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8: Patterns of Inheritance - Biology


8: Patterns of Inheritance

Introduction

Genetics is the study of heredity. Johann Gregor Mendel set the framework for genetics long before chromosomes or genes had been identified, at a time when meiosis was not well understood. Mendel selected a simple biological system and conducted methodical, quantitative analyses using large sample sizes. Because of Mendel’s work, the fundamental principles of heredity were revealed. We now know that genes, carried on chromosomes, are the basic functional units of heredity with the ability to be replicated, expressed, or mutated. Today, the postulates put forth by Mendel form the basis of classical, or Mendelian, genetics. Not all genes are transmitted from parents to offspring according to Mendelian genetics, but Mendel’s experiments serve as an excellent starting point for thinking about inheritance.

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    • Authors: Samantha Fowler, Rebecca Roush, James Wise
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    • Book title: Concepts of Biology
    • Publication date: Apr 25, 2013
    • Location: Houston, Texas
    • Book URL: https://openstax.org/books/concepts-biology/pages/1-introduction
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    Chapter 8: Introduction to Patterns of Inheritance

    Figure 8.1 Experimenting with thousands of garden peas, Mendel uncovered the fundamentals of genetics. (credit: modification of work by Jerry Kirkhart)

    Genetics is the study of heredity. Johann Gregor Mendel set the framework for genetics long before chromosomes or genes had been identified, at a time when meiosis was not well understood. Mendel selected a simple biological system and conducted methodical, quantitative analyses using large sample sizes. Because of Mendel’s work, the fundamental principles of heredity were revealed. We now know that genes, carried on chromosomes, are the basic functional units of heredity with the ability to be replicated, expressed, or mutated. Today, the postulates put forth by Mendel form the basis of classical, or Mendelian, genetics. Not all genes are transmitted from parents to offspring according to Mendelian genetics, but Mendel’s experiments serve as an excellent starting point for thinking about inheritance.


    8.3 Extensions of the Laws of Inheritance

    By the end of this section, you will be able to:

    • Identify non-Mendelian inheritance patterns such as incomplete dominance, codominance, multiple alleles, and sex linkage from the results of crosses
    • Explain the effect of linkage and recombination on gamete genotypes
    • Explain the phenotypic outcomes of epistatic effects among genes

    Mendel studied traits with only one mode of inheritance in pea plants. The inheritance of the traits he studied all followed the relatively simple pattern of dominant and recessive alleles for a single characteristic. There are several important modes of inheritance, discovered after Mendel’s work, that do not follow the dominant and recessive, single-gene model.

    Alternatives to Dominance and Recessiveness

    Mendel’s experiments with pea plants suggested that: 1) two types of “units” or alleles exist for every gene 2) alleles maintain their integrity in each generation (no blending) and 3) in the presence of the dominant allele, the recessive allele is hidden, with no contribution to the phenotype. Therefore, recessive alleles can be “carried” and not expressed by individuals. Such heterozygous individuals are sometimes referred to as “carriers.” Since then, genetic studies in other organisms have shown that much more complexity exists, but that the fundamental principles of Mendelian genetics still hold true. In the sections to follow, we consider some of the extensions of Mendelism.

    Incomplete Dominance

    Mendel’s results, demonstrating that traits are inherited as dominant and recessive pairs, contradicted the view at that time that offspring exhibited a blend of their parents’ traits. However, the heterozygote phenotype occasionally does appear to be intermediate between the two parents. For example, in the snapdragon, Antirrhinum majus (Figure 8.12), a cross between a homozygous parent with white flowers (C W C W ) and a homozygous parent with red flowers (C R C R ) will produce offspring with pink flowers (C R C W ). (Note that different genotypic abbreviations are used for Mendelian extensions to distinguish these patterns from simple dominance and recessiveness.) This pattern of inheritance is described as incomplete dominance, meaning that one of the alleles appears in the phenotype in the heterozygote, but not to the exclusion of the other, which can also be seen. The allele for red flowers is incompletely dominant over the allele for white flowers. However, the results of a heterozygote self-cross can still be predicted, just as with Mendelian dominant and recessive crosses. In this case, the genotypic ratio would be 1 C R C R :2 C R C W :1 C W C W , and the phenotypic ratio would be 1:2:1 for red:pink:white. The basis for the intermediate color in the heterozygote is simply that the pigment produced by the red allele (anthocyanin) is diluted in the heterozygote and therefore appears pink because of the white background of the flower petals.

