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| Extended Lecture Outline |
Chapter 16: Mendelian Heredity |
16.1 Heredity is determined by discrete, conserved "factors."
a. Heredity of common features is obvious in many organisms, especially in humans (Figure 16.1).
b. The rules governing inheritance were discovered by Mendel around 1850, but were not made widely known until 1900.
c. Mendel's success was determined by several factors:
1. He studied only a few characteristics.
2. He studied each characteristic independently (he luckily selected traits that were determined by unlinked genes).
3. He analyzed his results quantitatively.
4. He used purebred strains for making clearly defined crosses for the trait in question.
d. Figure 16.2 shows Mendel's experiment with yellow-seeded and green-seeded peas.
e. Generations of offspring from a cross are known as first filial (F1), second filial (F2), and so on.
f. When Mendel crossed pure-bred yellow-seeded peas with pure-bred green-seeded peas, all the F1 peas were green-seeded. When these were crossed with each other, Mendel discovered an approximate 3:1 ratio of green-seeded F2 peas to yellow-seeded F2 peas, and noted that the yellow-seeded peas reappeared.
g. Mendel's results led him to propose that hereditary factors are conserved from one generation to another, even if their effects are not seen in every generation.
h. Mendel also assumed that there are two hereditary factors for each characteristic, and termed the factor that appears dominant, and the factor that is hidden recessive.
i. Mendel's factors are now called alleles, which are alternative forms of each gene.
j. In genetics nomenclature, dominant alleles are abbreviated with capital letters (e.g. Y), and recessive alleles are abbreviated with lower case letters (e.g. y).
k. Figure 16.3 shows the inheritance of pea color in Mendel's experiment.
l. Individuals with identical alleles for the same gene (e.g. YY or yy) are called homozygotes, and are said to be homozygous for that gene.
m. Individuals with different alleles for the same gene (e.g. Yy) are called heterozygotes, and are said to be heterozygous for that gene.
n. Concepts 16.1 provides further clarification of the terms gene, allele, homozygous, heterozygous, genotype, and phenotype.
o. Mendel's First Law, the Law of Segregation, states that the two factors carried by each individual for each trait separate, or segregate, during the formation of germ cells. In modern terms, the two alleles, which are carried on different homologous chromosomes, are separated into different daughter cells during meiosis I.
p. A Punnett square is useful for showing the segregated alleles produced by each parent, and all the equally possible combinations of these alleles that could result from random fertilization.
q. The phenotype of an organism is its observable characteristics, and the genotype is the information encoded in its genome.
r. When referring to phenotypes and genotypes, it is conventional to limit the traits and genes to just the characters in question (e.g. pea color), rather than to include all the features of the organism.
s. Sidebar 16.1 and Figures 16.A and 16.B address variance in populations, normal distributions, and the "nature vs. nurture" debate.
16.2 Regular inheritance depends upon stability of the chromosome set.
a. Mendelian inheritance follows directly from meiosis and fertilization, two key events in sexual reproduction.
b. Every sexually reproducing organism has a characteristic set of chromosomes, and the genes located on each of them do not move from one chromosome to another, but are stable throughout both meiosis and fertilization events.
c. Figure 16.4 illustrates this idea using the seed-color gene, located on chromosome #1 of Mendel's pea plants.
16.3 The chance of two events happening together is the product of the chances that they will happen independently.
a. The product rule of probability states that the probability of two independent events happening is the product of their individual probabilities.
b. For example, the probability that it will rain today and you will get a letter from a friend is the product of each individual probability (Figure 16.5).
c. The sum, or addition, rule of probability states that the probability of an event that can happen in two or more mutually exclusive ways is the sum of the probabilities for each way it could happen.
d. For example, the probability of rolling an eleven with two dice is the sum of the two ways it could happen: 5 with 6, or 6 with 5. These events are mutually exclusive–they cannot both happen at the same time, but each is one way to roll an eleven–thus their probabilities must be added for the answer.
