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Chapter 11 : Patterns of Genetic Inheritance

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Abstract:

This chapter explains the basic aspects of heredity, such as the physical basis of heredity, the mechanics of transmission and the basic laws governing genetic inheritance. In sexually reproducing organisms, genes and chromosomes occur in maternal-paternal, or homologous, pairs. Only one member of the pair is transmitted from each parent to its offspring. Meiosis is the cell division process that creates gametes with only one member of each homologous pair. Mendel’s Law of Segregation describes the behavior of homologous chromosomes: in meiosis, they separate prior to gamete formation. Mendel’s Law of Independent Assortment describes the behavior of nonhomologous chromosomes: when homologous chromosomes separate in meiosis, they do so independently of the other chromosomes. Using fruit flies as their experimental organism and relying on findings in cytology,T.H.Morgan and his students proved that genes are located at very specific sites, or loci, on chromosomes. During meiosis, homologous chromosomes exchange genetic material. Chromatids cross over each other, break, and rejoin quite specifically. This is known as recombination, because the genetic material of the two chromosomes has been recombined. The frequency of crossing over allows scientists to determine the relative distance between two genes on a chromosome. When distances among many genes on a chromosome have been determined, scientists are able to create a linkage map that shows the relative positions of genes on a chromosome.

Citation: Kreuzer H, Massey A. 2005. Patterns of Genetic Inheritance, p 233-255. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch11

Key Concept Ranking

Sister Chromatids
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Chromosome Structure
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Chromosomes
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Figures

Image of Figure 11.1
Figure 11.1

Preformationist view of heredity and reproduction. The preformationists believed a tiny replica of a person was contained in either the sperm or the egg.

Citation: Kreuzer H, Massey A. 2005. Patterns of Genetic Inheritance, p 233-255. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch11
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Image of Figure 11.2
Figure 11.2

Graphic representation of two different models of inheritance: fluid blending and discrete particle.

Citation: Kreuzer H, Massey A. 2005. Patterns of Genetic Inheritance, p 233-255. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch11
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Citation: Kreuzer H, Massey A. 2005. Patterns of Genetic Inheritance, p 233-255. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch11
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Image of Figure 11.3
Figure 11.3

Hand pollination of pea flowers. In all flowers, fusion of male and female gametes occurs when mature pollen, produced by the male reproductive organ (anther), lands on the stigma and makes its way down to the eggs. Fertilization leads to fruit and seed formation. In peas, the male and female organs are encased in a special structure, the keel, and pea flowers usually self-pollinate before they open. Keel cut away to show the reproductive organs. Mendel prevented self-pollination by prying open the keel and removing a flower's anthers before its pollen had matured. He cross-pollinated two plants by placing mature pollen from another plant (white flower) on the stigma of the flower lacking anthers. He saved the seeds produced by the cross, planted them the next year, and recorded the results—all offspring had purple flowers. For all seven traits, Mendel also did the reciprocal cross (purple flower as male, white as female) and got the same results.

Citation: Kreuzer H, Massey A. 2005. Patterns of Genetic Inheritance, p 233-255. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch11
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Image of Figure 11.4
Figure 11.4

Inheritance in peas. Mendel's experiments provided data on seven traits that occurred in two very distinct forms.

Citation: Kreuzer H, Massey A. 2005. Patterns of Genetic Inheritance, p 233-255. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch11
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Image of Figure 11.5
Figure 11.5

Mendel's experimental results. Mendel obtained similar results for each of the seven traits he studied. We use seed shape in the example below. Crossing purebred parents with different phenotypes led to the production of offspring with only one phenotype, which for the seed shape trait was round. Mendel described the phenotype that disappeared as recessive and the phenotype displayed by all first-generation offspring as dominant. Plants grown from the first-generation all-round offspring self-pollinated, and their seeds displayed both parental types, round and wrinkled, in a 3:1 ratio. When the second generation self-pollinated, all plants with wrinkled seeds produced only wrinkled seeds, and one-third of the plants grown from round seeds had only round seeds. Two-thirds of those grown from round seeds had both round and wrinkled seeds. Within their pods, the seeds occurred in a 3:1 ratio, round to wrinkled.

Citation: Kreuzer H, Massey A. 2005. Patterns of Genetic Inheritance, p 233-255. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch11
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Image of Figure 11.6
Figure 11.6

Mendel's Law of Segregation. Mendel's experimental results led him to the conclusion that the hereditary material is packaged as discrete particles, or factors, that occur in pairs for each trait (e.g., RR and rr). Each gamete contains only one member of the pair. The gametes fuse in fertilization, and the resulting offspring once again has a pair of factors for each trait. The best way to visualize Mendel's conclusions is with a matrix, or Punnett square, using a letter to represent a gene for a trait. The letter chosen to signify a trait is usually the first letter of the dominant trait. Uppercase signifies a dominant allele, while the recessive allele is represented by the same letter in lower case. In the case shown, R refers to round seed shape and r refers to wrinkled seed shape. Production of first generation of 100% round seeds. Production of second generation in 3:1 ratio.

Citation: Kreuzer H, Massey A. 2005. Patterns of Genetic Inheritance, p 233-255. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch11
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Image of Figure 11.7
Figure 11.7

Mendel's Law of Independent Assortment. By observing the hereditary patterns of two different traits simultaneously, such as seed shape and color, Mendel inferred that during gamete formation the maternal and paternal genes for one trait, such as seed shape, segregate independently of the maternal and paternal genes for a different trait, such as seed color. Subsequent research by others showed that this law applies only to genes on different chromosomes or genes located very far from each other on the same chromosome.

