Chapter 16 Summary
Core Concepts Summary
16.1 The earliest theories of heredity incorrectly assumed the inheritance of acquired characteristics and blending of parental traits in the offspring.
The inheritance of acquired characteristics suggests that traits that develop during the lifetime of an individual can be passed on to offspring. With rare exceptions, this mode of inheritance does not occur. page 326
Blending inheritance is the incorrect hypothesis that characteristics in the parents are averaged in the offspring. This model predicts the blending of genetic material, which does not occur. Different forms of a gene maintain their separate identities even when present together in the same individual. page 326
16.2 The study of modern transmission genetics began with Gregor Mendel, who used the garden pea as his experimental organism and studied traits with contrasting characteristics.
Mendel started his experiments with true-
In crosses of one true-
Mendel explained this result by hypothesizing that there is a hereditary factor for each trait (now called a gene); that each pea plant carries two copies of the gene for each trait; and that one of two different forms of the gene (alleles) is dominant to the other one. page 329
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16.3 Mendel’s first key discovery was the principle of segregation, which states that members of a gene pair separate equally into gametes.
When Mendel allowed the progeny of the first cross to self-
Mendel reasoned that the parent in this generation must have two different forms of the same gene (A and a) and that these alleles segregate from each other to form gametes with each gamete getting A or a but not both. When the gametes combine at random, they produce progeny in the genotypic ratio 1 AA : 2 Aa : 1 aa, which yields a phenotypic ratio of 3 : 1 because A is dominant to a. This idea became known as the principle of segregation. page 330
The principle of segregation reflects the separation of homologous chromosomes that occurs in anaphase I of meiosis. page 332
Some traits show incomplete dominance, in which the phenotype of the heterozygous genotype is intermediate between those of the two homozygous genotypes. page 333
The expected frequencies of progeny of crosses can be predicted using the addition rule, which states that when two possibilities are mutually exclusive, the probability of either event occurring is the sum of their individual probabilities; and the multiplication rule, which states that when two possibilities occur independently, the probability of both events occurring is the product of the probabilities of each of the two events. page 333
16.4 Mendel’s second key finding was the principle of independent assortment, which states that different gene pairs segregate independently of one another.
In a cross with two traits, each with two contrasting characteristics, the two traits behave independently of each other. This idea became known as the principle of independent assortment. For example, in a self-
Independent assortment results from chromosome behavior during meiosis, specifically from the random orientation of different chromosomes on the meiotic spindle. page 337
In some cases, genes interact with each other, modifying the expected ratios in crosses. Epistasis is a gene interaction in which one gene affects the expression of another. page 338
16.5 The patterns of inheritance that Mendel observed in peas can also be seen in humans.
In a human pedigree, females are represented by circles and males as squares; affected individuals are shown as filled symbols and unaffected individuals as open symbols. Horizontal lines denote matings. page 339
Dominant traits appear in every generation and affect males and females equally. page 339
Recessive traits skip one or more generations and affect males and females equally. Affected individuals often result from matings between close relatives. page 340
Although a given individual has only two alleles of each gene, there can be many alleles of a particular gene in the population as a whole. page 340
Interpreting pedigrees can be difficult because of incomplete penetrance and variable expressivity. Penetrance is the percentage of individuals with a particular genotype who show the expected phenotype, and expressivity is the degree to which a genotype is expressed in the phenotype. page 341
Genetic testing enables the genotype of an individual to be determined for one or more genes. Such tests carry both benefits and risks. page 342
Self-Assessment
In his famous paper, Mendel writes that he set out to “determine the number of different forms in which hybrid progeny appear” and to “ascertain their numerical interrelationships.” How did his close attention to numbers lead him to discover segregation and independent assortment?
Self-Assessment 1 Answer
Both segregation and independent assortment are statistical principles governing inheritance. It was only by keeping careful records of the types and numbers of offspring from each cross that Mendel was able to recognize the 3:1 ratio of phenotypes in the F2 progeny of a single-
gene cross and the 9:3:3:1 ratio of phenotypes in the F2 progeny of a two- gene cross. These ratios are characteristic of segregation and independent assortment. He also took his experiments a step further, and by testcrosses and other means demonstrated the expected ratios of genotypes underlying the 3:1 and 9:3:3:1 ratios of phenotypes. Distinguish among gene, allele, genotype, and phenotype.