16.4 Not all genes exhibit dominance.
a. Mendel was fortunate to have studied traits that all exhibit simple dominance (e.g. green peas are dominant and yellow peas are recessive.)
b. Some crosses of purebred strains (e.g. red and white zinnias) produce offspring with an intermediate phenotype, and illustrate incomplete dominance (Figure 16.6).
c. A simple explanation for this phenomenon would be that the gene products act in an additive way: genotype RR codes for two doses of the enzyme producing red flower color, rr codes for no doses of red (giving a white flower), and Rr codes for just one dose of red, so that the flower is less red, or pink.
16.5 Genetic crosses show only a few simple patterns.
a. One should learn to recognize common phenotypic and genotypic ratios, such as 3:1 and 1:2:1, respectively, from a cross of two heterozygotes in a simple dominance system.
b. Table 16.1 shows six possible matings involving A and a, a pair of alleles, and represent the six basic genetic situations that should be easily recognized in analyzing crosses.
16.6 Genotypes can be determined with testcrosses.
a. In a simple dominance system (e.g. both PP and Pp produce purple flowers, and pp produces white flowers), the genotype of an individual with the dominant phenotype (purple) is not necessarily known; such an individual's genotype would be written P-.
b. A testcross mates an individual with dominant phenotype (e.g. purple) and unknown genotype, with an individual that is homozygous recessive (e.g. pp), to find the unknown genotype of the dominant individual.
c. A testcross can thus be said to reveal the hidden heterozygotes in a population of dominant phenotypes (Figure 16.7).
16.7 Two genes may be inherited quite independently of each other.
a. Genes that are on different chromosomes, such as those studied by Mendel, account for traits that are inherited independently.
b. In solving genetics problems, one should always think independently about such independently inherited traits, then combine the results according to the product and sum rules, in the final analysis.
c. Figure 16.8 illustrates this method for a Mendelian cross involving two independently inherited traits, seed color and seed shape.
d. In genetics nomenclature, a slash between two alleles (e.g. Y/Y) is a reminder that the alleles are on homologs of the same chromosome; similarly, Y/Y R/R would indicate that the genes Y and R are on different chromosomes, since two slashes are used in writing the genotype.
e. Figure 16.9 illustrates Mendel's Second Law, the Law of Independent Segregation (also known as the Law of Independent Assortment).
f. This law states that, during metaphase I, the orientation of one pair of homologs on the metaphase plate does not influence the orientation of any other pair of homologs, such that the maternal and paternal homologs in each tetrad separate to opposite poles independently.
g. This phenomenon accounts for increased genetic variation, as maternal and paternal alleles are mixed during meiosis I.
h. Figure 16.10 shows a Punnett square analysis of a two-gene cross of two double heterozygotes, R/r Y/y and R/r Y/y.
i. The use of probabilities, rather than the Punnett square, is encouraged as a faster and less cumbersome method of analyzing crosses involving multiple sets of genes.
16.8 Interactions between genes can produce unexpected ratios of phenotypes.
a. Epistasis is the term for gene interactions in general, and literally means "to stand upon," as most epistatic interactions involve one or more genes suppressing the effects of other genes.
b. One general category of epistatic interactions involves two genes encoding enzymes for a single pathway, and results in a 9:3:4 ratio, as both dominant alleles are necessary to jointly produce a product.
c. Another general category of epistatic interactions involves two genes encoding enzymes for a single pathway, and results in a 9:7 ratio, as just one dominant allele or the other is sufficient to produce the product.
d. Figure 16.11 illustrates a 9:7 phenotypic ratio resulting from epistasis.