    Figure 8.12 These pink flowers of a heterozygote snapdragon result from incomplete dominance. (credit: "storebukkebruse"/Flickr)

    A variation on incomplete dominance is codominance, in which both alleles for the same characteristic are simultaneously expressed in the heterozygote. An example of codominance occurs in the ABO blood groups of humans. The A and B alleles are expressed in the form of A or B molecules present on the surface of red blood cells. Homozygotes (I A I A and I B I B ) express either the A or the B phenotype, and heterozygotes (I A I B ) express both phenotypes equally. The I A I B individual has blood type AB. In a self-cross between heterozygotes expressing a codominant trait, the three possible offspring genotypes are phenotypically distinct. However, the 1:2:1 genotypic ratio characteristic of a Mendelian monohybrid cross still applies (Figure 8.13).

    Figure 8.13 This Punnet square shows an AB/AB blood type cross

    Multiple Alleles

    Mendel implied that only two alleles, one dominant and one recessive, could exist for a given gene. We now know that this is an oversimplification. Although individual humans (and all diploid organisms) can only have two alleles for a given gene, multiple alleles may exist at the population level, such that many combinations of two alleles are observed. Note that when many alleles exist for the same gene, the convention is to denote the most common phenotype or genotype in the natural population as the wild type (often abbreviated “+”). All other phenotypes or genotypes are considered variants (mutants) of this typical form, meaning they deviate from the wild type. The variant may be recessive or dominant to the wild-type allele.

    An example of multiple alleles is the ABO blood-type system in humans. In this case, there are three alleles circulating in the population. The I A allele codes for A molecules on the red blood cells, the I B allele codes for B molecules on the surface of red blood cells, and the i allele codes for no molecules on the red blood cells. In this case, the I A and I B alleles are codominant with each other and are both dominant over the i allele. Although there are three alleles present in a population, each individual only gets two of the alleles from their parents. This produces the genotypes and phenotypes shown in Figure 8.14. Notice that instead of three genotypes, there are six different genotypes when there are three alleles. The number of possible phenotypes depends on the dominance relationships between the three alleles.

    Figure 8.14 Inheritance of the ABO blood system in humans is shown.

    EVOLUTION IN ACTION

    Multiple Alleles Confer Drug Resistance in the Malaria Parasite

    Malaria is a parasitic disease in humans that is transmitted by infected female mosquitoes, including Anopheles gambiae, and is characterized by cyclic high fevers, chills, flu-like symptoms, and severe anemia. Plasmodium falciparum and P. vivax are the most common causative agents of malaria, and P. falciparum is the most deadly. When promptly and correctly treated, P. falciparum malaria has a mortality rate of 0.1 percent. However, in some parts of the world, the parasite has evolved resistance to commonly used malaria treatments, so the most effective malarial treatments can vary by geographic region.

    In Southeast Asia, Africa, and South America, P. falciparum has developed resistance to the anti-malarial drugs chloroquine, mefloquine, and sulfadoxine-pyrimethamine. P. falciparum, which is haploid during the life stage in which it is infective to humans, has evolved multiple drug-resistant mutant alleles of the dhps gene. Varying degrees of sulfadoxine resistance are associated with each of these alleles. Being haploid, P. falciparum needs only one drug-resistant allele to express this trait.

