AP Biology
Name:
Genetics Problems
A. MONOHYBRID CROSSES
I. COMPLETE DOMINANCE
1. For each genotype, indicate whether it is heterozygous (HE) or homozygous (HO):
AA _____, Aa _____, aa _____, Kk ______, CC _____, DD _____, ee _____, Bb _____, Vv _____.
2. For each of the genotypes below, determine the phenotype.
Purple flowers are dominant to white flowers.
PP = ___________________________
Pp = ___________________________
pp = ___________________________
Round seeds are dominant to wrinkled.
RR = ___________________________
Rr = ___________________________
rr = ___________________________
3. For each phenotype, list the genotypes.
Straight hair is dominant to curly.
____________ straight
____________ straight
____________ curly
Pointed heads are dominant to round heads.
____________ pointed
____________ pointed
____________ round
4. A homozygous dominant tall plant is crossed with a short plant. What percentage of the offspring
will be tall?
Key:
5. In guinea pigs, the allele for short hair is dominant. Show the cross for two heterozygous guinea
pigs. What percentage of offspring will have short hair?
Key:
6. In pea plants, purple flowers are dominant to white flowers. If two white flowered plants cross,
what percentage of their offspring will be white flowered?
Key:
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7. A testcross is used to determine if the organism showing a dominant trait is homozygous or
heterozygous. In a testcross, the individual of the unsure genotype is crossed with the recessive
individual. For example, if two purple heterozygous flowers mated and produced a purple offspring, a
test cross would determine whether the purple offspring was also heterozygous or whether it was
homozygous dominant.
What test cross results would you expect if the purple offspring was heterozygous? Show your work!
What results would you expect if the purple offspring was homozygous dominant? Show your work!
II. INCOMPLETE DOMINANCE
ANDULASIAN CHICKENS
Inheritance of feather color in Andulasian chickens follows the pattern of incomplete dominance.
Black Andalusian chickens have BB genes, white Andalusian chickens have WW genes and blue
Andalusian chickens have BW genes for feather color.
Key:
8. Determine the possible genotypes and phenotypes of offspring of one black and one white
Andalusian chicken.
9. Determine the possible genotypes and phenotypes of offspring of two blue Andalusian chickens.
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FOUR O’CLOCK COLOR
Petal color of four o’clock flowers is inherited by incomplete dominance. Plants with RR genes have
red flowers. Plants with WW genes have white flowers. Plants with RW genes have pink flowers.
Key:
10. Sarah planted four o’clock in her garden. All the seeds came from one set of parent plants. When
the flowers bloomed, Sarah saw that all her flowers were pink. What were the genes of the parent
plants? Show your reasoning.
11. When Sarah went to plant her garden the following year, she decided that she wanted more
colors of four o’clock than just pink. She decided that she wanted 25% red flowers, 25% white flowers
and 50% pink flowers. What genes must the parents plants have to produce offspring in these
percentages? Show your reasoning.
III. CODOMINANCE
Inheritance of cattle coat color follows the pattern of codominance. Red shorthorn cattle have RR
genes, white shorthorn cattle have WW genes, and roan have RW genes.
Key:
12. Determine the possible genotypes and phenotypes of offspring of one red parent and one roan
parent.
13. Determine the possible genotypes and phenotypes of one red parent and one white parent.
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IV. MULTIPLE ALLELES
14. Mario is surprised to learn that although his both parents are blood type A, he is blood type O.
What must his parents’ genotypes be? Prove your answer with a cross.
15. A wealthy couple is killed when their yacht is sunk by a hurricane. They leave an estate of 5
million dollars. Six months after their death a man claims he is their long lost son. The man claiming
to be their sun has blood type A. The wealthy man is type A and his wife is type O. You are the judge.
Does the man get the $5 million?
V. SEX-LINKED INHERITANCE
16. Green-red colorblindness is a sex-linked disease. If a woman that is a carrier marries a colorblind
male, what percentage of their male children will be colorblind? What about the female children?
Remember to start with a key.
Key:
17. In a cross between a white-eyed female fruit fly and red-eyed male, what percent of the female
offspring will have white eyes? (w = white eye allele that is X-linked and recessive, w+ = red-eye allele
that is dominant).
Key:
XW+Y =
XW Y =
XW+XW+ =
XW+XW =
XW XW =
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18. A female Drosophila of unknown genotype was crossed with a white-eyed male fly, half of the
male and half of the female offspring were red-eyed, and half of the male and half of the female
offspring were white-eyed. What was the genotype of the female fly?
VI. POLYGENIC INHERITANCE
19. Define polygenic inheritance and give an example of trait that follows this type of inheritance.
B. DIHYBRID CROSSES
20. In guinea pigs, black hair is dominant to brown hair and short hair is dominant to long hair. What
would be the genotypes of the following?
KEY:
a. A black, long haired variety, homozygous for both traits? ____________________
b. A brown, short haired variety, heterozygous for hair length? ____________________
c. A brown, long haired variety? ____________________
21. In summer squash, white fruit color (W) is dominant over yellow fruit color (w) and disk-shaped
fruit (D) is dominant over sphere-shaped fruit (d). If a heterozygote white, heterozygote disk-shaped
fruit is crossed with a yellow, heterozygote disk-shaped fruit, what will the phenotypic and genotypic
ratios be for the F1 generation?
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C. USING PROBABILITY
22. An XxYYZZ parent mated with an xxYyZz parent. Assuming independent assortment of these
three genes, what is the chance for the offspring to be xxYyZZ?
23. An organism with the genotype BbDDEEff is mated to one with the genotype BBDdEeff. Assuming
independent assortment of these four genes, what is the chance for the offspring to be
BBDdEEff?
24. The genotype of F1 individuals in a tetrahybrid cross is AaBbCcDd. Assuming independent
assortment of these four genes, what are the possibilities that F2 offspring will have the
AaBBCCdd genotype?