General Genetics Problem Set #1
Background: The possible gene combinations in the offspring that result from a genetic cross can be determined by drawing a diagram known as a Punnett square. A Punnett square shows the genes in the gametes produced by the parents along the top and left side of the square and the possible gene combinations in the offspring within the square.
Rules for Working Genetic Problems Using a Punnett Square:
1. Use the letters given or choose a letter to represent the alleles in a cross.
2. Write the genotypes of the parents, P1.
3. Write the possible gametes that the parent can produce.
4. Enter possible gametes at the top and sides of the square.
5. Combine the gamete possibilities in the Punnett Square.
6. Determine the genotypic ratio.
7. Determine the phenotypic ratio.
A. The following letters represent gene pairs. Indicate which of the following pairs of genes are dominant (D), recessive (R), homozygous (HO) and heterozygous (HE). Include ALL possibilities.
1. DD ______3. Dd ______5. ss ______
2. Dd ______4. Rr ______6. Yy ______
B. Use letters G and g to illustrate the following gene conditions in a pair of chromosomes.
7. Homozygous recessive ______9. Heterozygous ______
8. Homozygous dominant ______10. hybrid ______
C. Single Trait Problems.
*In pea plants, the gene for round seeds is dominant over the gene for wrinkled seeds.
11. What symbol will represent the gene for round seeds? (circle one) R or r
12. What symbol will represent the gene for wrinkled seeds? (circle one) R or r
13. What is the genotype for a pea plant that is homozygous round? ______
14. What is the genotype for a pea plant that is heterozygous round? ______
15. What is the genotype for a pea plant with wrinkled seeds? ______
16. What is the phenotype for a pea plant with the genotype Rr? ______
17. What is the phenotype for a pea plant with the genotype RR? ______
18. What is the phenotype for a pea plant with the genotype rr? ______
19. If a pea plant that is heterozygous for round seeds is crossed with a pea plant with wrinkled seeds, what are the expected genotypic and phenotypic ratios in the F1?
Parent Genotypes: ______X ______
F1Genotypic Ratio: ______
F1Phenotypic Ratio: ______
20. Cross a pea plant that is homozygous for round seeds with a pea plant that is homozygous for wrinkled seeds. What are the genotypes and phenotypes of the offspring of the F1 generation?
Parent Genotypes: ______X ______
F1Genotypic percentage or ratio: ______
F1Phenotypic percentage or ratio: ______
21. Cross two of the F1generation from the previous cross to obtain the F2 generation. What are the expected genotypic and phenotypic ratios in the F2 generation?
Parent Genotypes: ______X ______
F2 Genotypic percentage or ratio: ______
F2 Phenotypic percentage or ratio: ______
* In foxes, red coat color is governed by a dominant allele B, silver-black coat color by a recessive allele b. Determine the genotypic and phenotypic ratios expected from the following matings.
22. Cross a homozygous red fox and a heterozygous red fox.
Parent Genotypes: ______X ______
Genotypic percentage or ratio: ______
Phenotypic percentage or ratio: ______
23. Cross a heterozygous red fox with a silver-black fox.
Parent Genotypes: ______X ______
Genotypic percentage or ratio: ______
Phenotypic percentage or ratio: ______
24. Cross a homozygous red fox with a silver-black fox.
Parent Genotypes: ______X ______
Genotypic percentage or ratio: ______
Phenotypic percentage or ratio: ______
25. Frosty the Snowman is homozygous dominant for a carrot nose. Mrs. Frosty is homozygous recessive and does not have a carrot nose. She had to settle for a nose made out of coal. What will be the possible genotypic and phenotypic ratios of their kids?
Parent Genotypes: ______X ______
Genotypic percentage or ratio: ______
Phenotypic percentage or ratio: ______
26. After the Grinch joined all the Whos down in Whoville for Christmas one year, he married a Who by the name of Peggy Lou. The Grinch is heterozygous dominant for green fur, but Peggy Lou is recessive and has lovely purple fur. What will be the possible genotypic and phenotypic ratios of their little Whos?
Parent Genotypes: ______X ______
Genotypic percentage or ratio: ______
Phenotypic percentage or ratio: ______
27. Mickey Mouse is recessive for big ears. Three of his children, Ricky, Vicky, and Nicky area also recessive and have big ears like their famous father. One son, Ronald, however, has small ears. What must the genotype of the children’s mother, Minnie, be?
Minnie’s Genotype: ______
28. Dumbo the elephant is homozygous dominant for big ears. His wife, Mumbo, is recessive and does not have big ears (which, sadly means that she can’t fly). What will be the possible genotype of Baby Dumdodo?
Dumdodo’s Genotype: ______
29. Tarzan has normal skin, which is a dominant trait. He marries Jane, who is an albino (a recessive trait). While swinging from tree to tree in the jungle, Tarzan and Jane realize they have left their son, Boy, “Home Alone.” What are the genotypes of Tarzan, Jane and Boy? Oh, by the way, Boy looks just like his mother.
Tarzan’s genotype: ______
Jane’s genotype: ______
Boy’s genotype: ______
* In guinea pigs, black coat color is dominant to brown coat color. A brown guinea pig is crossed with a black guinea pig of unknown genotype.
30. Because a known recessive guinea pig is crossed with an unknown black guinea pig, what kind of cross is this? ______
31. If any of the offspring are brown, what is the genotype of the black parent? ______
32. If, after many offspring, all offspring are black, what is the probable genotype of the black parent guinea pig? ______