    In Southeast Asia, different sulfadoxine-resistant alleles of the dhps gene are localized to different geographic regions. This is a common evolutionary phenomenon that comes about because drug-resistant mutants arise in a population and interbreed with other P. falciparum isolates in close proximity. Sulfadoxine-resistant parasites cause considerable human hardship in regions in which this drug is widely used as an over-the-counter malaria remedy. As is common with pathogens that multiply to large numbers within an infection cycle, P. falciparum evolves relatively rapidly (over a decade or so) in response to the selective pressure of commonly used anti-malarial drugs. For this reason, scientists must constantly work to develop new drugs or drug combinations to combat the worldwide malaria burden. [2]

    Sex-Linked Traits

    In humans, as well as in many other animals and some plants, the sex of the individual is determined by sex chromosomes—one pair of non-homologous chromosomes. Until now, we have only considered inheritance patterns among non-sex chromosomes, or autosomes. In addition to 22 homologous pairs of autosomes, human females have a homologous pair of X chromosomes, whereas human males have an XY chromosome pair. Although the Y chromosome contains a small region of similarity to the X chromosome so that they can pair during meiosis, the Y chromosome is much shorter and contains fewer genes. When a gene being examined is present on the X, but not the Y, chromosome, it is X-linked.

    Eye color in Drosophila, the common fruit fly, was the first X-linked trait to be identified. Thomas Hunt Morgan mapped this trait to the X chromosome in 1910. Like humans, Drosophila males have an XY chromosome pair, and females are XX. In flies the wild-type eye color is red (X W ) and is dominant to white eye color (X w ) (Figure 8.15). Because of the location of the eye-color gene, reciprocal crosses do not produce the same offspring ratios. Males are said to be hemizygous, in that they have only one allele for any X-linked characteristic. Hemizygosity makes descriptions of dominance and recessiveness irrelevant for XY males. Drosophila males lack the white gene on the Y chromosome that is, their genotype can only be X W Y or X w Y. In contrast, females have two allele copies of this gene and can be X W X W , X W X w , or X w X w .

    Figure 8.15 In Drosophila, the gene for eye color is located on the X chromosome. Red eye color is wild-type and is dominant to white eye color.

    In an X-linked cross, the genotypes of F1 and F2 offspring depend on whether the recessive trait was expressed by the male or the female in the P generation. With respect to Drosophila eye color, when the P male expresses the white-eye phenotype and the female is homozygously red-eyed, all members of the F1 generation exhibit red eyes (Figure 8.16). The F1 females are heterozygous (X W X w ), and the males are all X W Y, having received their X chromosome from the homozygous dominant P female and their Y chromosome from the P male. A subsequent cross between the X W X w female and the X W Y male would produce only red-eyed females (with X W X W or X W X w genotypes) and both red- and white-eyed males (with X W Y or X w Y genotypes). Now, consider a cross between a homozygous white-eyed female and a male with red eyes. The F1 generation would exhibit only heterozygous red-eyed females (X W X w ) and only white-eyed males (X w Y). Half of the F2 females would be red-eyed (X W X w ) and half would be white-eyed (X w X w ). Similarly, half of the F2 males would be red-eyed (X W Y) and half would be white-eyed (X w Y).

    ART CONNECTION

    Figure 8.16 Crosses involving sex-linked traits often give rise to different phenotypes for the different sexes of offspring, as is the case for this cross involving red and white eye color in Drosophila. In the diagram, w is the white-eye mutant allele and W is the wild-type, red-eye allele.

    What ratio of offspring would result from a cross between a white-eyed male and a female that is heterozygous for red eye color?

    Discoveries in fruit fly genetics can be applied to human genetics. When a female parent is homozygous for a recessive X-linked trait, she will pass the trait on to 100 percent of her male offspring, because the males will receive the Y chromosome from the male parent. In humans, the alleles for certain conditions (some color-blindness, hemophilia, and muscular dystrophy) are X-linked. Females who are heterozygous for these diseases are said to be carriers and may not exhibit any phenotypic effects. These females will pass the disease to half of their sons and will pass carrier status to half of their daughters therefore, X-linked traits appear more frequently in males than females.