Self-Assessment 2 Answer
A gene is a unit of heredity. Alleles are the different forms of the gene. A genotype is the particular combination of alleles present in an individual, and the phenotype is the expression of that trait in an individual. For example, in Mendel’s experiments, he looked at the gene for seed color. This gene had two alleles, yellow (dominant) and green (recessive). The genotype of a seed could be AA or Aa and the resulting phenotype would be yellow seeds. The genotype of green seeds (phenotype) was aa.
Name and describe Mendel’s two laws.
Self-Assessment 3 Answer
Mendel’s two laws are: (1) The principle of segregation states that alleles will separate equally into different gametes, and (2) the principle of independent assortment, which states that different gene pairs segregate independently of one another.
Explain how the mechanics of meiosis and the movement of homologous chromosomes underlie Mendel’s principles of segregation and independent assortment.
Self-Assessment 4 Answer
The segregation of alleles reflects the separation of chromosomes in meiosis. Homologous chromosomes separate during anaphase I of meiosis I, leading to the segregation of the alleles. Chromosomes display independent assortment during meiosis when they are sorted into daughter cells randomly.
Explain how you can predict the genotypes and phenotypes of offspring if you know the genotypes of the parents.
Self-Assessment 5 Answer
By knowing the genotypes of the parents, you can determine the alleles that will occur in each gamete by applying Mendel’s principles of segregation and independent assortment. One systematic approach makes use of a Punnett square, which predicts the probability of every possible combination of one maternal allele with one paternal allele for a particular cross. For example, if a homozygous dominant red parent (RR) were crossed with a homozygous recessive white parent (rr), and R (red) is dominant over r (white), then the offspring predicted by the Punnett square would all have genotype Rr and a phenotype of red.
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Describe an instance in which you would use a testcross, and why.
Self-Assessment 6 Answer
In a testcross, one of the parents is homozygous for one or more recessive genes. You would use a testcross in a situation in which you wished to identify the genotype of an individual with the dominant phenotype for one or more genes. For example, among the progeny of the cross Aa Bb × Aa Bb, you cannot distinguish the genotypes AABB, AaBB, AABb, and AaBb, because each has the dominant phenotype of each gene. You can identify the genotypes by crossing to aa bb (a testcross), because each possible genotype of the other parent will produce a different combination of phenotypes among the offspring.
Define the multiplication and addition rules, and explain how these rules can help you predict the outcome of a cross between parents with known genotypes.
Self-Assessment 7 Answer
The addition rule applies when the possible outcomes being considered cannot occur simultaneously. The multiplication rule applies when outcomes can occur simultaneously and the occurrence of one has no effect upon the likelihood of the other. Using these two rules, the probability that a particular genotype will occur can be determined from a known parental cross.
What are some reasons why a single trait might not show a 3:1 ratio of phenotypes in the F2 generation of a cross between true-
breeding strains, and why a pair of traits might not show a 9:3:3:1 ratio of phenotypes in the F2 generation of a cross between true- breeding strains? Self-Assessment 8 Answer
A single trait might not show a 3:1 ratio of phenotypes in the F2 generation of a cross between true-
breeding strains because the trait is affected by more than one gene, or because it is affected by the environment, or because it does not show complete dominance, or because of incomplete penetrance or variable expressivity, or because one phenotype may survive better than the other. Similarly, a pair of traits might not show a 9:3:3:1 ratio of phenotypes in the F2 generation of a cross between true- breeding strains because the trait is affected by more than two genes, or because it is affected by the environment, or because one or both traits do not show complete dominance, or because of incomplete penetrance or variable expressivity, or because the genes have an epistatic interaction that modifies the 9:3:3:1 ratio, or because some phenotypes may survive better than others. Construct a human pedigree for a dominant and a recessive trait and explain the patterns of inheritance.
Self-Assessment 9 Answer
See the examples below. The first pedigree displays a recessive trait. It is revealed as recessive because it does not occur in every generation, and because two nonaffected individuals had two affected children. The second pedigree displays a dominant trait. It is revealed as dominant because it appears in every generation and affects about half of the offspring.
Discuss the benefits and risks of genetic testing and personal genomics.
Self-Assessment 10 Answer
The benefits of genetic testing include screening for potentially fatal or damaging diseases. For example, there are certain mutations in the BRCA2 gene that put an individual at an extremely high risk for breast cancer. If a patient is screened and found to have this mutation, preventative measures can be taken so hopefully he or she does not develop cancer. There are risks involved with genetic testing, however. Discrimination against individuals based on their genetic information is an issue, especially when it comes to health care. There are also direct-
to- consumer companies that provide you with your genetic information without the consent of your doctor. This information could be misleading if you do not understand how the tests were performed and what the probabilities mean.