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Genetics problems I 1. Assume that white color is dominant over yellow color in squash. If pollen from the anthers of a heterozygous white-fruited plant is placed on the pistil of a yellow-fruited plant, show, using ratios, the genotypes and phenotypes you would expect the seeds from this cross to produce. 2. In human beings, brown eyes are usually dominant over blue eyes. Suppose a blue- eyed man reproduces with a brown-eyed woman whose father was blue-eyed. What pro- portion of their children would you predict will have blue eyes? 3. If a brown-eyed man reproduces with a blue-eyed woman and they have ten children, all brown-eyed, can you be certain that the man is homozygous? If the eleventh child has brown eyes, will that prove what the father's genotype is? 4. A brown-eyed man whose father was brown-eyed and whose mother was blue-eyed re- produces with a blue-eyed woman whose father and mother were both brown-eyed. The couple has a blue-eyed son. For which of the individuals mentioned can you be sure of the genotypes? What are their genotypes? What genotypes are possible for the others? 5. If the litter resulting from the mating of two short-tailed cats contains three kittens without tails, two with long tails, and six with short tails, what would be the simplest way of explaining the inheritance of tail length in these cats? Show genotypes. 6. In the fruit fly Drosophila melanogaster, vestigial wings and hairy body are produced by two recessive genes located on different chromosomes. The normal alleles, long wings and hairless body, are dominant. Suppose a vestigial-winged hairy male is crossed with a homozygous normal female. What types of progeny would be expected? If the F1 from this cross are permitted to mate randomly among themselves, what progeny would be ex- pected in the F2? Show complete genotypes, phenotypes, and ratios for each generation. 7. Suppose a hairy female heterozygous for vestigial wing is crossed with a vestigial- winged male heterozygous for the hairy character. What will be the characteristics of the F1? 8. In some breeds of dogs, a dominant gene controls the characteristic of barking while trailing. In these dogs, another independent gene produces erect ears; it is dominant over its allele for drooping ears. Suppose a dog breeder wants to produce a pure-breeding strain of droop-eared barkers, but he knows that the genes for silent trailing and erect ears are present in his kennels. How should he proceed? 9. In hogs, a gene that produces a white belt around the animal's body is dominant over its allele for a uniformly colored body. Another gene produces a fusion of the two hoofs on each foot, a condition known as syndactyly; this gene is dominant over its allele, which produces normal hoofs. Suppose a uniformly colored hog homozygous for syn- dactyly is mated with a normal-footed hog homozygous for the belted character. What would be the phenotype of the F1? If the F1 were allowed to breed freely among them- selves, what genotype and phenotype ratios would you predict for the F2? 10. In watermelons, green color and short shape are dominant over their alleles for striped color and for long shape. Suppose a plant with long striped fruit is crossed with a plant

that is heterozygous for both of these characters. What phenotypes would this cross pro- duce and in what ratios? 11. In peas, a gene for tall plants (T) is dominant over its allele for short plants (t). The gene for smooth peas (S) is dominant over its allele for wrinkled peas (s). Calculate phe- notypic and genotypic ratios for the results of each of the following crosses: TtSs x TtSs, Ttss x ttss, ttSs x Ttss, TTss x ttSS. 12. A dominant gene, A, causes yellow color in rats. The dominant allele of another inde- pendent gene, R, produces black coat color. When the two dominants occur together (A_R_), they interact to produce gray. Rats of the genotype aarr are cream-colored. If a gray male and yellow female produce approximately 3/8 yellow, 3/8 gray, 1/8 cream, and 1/8 black offspring, what are the genotypes of the two parents? 13. What are the genotypes of a yellow male and black female rat that produce 46 gray and 53 yellow offspring? 14. In Leghorn chickens, colored feathers are due to a dominant gene, C; white feathers are due to its recessive allele, c. Another dominant gene, I, inhibits expression of color in birds with genotypes CC or Cc. Consequently both C_I_ and cc__ are white. A colored cock is mated with a white hen and produces many offspring, all colored. Give the geno- types of both parents and offspring. 15. If the dominant gene K is necessary for hearing, and the dominant gene M results in deafness no matter what other genes are present, what percentage of the offspring pro- duced by the cross kkMm x Kkmm will be deaf? (Assume that there is no linkage). 16. What fraction of the offspring of parents each with the genotype KkLlMm will be kkllmm? 17. Suppose two DdEeFfGgHh individuals are mated. What would be the predicted fre- quency of ddEEFfggHh offspring from such a mating? 18. If a man with blood type B, one of whose parents had blood type O, reproduces with a woman with blood type AB, what will be the theoretical percentage of their children with blood type B? 19. Both Mrs. Smith and Mrs. Jones had babies the same day in the same hospital. Mrs. Smith took home a baby girl, whom she names Shirley. Mrs. Jones took home a baby girl, whom she named Jane. Mrs. Jones began to suspect, however, that her child had been accidentally switched with the Smith baby in the nursery. Blood tests were made; Mr. Smith was type A, Mrs. Smith was type B, Mr. Jones was type A, Mrs. Jones was type A, Shirley was type O, and Jane was type B. Had a mixup occurred? 20. When Mexican Hairless dogs are crossed with normally-haired dogs, about half the pups are hairless and half have hair. When, however, two Mexican Hairless dogs are mated, some deformed puppies are born dead. Explain the results. 21. Suppose a pigeon breeder finds that about one fourth of the eggs produced by one of his prize pairs do not hatch. Of the young birds produced by this pair, two thirds are males. Give a possible explanation for these results. (Remember the mechanism of sex determination in birds.)

