Genetics Problems
AP Biology
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 ___________________________
Brown eyes are dominant to blue eyes.
BB ___________________________
Bb ___________________________
bb ___________________________
Round seeds are dominant to wrinkled.
RR ___________________________
Rr ___________________________
rr ___________________________
Bobtails are recessive (long tails dominant).
TT ___________________________
Tt ___________________________
tt ___________________________
3. For each phenotype, list the genotypes. (Remember to use the letter of the dominant trait)
Straight hair is dominant to curly.
____________ straight
____________ straight
____________ curly
Pointed heads are dominant to round heads.
____________ pointed
____________ pointed
____________ round
4. Set up the square for each of the crosses listed below. The trait being studied is round seeds
(dominant) and wrinkled seeds (recessive).
Key:
Rr x rr
What percentage of the offspring
will be round?
Rr x Rr
What percentage of the offspring
will be round?
RR x Rr
What percentage of the offspring
will be round?
More Complete Dominance Practice- show all work and remember to include keys!
5. A homozygous dominant tall plant is crossed with a short plant. What percentage of the offspring
will be tall?
6. 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?
7. 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?
8. A testcross is used to determine if the organism showing a dominant trait is homozygous or
heterozygous. For example, if two purple heterozygous flowers crossed 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? What results
would you expect if the purple offspring was homozygous dominant? Show your work! Remember
that in a testcross, the individual of the unsure genotype is crossed with the recessive individual.
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:
9. Determine the possible genotypes and phenotypes of offspring of one black and one white
Andalusian chicken.
10. Determine the possible genotypes and phenotypes of offspring of two blue Andalusian chickens.
11. Determine the possible genotypes and phenotypes of offspring of one blue and one white
Andalusian chicken.
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:
12. 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.
13. Matthew also planted four o’clock. All of his seeds came from one set of parents. When his plants
bloomed, 50% were white and 50% were pink. What were the genes of the parent plants? Show your
reasoning.
14. 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:
15. Determine the possible genotypes and phenotypes of offspring of one red parent and one roan
parent.
16. Determine the possible genotypes and phenotypes of two roan parents.
17. Determine the possible genotypes and phenotypes of one red parent and one white parent.
IV. MULTIPLE ALLELES
18. 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.
19. In a paternity case, both child and mother are blood type AB. What type of blood would definitely
prove that the man is not the father of this child?
20. Mr. Metcalf, a hospital director calls you in a state of shock. When he has finally calmed down
enough to talk coherently, you learn that one of his nurses came on duty the previous night after
spending a few hours with his friends at a local bar, and as a joke proceeded to exchange wrist bands
on the four babies in the nursery, all of whom are indistinguishable. As a first step in unraveling this
mess, you blood type the parents and babies, with the exception of Mr. and Mrs. Blatz who refuse to
do anything until they have talked with their lawyer. The results of the parents' blood tests are:
Mrs. Frink – AB, Mr. Frink - O
Mrs. Zeeb – B, Mr. Zeeb - B
Mrs. Youngblood – O, Mr. Youngblood O
Mrs. Blatz and Mr. Blatz – not typed
Assume that the individuals listed are the true parents, each set of parents having had one of the four
babies. Indicate the one family to which each of the following children belongs on the basis of blood
type. Prove your answers with crosses. Hint: you will need more than one cross for the Zeebs.
Baby 1 - B
Baby 2 - A
Baby 3 - AB
Baby 4 - O
Parents of baby 1?
Parents of baby 2?
Parents of baby 3?
Parents of baby 4?
V. SEX-LINKED INHERITANCE
21. Green-red colorblindness is a sex-linked disease. If a woman that is a carrier marries a normal
male, what percentage of their male children will be colorblind? What about the female children?
Remember to start with a key.
22. If a colorblind male marries a normal female, what percentage of their sons will be colorblind?
What percentage of their daughters will be colorblind?
DIHYBRID CROSSES
23. 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?
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?
24. 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 homozygous white, homozygous disk-shaped
fruit is crossed with a yellow, sphere-shaped fruit, what will the phenotypic and genotypic ratios be
for:
a. the F1 generation?
b. the F2 generation?
ANALYZING PEDIGREES
25. Tay-Sachs disease is a fatal genetic disorder caused by a recessive allele. The pedigree chart
below shows the history of the disease in a family. Identify the possible genotype(s) of each member
of the family.
Key:
= Normal male
= Tay-Sachs male
= Normal female
= Tay-Sachs female
26. Create a pedigree for you family. You will be supplied with PTC paper which contains the chemical
phenylthiocarbamide which has a bitter taste to most people. (The ability to taste the chemical is the
dominant trait). Test as many family members as you can: try to test siblings, parents, grandparents,
aunts, uncles, and cousins. The more members you test, the easier it will be to set up your pedigree.
After you draw the pedigree, determine possible genotype(s) of each family member.