Genetics Study Guide
For all questions, use these facts: the trait is fur color (f). Black fur is dominant over gray fur.
1. Write the letter of the dominant allele. ______
2. Write the letter of the recessive allele.______
3. Write the genotype for gray fur (2 alleles). ______
4. Write the genotype for Black fur (2 alleles). ______
5. Write out the purebred dominant genotype. ______.
6. Write out the hybrid genotype. ______
7. Write out the purebred recessive genotype. ______
8. Write the phenotype for #5.__________
9. Write the phenotype for #6. _________
10. Write the phenotype for #7. ________
11. If black fur is dominant to gray fur and a hybrid mouse is crossed with a gray mouse,
a. What are the possible genotypes of the offspring?
b. What are the possible phenotypes of the offspring?
12. Black fur is dominant to gray fur. If a gray mouse and a purebred black mouse are crossed, what are
the possible genotypes of the offspring?
What are the possible
phenotypes of the offspring?
You are experimenting with orchids, where red flowers are dominant over white flowers. (Use the
letter “R”). Complete the following crosses.
13. Cross purebred red orchids with white orchids.
a. Write the genotype probabilities.
b. Write the phenotype probabilities.
14. Cross 2 hybrid red orchids.
a. Write the genotype probabilities.
b. Write the phenotype probabilities.
15. Cross a white orchid with a hybrid red orchid.
a. Write the genotype probabilities.
b. Write the phenotype probabilities.
Directions: For each of the following problems (#16 – 22)
1. List the genotypes of the parents.
2. Diagram and complete a Punnett square
3. Give the phenotype percentages of the offspring.
16. In garden peas, round seed coats (R) is dominant over wrinkled seed coats (r). What will the results be of a
cross between a purebred round seed and a wrinkled seed.
17. In peas, yellow color (Y) is dominant to green (y). What will be the results of a cross-pollination of a hybrid
yellow and a hybrid yellow?
18. In humans, straight toes (S) is dominant over curled toes (s). What would be the result of a cross between a
curled toe and a hybrid straight toe?
19. In dogs, erect ears (E) is dominate over droopy ears (e). What are the results if two hybrid dogs have a litter
of puppies.
20. The ability to roll the tongue (R) is determined by a dominant gene while the recessive gene results in the
inability to roll the tongue (r). A man and his wife can both roll their tongues and are surprised to find that their
son cannot. Explain this by showing the genotypes of all three persons. (Note: you do not need to do a Punnett
Square for this problem).
21. In horses the allele for black coat (B) is dominant over the allele for a brown coat (b). A cross between a black
horse and a brown horse produces a brown foal. Is the black horse a hybrid or a purebred? Explain.
22. In guinea pigs, the allele for a smooth coat (S) is dominant over the allele for a rough coat (s). Explain how you
could find out whether a guinea pig with a smooth coat is a hybrid or a purebred.