    In some groups of organisms with sex chromosomes, the sex with the non-homologous sex chromosomes is the female rather than the male. This is the case for all birds. In this case, sex-linked traits will be more likely to appear in the female, in whom they are hemizygous.

    CONCEPT IN ACTION

    Linked Genes Violate the Law of Independent Assortment

    Although all of Mendel’s pea plant characteristics behaved according to the law of independent assortment, we now know that some allele combinations are not inherited independently of each other. Genes that are located on separate, non-homologous chromosomes will always sort independently. However, each chromosome contains hundreds or thousands of genes, organized linearly on chromosomes like beads on a string. The segregation of alleles into gametes can be influenced by linkage, in which genes that are located physically close to each other on the same chromosome are more likely to be inherited as a pair. However, because of the process of recombination, or “crossover,” it is possible for two genes on the same chromosome to behave independently, or as if they are not linked. To understand this, let us consider the biological basis of gene linkage and recombination.

    Homologous chromosomes possess the same genes in the same order, though the specific alleles of the gene can be different on each of the two chromosomes. Recall that during interphase and prophase I of meiosis, homologous chromosomes first replicate and then synapse, with like genes on the homologs aligning with each other. At this stage, segments of homologous chromosomes exchange linear segments of genetic material (Figure 8.17). This process is called recombination, or crossover, and it is a common genetic process. Because the genes are aligned during recombination, the gene order is not altered. Instead, the result of recombination is that maternal and paternal alleles are combined onto the same chromosome. Across a given chromosome, several recombination events may occur, causing extensive shuffling of alleles.

    Figure 8.17 The process of crossover, or recombination, occurs when two homologous chromosomes align and exchange a segment of genetic material.

    When two genes are located on the same chromosome, they are considered linked, and their alleles tend to be transmitted through meiosis together. To exemplify this, imagine a dihybrid cross involving flower color and plant height in which the genes are next to each other on the chromosome. If one homologous chromosome has alleles for tall plants and red flowers, and the other chromosome has genes for short plants and yellow flowers, then when the gametes are formed, the tall and red alleles will tend to go together into a gamete and the short and yellow alleles will go into other gametes. These are called the parental genotypes because they have been inherited intact from the parents of the individual producing gametes. But unlike if the genes were on different chromosomes, there will be no gametes with tall and yellow alleles and no gametes with short and red alleles. If you create a Punnett square with these gametes, you will see that the classical Mendelian prediction of a 9:3:3:1 outcome of a dihybrid cross would not apply. As the distance between two genes increases, the probability of one or more crossovers between them increases and the genes behave more like they are on separate chromosomes. Geneticists have used the proportion of recombinant gametes (the ones not like the parents) as a measure of how far apart genes are on a chromosome. Using this information, they have constructed linkage maps of genes on chromosomes for well-studied organisms, including humans.

    Mendel’s seminal publication makes no mention of linkage, and many researchers have questioned whether he encountered linkage but chose not to publish those crosses out of concern that they would invalidate his independent assortment postulate. The garden pea has seven chromosomes, and some have suggested that his choice of seven characteristics was not a coincidence. However, even if the genes he examined were not located on separate chromosomes, it is possible that he simply did not observe linkage because of the extensive shuffling effects of recombination.

    Epistasis

    Mendel’s studies in pea plants implied that the sum of an individual’s phenotype was controlled by genes (or as he called them, unit factors), such that every characteristic was distinctly and completely controlled by a single gene. In fact, single observable characteristics are almost always under the influence of multiple genes (each with two or more alleles) acting in unison. For example, at least eight genes contribute to eye color in humans.

