Turner’s syndrome also results in short stature and other physical characteristics. Klinefelter syndrome (XXY) occurs in 1 out of 700 live male births and results when an ovum containing an extra X chromosome is fertilized by a Y sperm. The Y chromosome stimulates the growth of male genitalia, but the additional X chromosome inhibits this development. An individual with Klinefelter syndrome has some breast development, infertility (this is the most common cause of infertility in males), and has low levels of testosterone.

Prenatal Testing

Prenatal testing consists of prenatal screening and prenatal diagnosis, which are aspects of prenatal care that focus on detecting problems with the pregnancy as early as possible. These may be anatomic and physiologic problems with the health of the zygote, embryo, or fetus, either before gestation even starts or as early in gestation as practical. Prenatal screening focuses on finding problems among a large population with affordable and noninvasive methods. The most common screening procedures are routine ultrasounds, blood tests, and blood pressure measurement. Prenatal diagnosis focuses on pursuing additional detailed information once a particular problem has been found, and can sometimes be more invasive.

Screening can detect problems such as neural tube defects, anatomical defects, chromosome abnormalities, and gene mutations that would lead to genetic disorders and birth defects, such as spina bifida, cleft palate, Downs Syndrome, Tay–Sachs disease, sickle cell anemia, thalassemia, cystic fibrosis, muscular dystrophy, and fragile X syndrome. Some tests are designed to discover problems which primarily affect the health of the mother, such as PAPP-A to detect pre-eclampsia or glucose tolerance tests to diagnose gestational diabetes. Screening can also detect anatomical defects such as hydrocephalus, anencephaly, heart defects, and amniotic band syndrome.

Common prenatal diagnosis procedures include amniocentesis and chorionic villus sampling. Because of the miscarriage and fetal damage risks associated with amniocentesis and CVS procedures, many women prefer to first undergo screening so they can find out if the fetus’ risk of birth defects is high enough to justify the risks of invasive testing. Screening tests yield a risk score which represents the chance that the baby has the birth defect the most common threshold for high-risk is 1:270. A risk score of 1:300 would, therefore, be considered low-risk by many physicians. However, the trade-off between the risk of birth defects and risk of complications from invasive testing is relative and subjective some parents may decide that even a 1:1000 risk of birth defects warrants an invasive test while others wouldn’t opt for an invasive test even if they had a 1:10 risk score.

There are three main purposes of prenatal diagnosis: (1) to enable timely medical or surgical treatment of a condition before or after birth, (2) to give the parents the chance to abort a fetus with the diagnosed condition, and (3) to give parents the chance to prepare psychologically, socially, financially, and medically for a baby with a health problem or disability, or for the likelihood of a stillbirth. Having this information in advance of birth means that healthcare staff, as well as parents, can better prepare themselves for the delivery of a child with a health problem. For example, Down Syndrome is associated with cardiac defects that may need intervention immediately upon birth.

The American College of Obstetricians and Gynecologists (ACOG) guidelines currently recommend that all pregnant women, regardless of age, be offered invasive testing to obtain a definitive diagnosis of certain birth defects. Therefore, most physicians offer diagnostic testing to all their patients, with or without prior screening and let the patient decide.

Watch it

Watch this video to learn more about prenatal testing and screening during pregnancy.

Try It


On Monday, October 5th, 2009, the Nobel Prize in Physiology or Medicine was awarded to three scientists for their work decades ago on understanding how integrity of the genetic code at the ends of the chromosomes is maintained as cells divide. Drs. Elizabeth H. Blackburn, Carol W. Greider, and Jack W. Szostak shared this most prestigious prize for work they did in the 70’s and 80’s on the “caps” at the ends of chromosomes. These “caps” are called telomeres (‘telos’ meaning ‘end’ and ‘meros’ meaning ‘part’ in Greek) and are added by an enzyme called telomerase. So why is understanding telomeres important? In order to appreciate their role, we need some background on chromosomes.

Putting chromosomes in perspective

Organisms are made up of many, many cells (around 100 trillion cells in an adult human) that all have specialized roles in the body to maintain health. Within each cell there are roughly 1 billion protein molecules, which are like miniature machines that perform the tasks essential for cell and organism survival. But how are all these proteins made in the right cells at the right time? This is where the genetic code and chromosomes come in. Genes are sequences of DNA that encode the instructions for making proteins, but these instructions are only used when signals in the cell direct the genes to turn on. Although only some of all possible proteins are present in each cell at any given time, the DNA encoding all proteins is present in all cells all the time (with a few exceptions such as eggs and sperm). All this DNA is housed within a compartment of the cell called the nucleus. In order to fit the massive amount of DNA in the human genome into the nucleus, it must be condensed into a compact structure. This has been achieved evolutionarily by compact nuclear structures called chromosomes. Human cells have 46 chromosomes, 23 inherited from mom and 23 from dad. Each chromosome is a tightly coiled, rod-like structure consisting of one linear, double-stranded molecule of DNA with many genes (ranging from 80 genes on the tiny Y chromosome to 4,220 genes on chromosome 1). Every time a cell divides, all chromosomes need to be replicated so that both the parent cell and the daughter cell have the proper 46 chromosomes with all their genes intact and capable of providing instructions for making the essential protein ‘machines’.