16.9 Genes are linked to one another.
a. Recall, genes on different chromosomes are inherited independently.
b. Genes on the same chromosome can be said to be linked; gene linkage was discovered by Thomas Hunt Morgan, through experiments with Drosophila.
c. Alfred Sturtevant, also using Drosophila as the experimental organism, produced the first linkage map in 1913.
d. Where the P1 parents are known to be true-breeding (Figure 16.12), one can analyze F2 offspring and recognize parental types, which resemble the F1 parents phenotypically, and recombinant types, which have new combinations of traits (Figure 16.13).
e. Parental arrangements of chromosomes tend to stay together, and recombinations, resulting from cross overs during meiosis, tend to be more rare.
f. The probability of a crossover event is proportional to the distance between the genes on the chromosome; this is how Sturtevant approximated the first gene map.
g. The frequency of recombination, denoted by R, can thus be calculated by determining the frequency of offspring that are recombinant types.
h. Distances between genes are measured in map units, where 1 percent recombination is called 1 map unit.
i. A series of test crosses, with heterozygous individuals that have known allele arrangements on their homologous chromosomes, can be used to determine map distances between genes, two genes at a time, and to resolve questions regarding the order of genes on the chromosome.
j. A mapping function can be used to convert R into map distances, since R values greater than 0.5 are not reliable indicators of map distances.
k. Methods 16.1 gives an example of a three-gene cross, where the gene order is known, as an alternative to mapping genes using various combinations of two-gene crosses.
16.10 Many genes have multiple effects.
a. Though it is common to refer to the "gene for eye color" in Drosophila, there is not necessarily just one gene that codes for such a trait.
b. Recall, genes code for proteins, typically enzymes.
c. In multicellular organisms, complex traits are not likely to be produced by single genes.
d. More often, traits, such as fur color in mice, result from the coordinated action of several genes.
e. No single gene is likely to have just one effect on an individual's traits, but usually has multiple effects.
f. Genes that have multiple effects (on various traits) are termed pleiotropic (Figure 16.14).
g. Chapter 22 addressed the adaptive value of alleles, and the pleiotropic effects of genes are very relevant to that discussion.
16.11 Any gene can have more than two alleles.
a. The alleles of a gene arise through mutation, and there may be several different alleles for each gene.
b. One gene that determines rabbit fur color (Figure 16.15), for instance, has four alleles.
c. Of course, any diploid individual can only carry two alleles for any one gene.
16.12 Special chromosomes often determine an organism's sex.
a. In general, an organism's sex is genetically determined.
b. Sex chromosomes are special chromosomes that carry genes determining the sex of most plants and animals.
c. In humans, there are 22 pairs of autosomes, and one pair of sex chromosomes: XX in females, and XY in males (Figure 16.16).
d. Female humans can thus only contribute an X chromosome to their gametes, and are termed homogametic.
e. Male humans can contribute either an X or a Y chromosome, are termed heterogametic, and determine the sex of the offspring.
f. All mammals, and fruit flies, have this XY system.
g. Insects have females that are XX and males that do not carry two sex chromosomes, and are thus XO.
h. In animals where the female is heterogametic, the sex chromosomes are designated Z and W, and homogametic males carry chromosomes termed ZZ.
i. Several complex factors determine differentiation between the two sexes, including the development of gonads into testes, rather than ovaries.
16.13 Genes on sex chromosomes show a distinctive pattern of inheritance.
a. Thomas Hunt Morgan discovered white-eyed Drosophila, and the fact that the gene for white eyes, w, is located on the X chromosome, and is thus called a sex-linked gene.
b. The nomenclature for sex-linked genes uses a superscript system: XwXw females have white eyes, and XwY males have white eyes, for example.
c. Figure 16.17 illustrates the behavior of a sex-linked gene in Drosophila.
d. A carrier for a trait is a heterozygote, having one wild-type allele, and one recessive allele, and which also displays the dominant phenotype.
e. A pedigree is a drawing for analyzing inheritance in animals, where males are traditionally represented by squares, and females by circles.
f. Queen Victoria's family pedigree illustrates one famous case of inheritance of a sex-linked trait, hemophilia (Figure 16.18).
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