    CONCEPT IN ACTION

    Eye color in humans is determined by multiple alleles. Use the Eye Color Calculator (http://openstaxcollege.org/l/eye_color_calc) to predict the eye color of children from parental eye color.

    In some cases, several genes can contribute to aspects of a common phenotype without their gene products ever directly interacting. In the case of organ development, for instance, genes may be expressed sequentially, with each gene adding to the complexity and specificity of the organ. Genes may function in complementary or synergistic fashions, such that two or more genes expressed simultaneously affect a phenotype. An apparent example of this occurs with human skin color, which appears to involve the action of at least three (and probably more) genes. Cases in which inheritance for a characteristic like skin color or human height depend on the combined effects of numerous genes are called polygenic inheritance.

    Genes may also oppose each other, with one gene suppressing the expression of another. In epistasis, the interaction between genes is antagonistic, such that one gene masks or interferes with the expression of another. “Epistasis” is a word composed of Greek roots meaning “standing upon.” The alleles that are being masked or silenced are said to be hypostatic to the epistatic alleles that are doing the masking. Often the biochemical basis of epistasis is a gene pathway in which expression of one gene is dependent on the function of a gene that precedes or follows it in the pathway.

    An example of epistasis is pigmentation in mice. The wild-type coat color, agouti (AA) is dominant to solid-colored fur (aa). However, a separate gene C, when present as the recessive homozygote (cc), negates any expression of pigment from the A gene and results in an albino mouse (Figure 8.18). Therefore, the genotypes AAcc, Aacc, and aacc all produce the same albino phenotype. A cross between heterozygotes for both genes (AaCc x AaCc) would generate offspring with a phenotypic ratio of 9 agouti:3 black:4 albino (Figure 8.18). In this case, the C gene is epistatic to the A gene.

    Figure 8,18 In this example of epistasis, one gene (C) masks the expression of another (A) for coat color. When the C allele is present, coat color is expressed when it is absent (cc), no coat color is expressed. Coat color depends on the A gene, which shows dominance, with the recessive homozygote showing a different phenotype than the heterozygote or dominant homozygote.

    8: Patterns of Inheritance - Biology

    We all have heard how we get certain traits from our parents such as the color of our eyes or how tall we are. These traits are passed on by genes in our DNA. Half of our DNA comes from our mother and half from our father.

    Scientists have discovered that genes are inherited in certain patterns. What genes your parents and grandparents have, affects what genes you have. On this page we will learn how those patterns work.

    We learned some of the basics about inheritance on the Mendel and Inheritance page. You can also go to our DNA page and our chromosome page to learn more.

    A few things you should know about genes and inheritance:

    Gene - Inside the DNA molecule are sections of information called genes. Each gene tells the cell how to make a certain protein which may determine a trait such as the color of the eyes.

    Allele - While the section of DNA is called a gene, a specific pattern in a gene is called an allele. For example, the gene would determine the hair color. The specific pattern of the hair color gene that causes the hair to be black would be the allele.

    Dominant and Recessive Genes

    Each child inherits two genes for each trait from their parents. Some genes are more dominant than others. For example, brown eyes are dominant over blue eyes. If someone has a brown eyed gene and a blue eye gene, they will have brown eyes. They will only have blue eyes if both genes are blue.

    The brown eyed gene is called the dominant gene and the blue eyed gene is the recessive gene.

    Writing out the Genes

    In order to write out the specific allele a person has for a gene, you write a letter representing the gene from the mother and a letter for the gene from the father. Dominant genes are written with capital letters and recessive genes with lower case letters. Here is an example:

    • Bb - one brown gene, one blue gene (this person will have brown eyes)
    • BB - both brown genes (this person will have brown eyes)
    • bb - both blue genes (this person will have blue eyes)

    The main way to figure out the pattern of inheritance that could come from two parents is using a Punnet square. A Punnet square shows all the possible combinations of genes from the parents.