What are telomeres?

During cell division, the DNA that is packed into the chromosomal structures must be duplicated, so that each cell gets a copy of the instructions after division. During this process, the double-stranded DNA structure is unzipped so that each strand can be used as a template for enzymes called polymerases to read as they make the extra copy of DNA. A long-standing conundrum before the work of this year’s Nobel winners was how the genes at the very end of the chromosomes can be copied. This was puzzling because it was known that one of the two DNA strands is copied in a manner that requires the polymerase to latch on to the template strand beyond the end of the last gene at the most extreme end of the chromosome. How could the genetic material remain intact if there was no sequence to latch onto? In other words, what is preventing genomic DNA at the very end of the chromosome from being lost? A discovery in the 70’s began to shed light on this question. Elizabeth Blackburn found a DNA sequence that was repeated multiple times within the telomere “caps” at the ends of chromosomes and, together with Jack Szostak, found that these evolutionarily conserved sequences are sufficient to prevent degradation of linear chromosomes. But exactly how does this work?

The repeating DNA sequence (in humans, 5′-TTAGGG-3′) that makes up telomeres is present in hundreds to thousands of copies at the end of the linear chromosome. This sequence solves the puzzle by providing ‘extra’ non-essential DNA that the polymerase can bind to replicate the genome in its entirety. In fact, with each cell division, 50-100 of the building blocks (called nucleotides) that make up the ‘extra’ repeated DNA sequence in the telomere are lost. Without the telomere DNA, the lost sequence would be from genomic DNA at the end of the chromosome, and dramatic mutations would occur with every cell division.

Since the repeat DNA sequence starts off as a finite length, won’t genomic DNA eventually be lost if enough cell divisions, each with 50-100 building blocks removed from the telomere sequence, occur? In fact, this very phenomenon is thought to provide the cell with a way to “count” the number of cell divisions it has been through so it “knows” when to stop dividing or, in other words, when to become “senescent”. However, cellular senescence is delayed in cells that must divide regularly by the action of an enzyme named ‘telomerase’, discovered by Carol Greider in the laboratory of Elizabeth Blackburn. Telomerase carries with it a template for the DNA repeats within telomeres and allows new repeats to be added. The telomerase enzyme is highly active in embryonic stem cells and rapidly dividing cells of the immune system so that they can continue to divide to do their job. Most other cells, however, have very little active telomerase. Thus, telomerase plays a key role in controlling which cells in our bodies are allowed to continue to divide and which are limited.

Telomeres in disease

Part of the reason these and related discoveries about the biology of telomeres and telomerase are worthy of the Nobel Prize is that they have tangible implications and, indeed, proven links to diseases. Telomeres are now known to be critical for keeping chromosomes from attaching to one another and for regulating the aging process. Moreover, telomerase activity has been shown to be aberrantly re-activated in cancer, allowing cancer cells to avoid becoming senescent. The tantalizing possibilities of inhibiting the loss of telomeres to prevent senescence in order to slow aging and of blocking the activity of telomerase in cancer cells in order to induce their senescence are currently areas of active investigation within the scientific community.

These discoveries provide yet another example of how basic science research can unexpectedly have a profound impact on our understanding of disease and our future therapeutic approaches. The 2009 Nobel Prize in Physiology or Medicine is also the first time two women have shared the Prize in the same year. Advances on both these fronts are encouraging, and provide us with an important reminder that pursuing answers to curiosity-driven questions, no matter how seemingly esoteric, really can contribute to improving the human condition.

–Carrie L. Lucas, Harvard Medical School

For More Information:

Carol Greider and Elizabeth Blackburn. “Telomeres, Telomerase, and Cancer.” Scientific American. Reprint of Feb. 1996 article:
< http://www.scientificamerican.com/article.cfm?id=telomeres-telomerase-and >

Nobel Prize press release:
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Primary Literature:

Szostak JW, Blackburn EH. Cloning yeast telomeres on linear plasmid vectors. Cell 1982 29:245-255

Greider CW, Blackburn EH. Identification of a specific telomere terminal transferase activity in Tetrahymena extracts. Cell 1985 43:405-13.

Greider CW, Blackburn EH. A telomeric sequence in the RNA of Tetrahymena telomerase required for telomere repeat synthesis. Nature 1989 337:331-7