Chapter 8 Glossary, Summary, and Practice Questions

KEY TERMS

allele one of two or more variants of a gene that determines a particular trait for a characteristic

codominance in a heterozygote, complete and simultaneous expression of both alleles for the same characteristic

continuous variation a variation in a characteristic in which individuals show a range of traits with small differences between them

dihybrid the result of a cross between two true-breeding parents that express different traits for two characteristics

discontinuous variation a variation in a characteristic in which individuals show two, or a few, traits with large differences between them

dominant describes a trait that masks the expression of another trait when both versions of the gene are present in an individual

epistasis an interaction between genes such that one gene masks or interferes with the expression of another

F₁the first filial generation in a cross; the offspring of the parental generation

F₂the second filial generation produced when F₁ individuals are self-crossed or fertilized with each other

genotype the underlying genetic makeup, consisting of both physically visible and non-expressed alleles, of an organism

hemizygous the presence of only one allele for a characteristic, as in X-linkage; hemizygosity makes descriptions of dominance and recessiveness irrelevant

heterozygous having two different alleles for a given gene on the homologous chromosomes

homozygous having two identical alleles for a given gene on the homologous chromosomes

hybridization the process of mating two individuals that differ, with the goal of achieving a certain characteristic in their offspring

incomplete dominance in a heterozygote, expression of two contrasting alleles such that the individual displays an intermediate phenotype

law of dominance in a heterozygote, one trait will conceal the presence of another trait for the same characteristic

law of independent assortment genes do not influence each other with regard to sorting of alleles into gametes; every

possible combination of alleles is equally likely to occur

law of segregation paired unit factors (i.e., genes) segregate equally into gametes such that offspring have an equal likelihood of inheriting any combination of factors

linkage a phenomenon in which alleles that are located in close proximity to each other on the same chromosome are more likely to be inherited together

model system a species or biological system used to study a specific biological phenomenon to gain understanding that will be applied to other species

monohybrid the result of a cross between two true-breeding parents that express different traits for only one characteristic

Pthe parental generation in a cross

phenotype the observable traits expressed by an organism

Punnett square a visual representation of a cross between two individuals in which the gametes of each individual are denoted along the top and side of a grid, respectively, and the possible zygotic genotypes are recombined at each box in the grid

recessive describes a trait whose expression is masked by another trait when the alleles for both traits are present in an individual

reciprocal cross a paired cross in which the respective traits of the male and female in one cross become the respective traits of the female and male in the other cross

recombination the process during meiosis in which homologous chromosomes exchange linear segments of genetic material, thereby dramatically increasing genetic variation in the offspring and separating linked genes

test cross a cross between a dominant expressing individual with an unknown genotype and a homozygous recessive individual; the offspring phenotypes indicate whether the unknown parent is heterozygous or homozygous for the dominant trait

trait a variation in an inherited characteristic

wild type the most commonly occurring genotype or phenotype for a given characteristic found in a population

X-linked a gene present on the X chromosome, but not the Y chromosome

CHAPTER SUMMARY

Mendel’s Experiments

Working with garden pea plants, Mendel found that crosses between parents that differed for one trait produced F₁ offspring that all expressed one parent’s traits. The traits that were visible in the F₁ generation are referred to as dominant, and traits that disappear in the F₁ generation are described as recessive. When the F₁ plants in Mendel’s experiment were self-crossed, the F₂ offspring exhibited the dominant trait or the recessive trait in a 3:1 ratio, confirming that the recessive trait had been transmitted faithfully from the original P parent. Reciprocal crosses generated identical F₁ and F₂ offspring ratios. By examining sample sizes, Mendel showed that traits were inherited as independent events.

Laws of Inheritance

When true-breeding, or homozygous, individuals that differ for a certain trait are crossed, all of the offspring will be heterozygous for that trait. If the traits are inherited as dominant and recessive, the F₁ offspring will all exhibit the same phenotype as the parent homozygous for the dominant trait. If these heterozygous offspring are self-crossed, the resulting F₂ offspring will be equally likely to inherit gametes carrying the dominant or recessive trait, giving rise to offspring of which one quarter are homozygous dominant, half are heterozygous, and one quarter are homozygous recessive. Because homozygous dominant and heterozygous individuals are phenotypically identical, the observed traits in the F₂ offspring will exhibit a ratio of three dominant to one recessive.

Mendel postulated that genes (characteristics) are inherited as pairs of alleles (traits) that behave in a dominant and recessive pattern. Alleles segregate into gametes such that each gamete is equally likely to receive either one of the two alleles present in a diploid individual. In addition, genes are assorted into gametes independently of one another. That is, in general, alleles are not more likely to segregate into a gamete with a particular allele of another gene.

Extensions of the Laws of Inheritance

Alleles do not always behave in dominant and recessive patterns. Incomplete dominance describes situations in which the heterozygote exhibits a phenotype that is intermediate between the homozygous phenotypes. Codominance describes the simultaneous expression of both of the alleles in the heterozygote. Although diploid organisms can only have two alleles for any given gene, it is common for more than two alleles for a gene to exist in a population. In humans, as in many animals and some plants, females have two X chromosomes and males have one X and one Y chromosome. Genes that are present on the X but not the Y chromosome are said to be X-linked, such that males only inherit one allele for the gene, and females inherit two.

