Genetics Problems

1. In a particular dog, short hair is dominant over long, and dark is dominant over light. One dog is heterozygous for hair length and shows the recessive trait for hair color. Another dog is homozygous dominant for hair length and heterozygous for hair color. Identify the possible phenotypes of the offspring and give the number of each expected.

2. In fruit flies, straight wings is dominant over curly. Show how you would determine if a straight winged fly were heterozygous or homozygous?

3. In guinea pigs rough coat "R" is dominant over smooth coat "r". Black coat "B" is dominant over white coat "b". R and B are independent genes.

a) Can two black coat parents produce white coat offspring? Can two white coat parents produce black coat offspring?

b)Cross a homozygous rough black animal with a smooth white one. What will be the appearance of the F₁; of the F₂; of the offspring of a cross of F₁ back with the rough black parent; with the smooth white parent?

4. In humans, the ability to taste phenylthiourea (PTU) is dominant. "Tasters" (TT) or (Tt) perceive an extremely bitter taste of PTU, while "non tasters" (tt) experience no sensation, or taste.

a) What are the genotypes of Mr. and Mrs. Meadowmuffin, who can taste PTU, and who have 3 children, one of whom is a non-taster?

b) What offspring phenotypes would be expected from the following crosses and in what ratios?

i) heterozygous x heterozygous

ii) homozygous taster x heterozygous

iii) heterozygous x non-taster

5. A plant of genotype TTYy was crossed with one of genotype ttYy. If the cross yielded 800 seeds, what number of what phenotypes probably resulted? What were their genotypes?

6. A dominant gene (S) in snapdragons produces bilaterally symmetrical ("irregular") flowers; the recessive allele (s) produces radially symmetrical flowers. Another pair of alleles controls color, yeilding red (A¹A¹), pink (A¹A²), or white (A²A²). If a cross is made between two snapdragons of genotype SsA¹A², what will be the phenotypes of the F₁? Give the proportions of each.

7. Two fruit flies with normal wings are crossed. Among 123 offspring, 88 have normal wings and 35 have curly wings. What inheritance pattern is shown by the normal and curly alleles? What were the genotypes of the two parents?

8. Flower color in certain plants is governed by epistasis. P= purple p= white, C= curved petals c= straight petals. A homozygous recessive petal shape prevents purple color. If parent plants are Ppcc x PPCc, what is the genotypic and phenotypic ratios of the offspring?

9. Assume that straight hair allele is recessive. The dominant allele gives curly hair. Two curly haired parents have a straight haired child.

(a) What are the genotypes of the parents?

(b) What is the genotype of the child?

(c) Give two genotypes for curly haired people.

(d) Using letter symbols show two different crosses for a curly haired individual crossed with a straight haired individual.

10. In pigeons, the checkered pattern results from the dominant gene C and plain coloration on the recessive gene c. Red color is controlled by a dominant gene R and by the brown r recessive gene.

(a) A homozygous checkered, red bird and a plain, brown bird are crossed. Summarize the expected F₁ and F₂ results, giving both genotype and phenotype.

(b) Male checkered, red birds with the genotype CcRr were mated with females of the following genotypes; (i) CCRR (ii) Ccrr (iii) ccrr. Summarize the expected first generation results of each cross, giving all ratios.

11. A rooster with gray feathers is mated with a hen of the same phenotype. Among their offspring, 15 chicks are grey, 6 are black, and 8 are white. What is the simplest explanation for the inheritance of color in the chicks? What offspring would you expect from the mating of a grey rooster and a black hen?

12. A black guinea pig crossed with an albino guinea pig produces 12 black offspring. When the albino was crossed with a second black one, 7 blacks and 5 albinos resulted. What is the best explanation for these results. Give genotypes for parents and all offspring as well as the inheritance pattern.

13. The allele that causes albinism (p) is recessive to the allele for normal pigmentation (P). A normal woman whose father is an albino marries an albino man whose parents are both normal. They have three children, two normal, one albino. Give the genotype for each individual.

14. If a color-blind male marries a carrier female, what are the chances that their sons will be color-blind? What are the chances that their daughters will be carriers?

15. A color-blind man and a woman with normal vision whose father was color blind have a son. What is the probability that their first son will be color blind? What is the probability that they will have a color-blind daughter? (note: this is not the same as asking “What is the probability of their daughter being color-blind)

16. Hemophilia is an X-linked recessive trait. A woman who is seemingly normal but whose father was a hemophiliac marries a normal man. What proportion of their sons will have hemophilia? Their daughters? What proportion of their daughters will be carriers? If the couple has four sons, what is the probability that all four will be born with hemophilia.

17. a) If a carrier female marries a normal male, what are the chances of their offspring having the disease hemophilia? What are the chances of their sons being normal? What are the chances of their daughters being a carrier?

b) If a woman's father had hemophilia, what are the chances that she is a carrier? If her mother was a carrier, what are the chances that she is a carrier? A hemophiliac?

18. The gene for wild-type eye color is dominant and sex-linked in Drosophila. The mating of a male with wild-type eye color with a female of the same phenotype produces offspring that are 3/4 wild-type eye color and 1/4 white-eyed. Indicate the genotypes of the P and F₁ generations.

19. John and Carol, planning their first child, each had a brother who died of a recessively inherited, autosomal disease which is fatal in early childhood. Find the chance of them having a child with the disease. If their first child is born with the disease, what is the probability that their second will?