    We will use the example of a plant that could have a purple flower or a white flower. The purple gene is dominant and we write it "P." The white gene is recessive, so we write it "w." Here is an example of a Punnet square where one parent has two purple genes "P" and the other parent has two white "w" genes.

    Each child has the same gene pattern "Pw". They all have the dominant P gene and will all have purple flowers.

    Here is another example where each parent has a purple gene and a white gene (Pw):

    In this case, you can see that 75% of the children will have a dominant "P" gene and will have a purple flower. However, 25% of the children have "ww" genes and will have a white flower.

    More Punnet Square Examples

    In this example, one parent is PP and the other Pw.

    All of the children will have purple flowers, but because one parent has a recessive "w" gene, 50% of the children will pass on the "w" gene.

    Now look at what happens if only one parent has a single dominant P gene where one parent is "Pw" and the other "ww".

    You can see that 50% of the children will have white flowers and 50% purple.


    INHERITANCE PATTERN

    The law of Mendel states that the allele of gene present on the homologous chromosomes segregates during meiosis in such a way that each gamete get one allele not both. The genes in each parent are incorporated into separate gametes during gamete formation. The homologous chromosomes move towards opposite poles of the cell during anaphase I of meiosis. Therefore, the gametes have only one member of each chromosome pair. The allele of genes present on one member of a pair of homologous chromosomes enters into one gamete. The other allele of that gene is present on the other member. These alleles are segregated into a different gamete. There is random combination of gametes during fertilization. It brings homologous chromosomes together again.

    Proof of Principle of segregation

    There are two types of fruit flies (Drosophila):

    • Wild-type fruit flies: They have normal wings.
    • Vestigial wings: They have very reduced wings.

    These flies are taken from the true breed stock. These flies have been inbred for

    many generations. Thus they breed true for wild type wings or vestigial Wings

    The F1 flies are allowed to mate with each other. The progeny of F2 generation has:

    The vestigial characteristic disappears in the F1 generation. But it reappears in the F2 generation. The ratio of wild-type files to vestigial-winged flies in the F2 generation is approximately 3:1.

    Reciprocal crosses

    The cross with same characteristics but a reversal of the sexes of the individuals is called reciprocal cross. This cross gives similar results.

    Interpretation of the crosses

    . Gene and alleles: Genes of a particular trait eXist in alternative forms The alternative forms of gene are called alleles.

    • Dominant and recessive alleles: The vestigial allele is present in the Fi generation in the fruit fly. It is masked by the wild-type allele for wing shape. But it retains its uniqueness because it is expressed again in some members of the F2 generation.
    • Dominant allele hides the expression of recessive alleles. The wild-type allele is dominant. It masks the expression of the vestigial allele.
    • Recessive alleles are those whose expression can be masked. The allele of vestigial wing is recessive. Representation of alleles: Crosses are expressed by letter or letters. These letters are descriptive of the trait in question. The first letter of the description of the dominant allele is commonly used. In fruit flies, the mutants are compared with a wild-type. The symbol is taken from the allele that was derived by a mutation from wild condition. A superscript “+” is written next to the symbol. It represents the wild-type allele. A capital letter means that the mutant allele is dominant. A small letter means the mutant allele is recessive
    • 4. Phenotypes and genotypes:
      • The physical expression of a gene is called phenotype. The physical expression of alleles does not indicate the genetic makeup of an organism.
      • The genetic makeup is called the genotype: The F1 generations have

      5.Homozygous and heterozygous: An organism is homozygous . if it carries two identical genes for a given trait. Thus all members of the parental generation are homozygous. An organism is heterozygous if the genes are different. Only true-breeding flies are crossed. All members of the F1 generation are heterozygous.

      6.Punnett square: Punnett square is used to predict the result of crosses. The first step is to determine the kinds of gametes.