22. Red-green color blindness is inherited as a sex-linked recessive. If a color-blind woman reproduces with a man who has normal vision, what would be the expected phe- notypes of their children with reference to this character? 23. Suppose that gene b is sex-linked, recessive, and lethal. A man reproduces with a woman who is heterozygous for this gene. If this couple had many normal children, what would be the predicted sex ratio of these children? 24. A husband and wife both have normal color vision, but their daughter has red-green color blindness, a sex-linked recessive trait. The man sues his wife for divorce of the grounds of infidelity. Can genetics provide evidence supporting his case? 25. It is exceedingly difficult to determine the sex of very young chickens, but it is easy to tell, by visual observation, whether or not they are barred. The barred pattern is inher- ited as a sex-linked dominant. Set up a cross so that the sex of all chicks can be deter- mined when they hatch. (Remember that chickens are birds.) 26a. In cats, short hair is dominant over long hair; the gene involved is autosomal. An- other gene, B1, which is sex-linked, produces yellow coat; its allele B2 produces black coat color; and the heterozygous combination B1B2 produces tortoise-shell coat color. If a long-haired black male is mated with a tortoise-shell female homozygous for short hair, what kind of kittens will be produced in the F1? 26b. If the F1 cats are allowed to interbreed freely among themselves, what are the chances of obtaining a long-haired yellow male? 27. In Drosophila melanogaster, there is a dominant gene for gray body color and another dominant gene for normal wings. The recessive alleles of these two genes result in black body color and vestigial wings respectively. Flies homozygous for gray body and normal wings were crossed with flies that had black bodies and vestigial wings. The F1 progeny were then test-crossed, with the following results: Gray, normal (236), Black, vestigial (253), Gray, vestigial (50), Black, Normal (61). Would you say these two genes are linked? If so, how many units apart are they on the chromosome? 28. The cross-over frequency between linked genes A and B is 40%; between B and C, 20%; between C and D, 10%; between C and A, 20%; between D and B, 10%. What is the sequence of the genes on the chromosome? a. In silkworms, yellow (Y) cocoons are dominant over white (y). If a silkworm mother from a pure yellow cocoon is crossed with a male from a pure white one, what will be the appearance of the cocoons in the offspring? b. What results when these hybrids are crossed? c. What results when the yellow-cocoon hybrid moths are crossed back to the pure yel- low-cocoon parents? d. What results when the yellow-cocoon hybrid moths are crossed back to the white par- ent? e. What kind of gametes will be produced by (a) pure yellow-cocoon parents, (b) hybrid yellow-cocoon parents, or (c) white-cocoon parents?

