Chapter 14

DRAW IT Two pea plants heterozygous for the characters of pod color and pod shape are crossed. Draw a Punnett square to determine the phenotypic ratios of the offspring.

A man with type A blood marries a woman with type \(\mathrm{B}\) blood. Their child has type O blood. What are the genotypes of these three individuals? What genotypes, and in what frequencies, would you expect in future offspring from this marriage?

A man has six fingers on each hand and six toes on each foot. His wife and their daughter have the normal number of digits. Remember that extra digits is a dominant trait. What fraction of this couple's children would be expected to have extra digits?

DRAW IT A pea plant heterozygous for inflated pods (Ii) is crossed with a plant homozygous for constricted pods (ii). Draw a Punnett square for this cross to predict genotypic and phenotypic ratios. Assume that pollen comes from the ii plant.

Hemochromatosis is an inherited disease caused by a recessive allele. If a woman and her husband, who are both carriers, have three children, what is the probability of each of the following? (a) All three children are of normal phenotype. (b) One or more of the three children have the disease. (c) All three children have the disease. (d) At least one child is phenotypically normal. (Note: It will help to remember that the probabilities of all possible outcomes always add up to 1.)

The genotype of \(\mathrm{F}_{1}\) individuals in a tetrahybrid cross is AaBbCcDd. Assuming independent assortment of these four genes, what are the probabilities that \(\mathrm{F}_{2}\) offspring will have the following genotypes? $$\begin{array}{ll}{\text { (a) }} & {\text { abbccdd }} \\ {\text { (b) } A a B b C c D d} & {\text { (e) AaBBCCdd }} \\ {\text { (c) } A A B B C C D D}\end{array}$$

What is the probability that each of the following pairs of parents will produce the indicated offspring? (Assume independent assortment of all gene pairs.) $$\begin{array}{l}{\text { (a) } A A B B C C \times a a b b c c \rightarrow A a B b C c} \\ {\text { (b) } A A B b C c \times A a B b C c \rightarrow A A b b C C} \\ {\text { (c) } A a B b C c \times A a B b C c \rightarrow A a B b C c} \\ {\text { (d) } a a B b C C \times A A B b c c \rightarrow A a B b C c}\end{array}$$

In \(1981,\) a stray black cat with unusual rounded, curled-back ears was adopted by a family in California. Hundreds of descendants of the cat have since been born, and cat fanciers hope to develop the curl cat into a show breed. Suppose you owned the first curl cat and wanted to develop a true breeding variety. How would you determine whether the curl allele is dominant or recessive? How would you obtain true-breeding curl cats? How could you be sure they are true-breeding?

In tigers, a recessive allele of a particular gene causes both an absence of fur pigmentation (a white tiger) and a cross-eyed condition. If two phenotypically normal tigers that are heterozygous at this locus are mated, what percentage of their offspring will be cross-eyed? What percentage of cross-eyed tigers will be white?

In maize (corn) plants, a dominant allele I inhibits kernel color, while the recessive allele i permits color when homozygous. At a different locus, the dominant allele \(P\) causes purple kernel color, while the homozygous recessive genotype \(p p\) causes red kernels. If plants heterozygous at both loci are crossed, what will be the phenotypic ratio of the offspring?

Imagine that you are a genetic counselor, and a couple planning to start a family comes to you for information. Charles was married once before, and he and his first wife had a child with cystic fibrosis. The brother of his current wife, Elaine, died of cystic fibrosis. What is the probability that Charles and Elaine will have a baby with cystic fibrosis? (Neither Charles, Elaine, nor their parents have cystic fibrosis.)

EVOLUTION CONNECTION Over the past half century, there has been a trend in the United States and other developed countries for people to marry and start families later in life than did their parents and grandparents. What effects might this trend have on the incidence (frequency) of late-acting dominant lethal alleles in the population?

SCIENTIFIC INQUIRY You are handed a mystery pea plant with tall stems and axial flowers and asked to determine its genotype as quickly as possible. You know that the allele for tall stems \((T)\) is dominant to that for dwarf stems \((t)\) and that the allele for axial flowers \((A)\) is dominant to that for terminal flowers (a). (a) Identify all the possible genotypes for your mystery plant. (b) Describe the one cross you would do, out in your garden, to determine the exact genotype of your mystery plant. (c) While waiting for the results of your cross, you predict the results for each possible genotype listed in part a. Explain how you do this and why this is not called "performing a cross." a Explain how the results of your cross and your predictions will help you learn the genotype of your mystery plant.

WRITE ABOUT A THEME: INFORMATION The continuity of life is based on heritable information in the form of DNA. In a short essay \((100-150\) words), explain how the passage of genes from parents to offspring, in the form of particular alleles, ensures perpetuation of parental traits in offspring and, at the same time, genetic variation among offspring. Use genetic terms in your explanation.