*12. In cucumbers, orange fruit color (R) is dominant over cream fruit color (r). A cucumber plant homozygous for orange fruits is crossed with a plant homozygous for cream fruits. The F1 are intercrossed to produce the F2.
(a) Give the genotypes and phenotypes of the parents, the F1, and the F2.
(b) Give the genotypes and phenotypes of the offspring of a backcross between the F1 and the orange parent.
(c) Give the genotypes and phenotypes of a backcross between the F1 and the cream parent.
*13. In rabbits, coat color is a genetically determined characteristic. Some black females always produce black progeny, whereas other black females produce black progeny and white progeny. Explain how these outcomes occur. *14. In cats, blood type A results from an allele (7A) that is dominant over an allele (iB) that produces blood type B. There is no O blood type. The blood types of male and female cats that were mated and the blood types of their kittens follow. Give the most likely genotypes for the parents of each litter.
Male parent Female parent Kittens
(a) blood type A blood type B 4 kittens with blood type A, 3 with blood type B
(b) blood type B blood type B 6 kittens with blood type B
(c) blood type B blood type A 8 kittens with blood type A
(d) blood type A blood type A 7 kittens with blood type A, 2 kittens with blood type B
Male parent Female parent Kittens
(e) blood type A blood type A 10 kittens with blood type A
(f) blood type A blood type B 4 kittens with blood type A, 1 kitten with blood type B
15. In sheep, lustrous fleece (L) results from an allele that is dominant over an allele for normal fleece (l). A ewe (adult female) with lustrous fleece is mated with a ram (adult male) with normal fleece. The ewe then gives birth to a single lamb with normal fleece. From this single offspring, is it possible to determine the genotypes of the two parents? If so, what are their genotypes? If not, why not?
*16. In humans, alkaptonuria is a metabolic disorder in which affected persons produce black urine (see the introduction to this chapter). Alkaptonuria results from an allele (a) that is recessive to the allele for normal metabolism (A). Sally has normal metabolism, but her brother has alkaptonuria. Sally's father has alkaptonuria, and her mother has normal metabolism.
(a) Give the genotypes of Sally, her mother, her father, and her brother.
(b) If Sally's parents have another child, what is the probability that this child will have alkaptonuria?
(c) If Sally marries a man with alkaptonuria, what is the probability that their first child will have alkaptonuria?
17. Suppose that you are raising Mongolian gerbils. You notice that some of your gerbils have white spots, whereas others have solid coats. What type of crosses could you carry out to determine whether white spots are due to a recessive or a dominant allele?
*18. Hairlessness in American rat terriers is recessive to the presence of hair. Suppose that you have a rat terrier with hair. How can you determine whether this dog is homozygous or heterozygous for the hairy trait?
19. In snapdragons, red flower color (R) is incompletely dominant over white flower color (r); the heterozygotes produce pink flowers. A red snapdragon is crossed with a white snapdragon, and the F1 are intercrossed to produce the F2.
(a) Give the genotypes and phenotypes of the F1 and F2, along with their expected proportions.
(b) If the F1 are backcrossed to the white parent, what will the genotypes and phenotypes of the offspring be?
(c) If the F1 are backcrossed to the red parent, what are the genotypes and phenotypes of the offspring?
20. What is the probability of rolling one six-sided die and obtaining the following numbers?
*21. What is the probability of rolling two six-sided dice and obtaining the following numbers?
(d) Two of the same number (two 1s, or two 2s, or two 3s, etc.)
(e) An even number on both dice
(f) An even number on at least one die
*22. In a family of seven children, what is the probability of obtaining the following numbers of boys and girls?
(b) All children of the same sex
(c) Six girls and one boy
(d) Four boys and three girls
(e) Four girls and three boys
23. Phenylketonuria (PKU) is a disease that results from a recessive gene. Two normal parents produce a child with PKU.
(a) What is the probability that a sperm from the father will contain the PKU allele?
(b) What is the probability that an egg from the mother will contain the PKU allele?
(c) What is the probability that their next child will have PKU?
(d) What is the probability that their next child will be heterozygous for the PKU gene?
*24. In German cockroaches, curved wing (cv) is recessive to normal wing (cv+). A homozygous cockroach having normal wings is crossed with a homozygous cockroach having curved wings. The F1 are intercrossed to produce the F2. Assume that the pair of chromosomes containing the locus for wing shape is metacentric. Draw this pair of chromosomes as it would appear in the parents, the F1, and each class of F2 progeny at metaphase I of meiosis. Assume that no crossing over takes place. At each stage, label a location for the alleles for wing shape (cv and cv+) on the chromosomes.
*25. In guinea pigs, the allele for black fur (B) is dominant over the allele for brown (b) fur. A black guinea pig is crossed with a brown guinea pig, producing five F1 black guinea pigs and six F1 brown guinea pigs.