According to Mendel’s law of independent assortment, genes sort independently of each other into gametes during meiosis. This occurs because chromosomes, on which the genes reside, assort independently during meiosis and crossovers cause most genes on the same chromosomes to also behave independently. When genes are located in close proximity on the same chromosome, their alleles tend to be inherited together. This results in offspring ratios that violate Mendel’s law of independent assortment. However, recombination serves to exchange genetic material on homologous chromosomes such that maternal and paternal alleles may be recombined on the same chromosome. This is why alleles on

a given chromosome are not always inherited together. Recombination is a random event occurring anywhere on a chromosome. Therefore, genes that are far apart on the same chromosome are likely to still assort independently because of recombination events that occurred in the intervening chromosomal space.

Whether or not they are sorting independently, genes may interact at the level of gene products, such that the expression of an allele for one gene masks or modifies the expression of an allele for a different gene. This is called epistasis.

ART CONNECTION QUESTIONS

• Figure 8.9 In pea plants, round peas (R) are dominant to wrinkled peas (r). You do a test cross between a pea plant with wrinkled peas (genotype rr) and a plant of unknown genotype that has round peas. You end up with three plants, all which have round peas. From this data, can you tell if the parent plant is homozygous dominant or heterozygous?
• Figure 8.10 In pea plants, purple flowers (P) are dominant to white (p), and yellow peas (Y) are dominant to

REVIEW QUESTIONS

• Imagine that you are performing a cross involving seed color in garden pea plants. What traits would you expect to observe in the F₁ offspring if you cross true-breeding parents with green seeds and yellow seeds? Yellow seed color is dominant over green.
• only yellow-green seeds
• only yellow seeds
• 1:1 yellow seeds:green seeds
• 1:3 green seeds:yellow seeds
• Imagine that you are performing a cross involving seed texture in garden pea plants. You cross true-breeding round and wrinkled parents to obtain F1 offspring. Which of the following experimental results in terms of numbers of plants are closest to what you expect in the F2 progeny?

• 810 round seeds
• 810 wrinkled seeds
• 405:395 round seeds:wrinkled seeds
• 610:190 round seeds:wrinkled seeds
• The observable traits expressed by an organism are described as its .
• phenotype
• genotype
• alleles
• zygote
• A recessive trait will be observed in individuals that are

for that trait.

• heterozygous
• homozygous or heterozygous
• homozygous
• diploid
• What are the types of gametes that can be produced by an individual with the genotype AaBb?
• Aa, Bb
• AA, aa, BB, bb

green (y). What are the possible genotypes and phenotypes for a cross between PpYY and ppYy pea plants? How many squares would you need to complete a Punnett square analysis of this cross?

• Figure 8.16 What ratio of offspring would result from a cross between a white-eyed male and a female that is heterozygous for red eye color?

• AB, Ab, aB, ab
• AB, ab
• What is the reason for doing a test cross?
• to identify heterozygous individuals with the dominant phenotype
• to determine which allele is dominant and which is recessive
• to identify homozygous recessive individuals in the F2
• to determine if two genes assort independently
• If black and white true-breeding mice are mated and the result is all gray offspring, what inheritance pattern would this be indicative of?
• dominance
• codominance
• multiple alleles
• incomplete dominance
• The ABO blood groups in humans are expressed as the IA, IB, and i alleles. The IA allele encodes the A blood group antigen, IB encodes B, and i encodes O. Both A and B are dominant to O. If a heterozygous blood type A parent (IAi) and a heterozygous blood type B parent (IBi) mate, one quarter of their offspring are expected to have the AB blood type (IAIB) in which both antigens are expressed equally. Therefore, ABO blood groups are an example of:
• multiple alleles and incomplete dominance
• codominance and incomplete dominance
• incomplete dominance only
• multiple alleles and codominance
• In a cross between a homozygous red-eyed female fruit fly and a white-eyed male fruit fly, what is the expected outcome?
• all white-eyed male offspring

• all white-eyed female offspring
• all red-eyed offspring
• half white-eyed make offspring
• When a population has a gene with four alleles circulating, how many possible genotypes are there?
• 
  3
• 6
• 10
• 16

CRITICAL THINKING QUESTIONS

• Describe one of the reasons that made the garden pea an excellent choice of model system for studying inheritance.
• Use a Punnett square to predict the offspring in a cross between a dwarf pea plant (homozygous recessive) and a tall pea plant (heterozygous). What is the phenotypic ratio of the offspring?

• Use a Punnett square to predict the offspring in a cross between a tall pea plant (heterozygous) and a tall pea plant (heterozygous). What is the genotypic ratio of the offspring?
• Can a male be a carrier of red-green color blindness?
• Could an individual with blood type O (genotype ii) be a legitimate child of parents in which one parent had blood type A and the other parent had blood type B?