      One of the two axes of a square is designated for each parent. The different kinds of gametes are listed along the axis. The gametes are combined in the interior of the square. It shows the results of random fertilization. The figure indicates that the F1 flies are heterozygous. These flies have one wild-type allele and one vestigial allele.

      The two phenotypes of the F2 generation shows 3:1 ratio in Punnett square. The phenotypic ratio expresses the – results of a cross. This result is obtained from the relative numbers of progeny in each class. The Punnett square has thus explained in another way the F2 results in figure. It also shows that F2 individuals may have one of three different genotypes. The genotypic ratio expresses the results of a cross according to the relative numbers of progeny in each genotypic category. These are 1 vg + vg: 2 vg + vg: 1 vg vg.

      INDEPENDENT ASSORTMENT

      It states that, “When alleles of more than one trait are followed together in cross, the alleles of these traits assort independently to each other during amete formation.” It is also possible to make crosses using flies with two pairs

      • Mutants: These are flies with vestigial wings and sepia eyes. Sepia eyes are dark brown. These are represented as vg, se
      • Wild type: The flies are wild for these characteristics. The wild-type eyes are red. These are represented as vg, se +

      The flies in the parental generation are homozygous for each trait. Therefore, ach parent produces only one kind of gamete. Gametes have one allele for each trait. Fertilization takes place. It produces offspring heterozygous for both raits. The F1 flies have the wild type phenotype. Thus, wild-type eyes are dominant to sepia eyes. The F1 flies are hybrids. The crosi involves two pairs of enes and two traits. Therefore, it is a dihybrid.

      he F1 hybrids are crossed to obtain F2. The F2 gives 9:3:3:1 ratio. It is a typical ‘hybrid cross ratio. This ratio shows that the alleles have assorted independently.

      Interpretation of independent assortment from meiosis he distribution of genes of one trait does not influence distribution of other gene during gamete formation. F1 gamete have vg gene for wing condition. It also has the se or se + gene for eye color. All the combinations of the eye color and wing condition genes are present. These combinations can form equally. This shows the principle of independent assortment and the pairs of factors segregate i dependently of one another.

      The steps of meiosis explain the principle of independent assortment. Meiosis reduces haploid daughter cells. These cells have one member of each homologous pair of chromosomes. The homologous chromosomes line up at metaphase I. They then segregate from each other. The behavior of one pair of chromosomes does not influence the behavior of any other pair. The maternal and paternal chromosomes are distributed randomly among cells after meiosis.

      Possible combinations of chromosomes in haploid cells after seg­egation of homologous chromosomes during meiosis I. All possible combinations of one member of each pair is represented.

      Meiosis II results in separation of chromatids but no further reduction in chromosome number.

      THER INHERITANCE PATTERN

      Multiple alleles

      The presence of more than two alleles in a single gene in different combination is called multiple alleles. Normally two genes determine the traits i one individual. One gene is carried on each chromosome of a homologous pair. Some populations have many different alleles. They can transfer these alleles to any member of the population. These are called multiple alleles.

      Genes for a particular trait are present at the same position on a chromosome. e position of gene on the chromosome is ‘called its locus. Numerous human loci have multiple alleles. Three alleles, symbolized I A , 1 8 and i, determine the ABO blood types.

      • The allele i is recessive to I A and to 1 8 .
      • I A and 1 8 are neither dominant nor recessive to each other. When I A and 1 8 are present together, both are expressed.

      •smorrrs . PIIIINOTYPS

      re, es A

      A and B

      HamozYgous Dominant

      a WO % =- 1 0- 1 Thoduct •• Dominant

      FdliklotYiN

      Hstscorivous

      Substrata %• = if s Product – Dominant

      Plo Pharx4YIN

      a – No product

      Homozygous Reassaiss

      ____ a- No enzyme a

      a Submit* —s- No product u• Racossive

      Incomplete Dominance and Codominance

      The interaction between two alleles that are expressed more or less equally, and the heterozygote is different from their homozygote is called incomplete dominance.