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f . If two yellow-cocoon parents produce about 3/4 yellow-cocoon offspring, what are the genotypes of the parents? g . How would you determine whether yellow-cocoon moths are pure (homozygous) or hybrid (heterozygous) if you have the live moths? (Assume you have access to white-co- coon moths also.) i. How would you determine whether white-cocoon moths are pure or hybrid? 30 . In humans, curly hair (S) is dominant over straight hair (s); brown eyes (B) over blue eyes (b); and right handedness (R) over left-handedness (r). Color blindness (c) is reces- sive and sex-linked. A curly-haired, brown-eyed, left-handed man reproduces with a curly-haired, blue-eyed, right-handed woman. Both have normal color vision. They have five children, as follows: a- a daughter, curly-haired, blue-eyed, right-handed, normal vi- sion b- a daughter, straight-haired, blue-eyed, right-handed, normal vision c- a son, curly- haired, brown-eyed, right-handed, color blind d- a son, curly-haired, brown-eyed, right- handed, normal vision e- a son, curly-haired, blue-eyed, right-handed, normal vision What is the genotype of each parent? 31. Vermillion eye in Drosophila is a recessive sex-linked character. What will be the na- ture of the offspring of a vermillion female and a wild type (red) male? 32. White eye in Drosophila is a recessive sex-linked character. Vestigial wing is a reces- sive autosomal character. What will be the appearance of the F1 and F2 generations from a cross between a white-eyed homozygous long-winged female and a red-eyed vestigial male? 33. In cats, yellow is due to a factor (Y), and black to its allele (y). The heterozygous con- dition Yy results in tortoise shell. The factors Y and y are sex-linked. What kinds of off- spring would be expected to result from the cross of a black male with a yellow female? 34. What kinds of offspring would be expected to result from the cross of a black male with a tortoise shell female? 35. A yellow male is mated with a tortoise shell female. If the female has a litter consist- ing of four male kittens, what colors would they most probably be? 36. In humans, aniridia (a type of blindness) is due to a dominant factor. Optic atrophy (another type of blindness) is due to a recessive sex-linked factor. A man blind from optic atrophy reproduces with a woman blind from aniridia. Would any of their children be ex- pected to be blind? Which type of blindness would they have? 37. In humans, assume that brown eyes (B) are dominant over blue eyes (b) and that right-handedness (R) dominates left-handedness (r). What offspring may be expected from the partnership of a right-handed, blue-eyed man whose father was left-handed, and a brown-eyed woman from a family in which all of the members have been brown-eyed and right-handed for several generations? 38. A brown-eyed, right-handed man reproduces with a brown-eyed, right-handed woman. Their first child is blue-eyed and left-handed. If other children are born to this couple, what probably will be their appearance with respect to these two traits?

39a-c. Parent A is feeble-minded (n) and deaf (d); only one of his parents was deaf. Par- ent B is normal-minded (N) and deaf (d); only one her parents was deaf, but one was fee- ble-minded. What is the genotype of parents A and B? What are the genotypes of their possible offspring? d. What is the ratio of probability that the first child will be (1) normal-minded, deaf, (2) normal-minded, normal hearing, (3) feeble-minded, deaf, (4) feebel-minded, normal hear- ing? e. If six children from these parents fall into groups 1 & 3, what is the expectation from the seventh child? 40 a. In cocker spaniels, black is due to a dominant factor B, red to its recessive allele b. Solid color is dependent upon a dominant factor S, white spotting upon its recessive al- lele s. A red male was mated to a black-and-while female. They had five puppies, as fol- lows: 1 black, 1 red, 1 black/white, 2 red/white. What were the parents' genotypes? 40b. A black male was mated to a black female. They had six puppies, all black. What were the probable genotypes of the parents? Why can the genotypes in this case not be given with the same accuracy as those in 9a? 41 . In poultry, white may be due to homozygous condition of either or both of the reces- sive factors c and o. Color depends upon the presence of both dominants C and O. White males of genotype CCoo were crossed with white females of genotype ccOO. The col- ored F1 birds were crossed to double-recessives ccoo. From many such crosses the total results were as follows: 68 colored, 204 white. Are these factors linked? If so, what is the percent of crossing over? 42. A black-bodied, purple-eyed Drosophila, mated with a homozygous gray, red-eyed fly, produced all gray, red-eyed offspring. One of the female offspring mated with a black, purple-eyed male and produced 104 gray, red, 94 black-purple, 6 gray, purple, and 7 black, red offspring. What is the percentage of crossing over between body color and eye color? 43. A colorblind woman reproduces with a man with normal vision. What kind of chil- dren would be expected from such a union? Color blindness is recessive and sex-linked. 44. A woman with normal vision reproduces with a man with normal vision and they have a colorblind son. The husband dies and she reproduces with a colorblind man. Show the types of children that might be expected from this second marriage and the proportion of each.

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