Citation: Kreuzer H, Massey A. 2005. Patterns of Genetic Inheritance, p 233-255. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch11
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Image of Figure 11.8
Figure 11.8

The language of genetics. Genetics has many unique terms that beginning students often have a difficult time remembering. Some of those terms are defined and graphically presented here. We have shown three pairs of alleles on chromosome 1, two heterozygous pairs and one homozygous pair. Chromosome 2, which has genes for traits not found on chromosome 1, has two pairs of alleles, one heterozygous and one homozygous.

Citation: Kreuzer H, Massey A. 2005. Patterns of Genetic Inheritance, p 233-255. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch11
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Image of Figure 11.9
Figure 11.9

Schematic of meiosis in roundworms. These cells, which are redrawn from Boveri's 1887 sketches of his observations, show the level of detail microscopes provided to researchers at that time. Even though we have used colors to indicate homologous pairs, this degree of discernment was impossible for Boveri. Note that the progenitor cell of the gametes contains four chromosomes. During meiosis, homologous chromosomes align, and the members of a pair separate as the cell divides, halving the number of chromosomes from four to two. In the last drawing, each gamete contains both a maternal and a paternal chromosome, but in roundworms, it is equally likely that both chromosomes in a gamete are of paternal or maternal origin.

Citation: Kreuzer H, Massey A. 2005. Patterns of Genetic Inheritance, p 233-255. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch11
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Image of Figure 11.10
Figure 11.10

Sex-linked inheritance. T. H. Morgan observed a unique pattern of inheritance for eye color in fruit flies. Morgan mated a red-eyed female from a purebred line to a white-eyed male. The first-generation offspring displayed the dominant phenotype, red eyes, as predicted by Mendel. When the first-generation offspring mated with each other, all of the flies having white eyes were males. This suggested that a relationship existed between the inheritance of eye color and the inheritance of sex. Morgan reasoned that the gene for eye color was carried on the X chromosome but that no corresponding gene locus for eye color occurred on the much smaller Y chromosome. He confirmed that prediction by crossing the original white-eyed male with a red-eyed (heterozygous) female from the first generation.

Citation: Kreuzer H, Massey A. 2005. Patterns of Genetic Inheritance, p 233-255. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch11
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Image of Figure 11.11
Figure 11.11

Linkage. In the example shown, the two traits being investigated for linkage are eye color and wing shape. The two eye color alleles are red and purple. The red-eye trait (R) is dominant to purple eyes (r).The two alleles for wing shape are normal and vestigial. Normal wing shape (N) is dominant to vestigial wings (n).Morgan crossed red-eyed, normal-winged individuals, who were heterozygous at both loci, with flies that had purple eyes and vestigial wings. The results indicated that the genes for eye color and wing shape are linked, because the numbers Morgan observed deviated significantly from the1:1:1:1 ratio Morgan expected. Had the genes for eye color and wing shape been on separate chromosomes, the numbers would have been distributed equally among the four phenotypes.

Citation: Kreuzer H, Massey A. 2005. Patterns of Genetic Inheritance, p 233-255. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch11
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Image of Figure 11.12
Figure 11.12

Crossing over. The micrograph shows chromatids of homologous chromosomes crossing over each other when they align in meiosis. (Photograph by Bernard John, courtesy of John Kimball.)

Citation: Kreuzer H, Massey A. 2005. Patterns of Genetic Inheritance, p 233-255. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch11
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Image of Figure 11.13
Figure 11.13

Crossing over between homologous chromosomes. Early in meiosis, after DNA replication, homologous chromosomes align so that genes for the same trait are located opposite each other. While paired, the double-stranded chromosomes exchange segments quite precisely. If the gene-for-gene exchange involves only one chromosome arm, two of the four possible gametes, numbered 1 to 4, contain the chromosome found in the parental cell and two contain a chromosome, known as a recombinant, containing a mixture of maternal and paternal alleles. In a single crossover, one chromatid exchanges genes with its homolog at a single point. One chromatid can also cross over its homolog at a number of loci. A single chromatid in each chromosome crosses at two points in what is known as a double crossover. Other times, both arms of a chromosome are exchanged. If the exchange occurs at different places along the sister chromatids, all four gametes will contain recombinant chromosomes; none contains a chromosome that is genetically identical to the parental chromosome. The gametes shown correspond to the data in Table 11.4 .

Citation: Kreuzer H, Massey A. 2005. Patterns of Genetic Inheritance, p 233-255. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch11
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Image of Figure 11.14
Figure 11.14

Linkage map. This genetic linkage map is constructed from the genetic results shown in Table 11.4 . Linkage maps show the relative positions of different gene loci on a chromosome. A large number of recombinants indicates a high frequency of crossing over between two loci. Therefore, in creating a map from results of genetic crosses, the higher the percentage of individuals with the recombinant phenotypes, the greater the distance between the two loci. This map corresponds to the chromosomes in Figure 11.13 that illustrate crossing over.

Citation: Kreuzer H, Massey A. 2005. Patterns of Genetic Inheritance, p 233-255. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch11
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References

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Tables

Generic image for table
Table 11.1

Mendel's results

Citation: Kreuzer H, Massey A. 2005. Patterns of Genetic Inheritance, p 233-255. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch11
Generic image for table
Table 11.2

Mendel's conclusions

Citation: Kreuzer H, Massey A. 2005. Patterns of Genetic Inheritance, p 233-255. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch11
Generic image for table
Table 11.3

Genetic linkage

Citation: Kreuzer H, Massey A. 2005. Patterns of Genetic Inheritance, p 233-255. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch11
Generic image for table
Table 11.4

Genetic linkage of traits

Citation: Kreuzer H, Massey A. 2005. Patterns of Genetic Inheritance, p 233-255. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch11

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