(a) How many copies of the black allele (B) will be present in each cell from an F1 black guinea pig at the following stages: G1, G2, metaphase of mitosis, metaphase I of meiosis, metaphase II of meiosis, and after the second cytokinesis following meiosis? Assume that no crossing over takes place.
(b) How may copies of the brown allele (b) will be present in each cell from an F1 brown guinea pig at the same stages? Assume that no crossing over takes place.
26. In watermelons, bitter fruit (B) is dominant over sweet fruit (b), and yellow spots (S) are dominant over no spots (s). The genes for these two characteristics assort independently. A homozygous plant that has bitter fruit and yellow spots is crossed with a homozygous plant that has sweet fruit and no spots. The F1 are intercrossed to produce the F2.
(a) What will be the phenotypic ratios in the F2?
(b) If an F1 plant is backcrossed with the bitter, yellow spotted parent, what phenotypes and proportions are expected in the offspring?
(c) If an F1 plant is backcrossed with the sweet, nonspotted parent, what phenotypes and proportions are expected in the offspring?
27. In cats, curled ears (Cu) result from an allele that is dominant over an allele for normal ears (cu). Black color results from an independently assorting allele (G) that is dominant over an allele for gray (g). A gray cat homozygous for curled ears is mated with a homozygous black cat with normal ears. All the F1 cats are black and have curled ears.
(a) If two of the F1 cats mate, what phenotypes and proportions are expected in the F2?
(b) An F1 cat mates with a stray cat that is gray and possesses normal ears. What phenotypes and proportions of progeny are expected from this cross?
*28. The following two genotypes are crossed: AaBbCcddEe X AabbCcDdEe. What will the proportion of the following genotypes be among the progeny of this cross?
29. In mice, an allele for apricot eyes (a) is recessive to an allele for brown eyes (a+). At an independently assorting locus, an allele for tan (t) coat color is recessive to an allele for black (t+) coat color. A mouse that is homozygous for brown eyes and black coat color is crossed with a mouse having apricot eyes and a tan coat. The resulting F1 are intercrossed to produce the F2. In a litter of eight F2 mice, what is the probability that two will have apricot eyes and tan coats?
30. In cucumbers, dull fruit (D) is dominant over glossy fruit (d), orange fruit (R) is dominant over cream fruit (r), and bitter cotyledons (B) are dominant over nonbitter cotyledons (b). The three characters are encoded by genes located on different pairs of chromosomes. A plant homozygous for dull, orange fruit and bitter cotyledons is crossed with a plant that has glossy, cream fruit and nonbitter cotyledons. The F1 are intercrossed to produce the F2.
(a) Give the phenotypes and their expected proportions in the F2.
(b) An F1 plant is crossed with a plant that has glossy, cream fruit and nonbitter cotyledons. Give the phenotypes and expected proportions among the progeny of this cross.
31. A and a are alleles located on a pair of metacentric chromosomes. B and b are alleles located on a pair of acrocentric chromosomes. A cross is made between individuals having the following genotypes: AaBb X aabb.
(a) Draw the chromosomes as they would appear in each type of gamete produced by the individuals of this cross.
(b) For each type of progeny resulting from this cross, draw the chromosomes as they would appear in a cell at G1, G2, and metaphase of mitosis.
32. Ptosis (droopy eyelid) may be inherited as a dominant human trait. Among 40 people who are heterozygous for the ptosis allele, 13 have ptosis and 27 have normal eyelids.
(a) What is the penetrance for ptosis?
(b) If ptosis exhibited variable expressivity, what would it mean?
33. In sailfin mollies (fish), gold color is due to an allele (g) that is recessive to the allele for normal color (G). A gold fish is crossed with a normal fish. Among the offspring, 88 are normal and 82 are gold.
(a) What are the most likely genotypes of the parents in this cross?
(b) Assess the plausibility of your hypothesis by performing a chi-square test.
34. In guinea pigs, the allele for black coat color (B) is dominant over the allele for white coat color (b). At an independently assorting locus, an allele for rough coat (R) is dominant over an allele for smooth coat (r). A guinea pig that is homozygous for black color and rough coat is crossed with a guinea pig that has a white and smooth coat. In a series of matings, the F1 are crossed with guinea pigs having white, smooth coats. From these matings, the following phenotypes appear in the offspring: 24 black, rough guinea pigs; 26 black, smooth guinea pigs; 23 white, rough guinea pigs; and 5 white, smooth guinea pigs.
(a) Using a chi-square test, compare the observed numbers of progeny with those expected from the cross.
(b) What conclusions can you draw from the results of the chi-square test?
(c) Suggest an explanation for these results.
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