      In cattle, the alleles for red coat color and for white coat color’ interact to produce an intermediate coat color. This colour is called roan. Both red and the white – allele are not dominant. Therefore, capital letters and a superscript are used to represent genes. Thus, red cattle are symbolized RR, white cattle are symbolized

      Codominance

      The phenomenon in which heterozygote expresses the phenotypes of both mozygote is called Codominance. In the ABO blood types, the 1 A , I B h terozygote expresses both alleles.

      ENVIRONMENTAL EFFECTS AND GENE EXPRESSION

      Mutations and the various mechanisms of genetic recombination cause genetic recombination. These variations make natural selection possible. And natural selection al s over many generations. It can increase the proportion of mutants in a b cterial population. These mutants are adapted to some new environmental cendition. They can resist specific antibiotic. All variations are produced by

      environment. Thus environment greatly affects the gene expression. For

      e ample: E. coli bacterium lives in the changing environment of a human gut. It d pends in its host for nutrients.

      (a Sometimes, its host does not provide it tryptophan amino acid. This amino acid is necessary for its survival. Therefore, the bacteria activate its genes of tryptophan synthesis. It-stimulates the metabolic pathway to make its own tryptophan from another compound.

      (b Later, if the human host eats a tryptophan-rich meal, the cell stops producing tryptophan for itself. It saves the cell from wasting its resources to produce a substance. This substance is now available in the surroundings solution. Thus it stops the expression of genes of tryptophan. It shows that the environment directly affect the expression of tryptophan gene.

      Control of metabolism: Metabolic control occurs at two levels

      1. Cells can vary the numbers of specific enzymes. Thus the cell can regulate the expression of a gene of that enzyme.
      2. Second. cells can vary the activities of enzymes already present. It is more immediate control. It depends on the sensitivity of many enzymes. These enzymes can increase or decrease their catalytic activity. •

      For example, end product inhibits the activity of the first enzyme of the tryptophan synthesis pathway. Thus, if tryptophan accumulates in a cell, it shuts down its own synthesis. It is a feedback inhibition. It allows a cell to adapt to short-term fluctuotions in levels of a substance it needs. If the environment continues to provic.7 all the tryptophan the cell needs, then regulation of gene expression also cores into play: The cell stops making enzymes of the tryptophan pathway. Tnis control of enzyme occurs at transcription level. The synthesis Of messenger RNA coding for these enzymes is stopped. Thus many genes of the bacterial genome are switched on or off by changes in the metabolic status of the cell. It is the basic mechanism for this control of gene expression.


      Understanding Genetics: A District of Columbia Guide for Patients and Health Professionals.

      The basic laws of inheritance are important in understanding patterns of disease transmission. The inheritance patterns of single gene diseases are often referred to as Mendelian since Gregor Mendel first observed the different patterns of gene segregation for selected traits in garden peas and was able to determine probabilities of recurrence of a trait for subsequent generations. If a family is affected by a disease, an accurate family history will be important to establish a pattern of transmission. In addition, a family history can even help to exclude genetic diseases, particularly for common diseases where behavior and environment play strong roles.

      Most genes have one or more versions due to mutations or polymorphisms referred to as alleles. Individuals may carry a ‘normal’ allele and/or a 𠆍isease’ or ‘rare’ allele depending on the impact of the mutation/polymorphism (e.g., disease or neutral) and the population frequency of the allele. Single-gene diseases are usually inherited in one of several patterns depending on the location of the gene and whether one or two normal copies of the gene are needed for the disease phenotype to manifest.

      The expression of the mutated allele with respect to the normal allele can be characterized as dominant, co-dominant, or recessive. There are five basic modes of inheritance for single-gene diseases: autosomal dominant, autosomal recessive, X-linked dominant, X-linked recessive, and mitochondrial.


      Watch the video: OpenStax Concepts of Biology Ch 8 Patterns of Inheritance (December 2021).