GENETIC PROBLEMS Monohybrid Crosses 1. A brown mink crossed with a silver-blue mink produced all brown offspring. When these F1 minks were crossed among themselves they produced forty-seven brown animals and fifteen silver-blue animals. Which of these characters is dependent upon a dominant gene? 2. In sheep, white is due to a dominant gene (W), black to its recessive allele (w). A white ewe mated to a white ram produces a black lamb. If they produce another offspring, could it be white? List the genotypes of all the animals mentioned in this problem. 3. In dogs, wire hair is due to a dominant gene, smooth hair to its recessive allele. Two wire-haired dogs produce a male pup which is wire-haired. To find out most quickly whether he carries the gene for smooth hair, he should be mated to what kind of female? 4. In man, normal pigmentation is due to a dominant gene (A), albinism to its recessive allele (a). A normal man marries an albino woman. Their first child is an albino. What are the genotypes of these three persons? If there are more children, what would they probably be like? 5. An albino man marries a normally pigmented woman. They have nine children, all normally pigmented. What are the probable genotypes of the parents and of the children? 6. A normally pigmented man whose father was an albino marries an albino woman both of whose parents were normally pigmented. They have three children, two normally pigmented and one albino. List the genotypes of all these persons. 7. Albinism frequently skips generations in human pedigrees, while polydactyly (extra fingers) does not. How do you explain these facts? 8. In man, brown eyes (B) are dominant over blue (b). A brown-eyed man marries a blueeyed woman and they have eight children, all brown eyed. What are the genotypes of all the individuals in the family? 9. A blue-eyed man both of whose parents were brown-eyed marries a brown-eyed woman whose father was brown-eyed and whose mother was blue-eyed. They have one child, who is blue-eyed. What are the probable genotypes of all the individuals mentioned? 10. In man, a series of alleles has been associated with the blood typing groups as follows: IAIA = A; IBIB = B; IAIB = AB; What are the phenotypes from the following crosses? a. IAIA x IBIB b. IAIB x ii c. IAi x IBi ii = O d. IAi x ii 11. A man has type A blood and his wife has type B blood. A physician types the blood of their four children and is amazed to find one of each of the four blood types among them. He is not familiar with genetics and calls upon you to explain how such a thing could happen. What would you tell him? 12. A couple preparing for marriage have their blood typed along with the other required blood tests. Both are AB. They ask you what types of blood their children may have. What would you tell them and how would you explain your conclusions? 13. A woman sues a man for the support of her child. She has type A blood, her child type O, and the man type B. Could the man be the father? Explain. 14. A case was brought before a certain judge in which a woman of blood group O presented a baby of blood group O which she claimed as her child and brought suit against a man of blood group AB whom she claimed was the father of the child. If you were the judge, what would your decision be? Explain. Dihybrid Crosses 15. In horses, black is dependent upon a dominant gene, B, and chestnut upon its recessive allele, b. The trotting gait is due to a dominant gene, T, and the pacing gait to its recessive allele, t. If a homozygous black pacer is mated to a homozygous chestnut trotter, what will be the appearance of the F1 generation? 16. If two F1 individuals from the preceding problem were mated, what kinds of offspring could they have and in what proportions? 17. If an F1 male from Problem 15 were mated to a homozygous female black pacer, what kinds of offspring could be produced and in what proportions? 18. In rabbits, black is due to a dominant gene, B, and brown to its recessive allele, b. Short hair is due to a dominant gene, L, and long hair to its recessive allele, l. In a cross between a homozygous black short-haired male and a homozygous brown long-haired female, what would be the genetic constitution and the appearance of the F1 generation? Of the F2 generation? 19. Suppose you had a black short-haired male rabbit and a brown long-haired female rabbit and wished to develop a homozygous strain of black long-haired rabbits from them. Outline the breeding procedure necessary to establish such a strain. 20. In the garden, Phlox white flower color is due to a dominant gene, W, and cream to its recessive allele, w. Salver-shaped flowers are due to a dominant gene, S, and funnelshaped flowers to its recessive allele, s. A plant producing white funnel-shaped flowers is crossed to one producing cream slaver-shaped flowers. Of the offspring, one quarter produced white salver-shaped flowers, one quarter produced white funnel-shaped flowers, one quarter produced cream salver-shaped flowers, and one quarter produced cream funnel-shaped flowers. What were the genotypes of the parents? 21. A plant producing white salver-shaped flowers is crossed with one producing cream funnel-shaped flowers. Of the 76 offspring, 37 produce white salver-shaped flowers, and 39 produce cream salver-shaped flowers. What were the genotypes of the parents? 22. A plant producing white funnel-shaped flowers is crossed with one producing cream salver-shaped flowers. Ninety-six offspring are produced, all having white salver-shaped flowers. What are the genotypes of the parents? What would result in the F2 generation of this cross? 23. In snapdragons, red flower color, R, is incompletely dominant over white, r, the heterozygous condition being pink; and normal broad leaves, B, are incompletely dominant over narrow grass-like ones, b, the heterozygous condition being intermediate leaf breadth. If a red-flowered, broad-leaved plant is crossed with a white-flowered, narrow-leaved one, what will be the appearance of the F1 and the F2 ? Trihybrid Crosses 24. In guinea pigs, rough coat is due to a dominant gene, R, and smooth to its recessive allele, r. Short hair is dependent upon a dominant gene, L, and long hair upon its recessive allele, l. Black is due to a dominant gene, B, and white to its recessive allele, b. A homozygous rough short-haired black guinea pig is crossed to a smooth long-haired white one. What would be the appearance of the F1 ? Of the F2 ? 25. A rough long-haired black male guinea pig is mated to a rough short-haired white female. After they have produced several litters, their offspring are found to be as follows: 15 rough short-haired black, 13 rough long-haired black, 4 smooth short-haired black, and 5 smooth long-haired black. What were the genotypes of the parents? 26. In addition to the three pairs of genes previously mentioned in guinea pigs, there is another pair in which S is a gene for solid color, and s for spotting. How many different kinds of gametes will a guinea pig of the genotype BBLlrrSs produce? 27. If a male guinea pig of the genotype BbLlRrSs were mated to a female of the genotype bbllrrss, how many different kinds of sperms would be produced by the male? How many different kinds of eggs would be produced by the female?