Genetics Practice Problems
Answer on a separate page and SHOW ALL WORK ( yes that includes gamete Punnetts and everything!)
1. In pea plants, tall (T) is dominant to dwarf (t). Show the cross between a heterozygous tall and a dwarf plant. What are the
expected phenotypes and in what proportion?
2. Show the cross between two heterozygous tall plants. What are the expected phenotypes and in what proportion?
3. In pea plants, inflated pods are dominant to constricted pods. Show the cross between a pure-bred inflated pod plant with
a pure-bred constricted pod plant. What are the expected phenotypes and in what proportion?
4. If two constricted pod plants are crossed what are the expected phenotypes of the offspring and in what proportion?
5. A tall plant of unknown genotype is test-crossed. Of the offspring, 200 are dwarf and 200 are tall. What is the genotype of
the unknown parent? Show the cross to prove it.
6. In humans, tongue rolling is a dominant trait, those with the recessive condition cannot roll their tongues. Bob can roll his
tongue, but his mother could not. He is married to Sally, who cannot roll her tongue. What is the probability that their first born
child will not be able to roll his tongue?
7. In goats, a recessive gene causes the goats to "faint" when they are startled. A farmer breeds two goats (that have never
fainted) and their first offspring faints two days after its birth. What must the parent's genotypes have been? Show the cross
to prove it.
8. In guinea pigs, short hair is dominant to long hair. Also in guinea pigs, black eyes are dominant to red eyes. A male guinea
pig that is heterozygous for both traits is crossed with a female that is long haired and red eyed. What are the expected
phenotypes of their offspring and in what proportion?
9. If both parents are heterozygous for both traits, what are the expected phenotypes of their offspring and in what
proportion?
10. A black Labrador of genotype BbEe is crossed with a yellow lab of genotype Bbee. What are the expected phenotypes of
their offspring and in what proportion?
11. In cats, coat colour is an example of codominance. Colours can be yellow (YY), black (yy), or tortoise –shell (Yy).
What kinds of offspring will be expected from the following crosses:
a) yellow female x black male
b) tortoise-shell female x yellow male
c) tortoise-shell female x black male
12. In a certain cactus, prickly spines can be two pronged or one pronged. If a pure-bred one-pronged cactus is crossed with
a pure bred two-pronged cactus, the F1 generation has a mixture of spines, some are two-pronged, some are one-pronged.
a. Is this an example of codominance or incomplete dominance?
b. Show the F2 generation (a cross between the two F1's). What are the phenotypes of the offspring and in what proportion.
13. In this same cactus, purple flowers are dominant to yellow flowers. A cactus that has both two-pronged and one-pronged
spines has yellow flowers. It is crossed with a two-pronged cactus with purple flowers (heterozygous). What are the
phenotypes of the offspring and in what proportion?
15. Coat color in cats is a codominant trait and is also located on the X chromosome. Cats can be black, yellow or calico. A
calico cat has black and yellow splotches. In order to be calico. the cat must have an allele for the black color and an allele
for the yellow color. Use a punnet square to show why there are no male calico cats.
16. A female calico cat is crossed with a male black cat. What are the phenotypes of the offspring and in what proportion.
17. Also located on the X chromosome of a cat is a gene that codes for deafness. This gene is recessive. A black female cat
that is is heterozygous for deafness (Dd) is crossed with a yellow male cat that is not deaf. Show the cross. What are the
phenotypes of the offspring and in what proportion. Hint: place two letters on the X chromosome in your cross.
18. In mice, the spinning behavior is caused by a dominant gene that effects the mouse's equilibrium. This gene is lethal if
two alleles are present. Two "spinning mice" are mated together. Show the cross. What are the phenotypes of the offspring
and in what proportion. (Be sure to eliminate any offspring that do not survive.)
19. In one family, all three siblings have type B blood.
a. Use a punnett square (or two!) to show how two different sets of parent genotypes could produce this result.
b. Which of the two sets of potential parents in your answer to (a) is more likely to be the parents of these siblings.
Explain why.
20.
= Huntington’s
Disease
I
II
1
2
1
2
4
5
3
6
7
8
III
1
2
3
4
5
1. Which members of the family above are afflicted with Huntington’s Disease? ____________________________
2. There are no carriers for Huntington’s Disease- you either have it or you don’t.
With this in mind, is Huntington’s disease caused by a dominant or recessive trait? ___________________
3. How many children did individuals I-1 and I-2 have? _______________________________________________
4. How many girls did II-1 and II-2 have? ___________ How many have Huntington’s Disease? _______________
5. How is individual III-2 and II-4 related? _____________________ I-2 and III-5? _______________________
21. Mr. and Mrs. Jones have 4 children. The blood types of their children are type o, type A, type B, and type AB. What are
the genotypes with respect to blood type of Mr. and Mrs. Jones and their children?
22. Mr. Smith has blood type A. His wife has blood type B. Their newborn child has blood type O. Mr. Smith is filling a
paternal suit against his wife. Will Mr. Smith win in court? Why?
23. You have just been involved in a serious automobile accident and are in need of a blood transfusion. The doctor at the
hospital has discovered that your blood type is type O. What type of blood can the doctor transfuse into you and why?
24. Another woman has the same problem. Her blood type is A, her child’s is B. She again has three candidates for
fatherhood. Their blood types are: Man #1, B; Man #2, AB; Man #3, O. Based on blood types, the mother says it must
have been #1.
a. Do you agree? Why or why not?
b. This child, a son this time, is also colorblind. The only one of the men in question to share this characteristic is #2. The
mother is not colorblind. Can you now determine who the father of the little boy is, assuming it must be one of these
men? Explain your answer.