Discussion:
Genetic Test Crosses
1.) In dogs, black (B) is dominant to chestnuts (b), and solid color (S) is dominant to spotted (s). What are the genotypes of the of the parents that would produce a cross with 3/8 black solid, 3/8 black spotted, 1/8 chestnut solid, and 1/8 chestnut spotted puppies?
2.) When hairless hamsters are mated with normal-haired hamsters, about one-half of the offspring are hairless and one-half are normal. When hairless hamsters are crossed with each other, the ratio of normal-haired to hairless is 1:2. How do you account for the results of the first cross? How would you explain the unusual ratio obtained in the second cross?
3.) Two pigs whose tails are exactly 25 cm in length are bred over 10 years and they produce 96 piglets with the following tail lengths: 6 piglets at 15 cm, 25 at 20 cm, 37 at 25 cm, 23 at 30 cm, and 5 at 35 cm.
a. How many pairs of genes are regulating the tail length character. (Hint: count the # of phentotypic classes, or determine the sum of the ratios of the classes)
b.) What offspring phenotypes would you expect from mating between a 15-cm and 30-cm pig?
4.) Polydactyly (extra fingers and toes) is due to a dominant gene. A father is polydactyl, the mother has a normal phenotype, and they have had one normal child. What is the genotype of the father? Of the mother? What is the probability that a second child will have the normal number of digits?
5.) True-breeding tall red-flowered plants are crossed with dwarf white-flowered plants. The resulting F1 generation consists of all tall pink-flowered plants. Assuming that height and flower color are each determined by a single gene locus on different chromosomes, predict the results of an F1 cross of the TtRr plants. List the phenotypes and predicted ratios for the F2 generation.
6.) Two true-breeding varieties of garden peas are crossed. One parent had red, axial flowers, and the other had white, terminal flowers. All F1 individuals had red, terminal flowers. If 100 F2 offspring were counted, how many of them would you expect to have red, axial flowers?