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Cross Application Problems
Name: ______________________
Period: ______________________
Objective: To practice solving genetics problems by setting up both monohybrid and dihybrid crosses.
Part I Genotypes and Phenotypes:
1.
How many traits are investigated in a monohybrid cross? ___________
2.
For each genotype, indicate whether it is heterozygous or homozygous
AA ______________
Bb ______________
Ee ________________
ff _________________
ii ________________
JJ ________________
Mm _______________
nn _________________
3. For each of the genotypes below, determine the phenotype.
a. 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 to long tails
TT ___________________________
Tt ___________________________
tt ___________________________
4. For each phenotype, list the genotypes. (Remember to use the letter of the dominant trait)
Straight hair is dominant to curly.
____________ straight (homozygous)
____________ straight (heterozygous)
____________ curly
Pointed heads are dominant to round heads.
____________ pointed (homozygous)
____________ pointed (heterozygous)
____________ round
Part II Punnett Squares:
5. Set up the square for each of the crosses listed below. The trait being studied is round seeds (dominant) and
wrinkled seeds (recessive)
a. Rr x rr
i.
What is the phenotypic ratio of the offspring? ___________
ii.
What is the genotypic ratio of the offspring? ___________
b. Rr x Rr
i.
What is the phenotypic ratio of the offspring? ___________
ii.
What is the genotypic ratio of the offspring? ___________
i.
What is the phenotypic ratio of the offspring? ___________
ii.
What is the genotypic ratio of the offspring? ___________
a. RR x Rr
Part III Practicing Crosses: Show all of your work with PUNNETT SQUARES.
6. In plants a tall phenotype (T) is dominant to a short phenotype (t).
a. A homozygous tall plant is crossed with a homozygous short plant.
What percentage of the offspring will be tall? ___________
b. Two heterozygous plants are crossed.
What percentage of the offspring will be short? ______
7. In plants round seeds (R) are dominant to wrinkled seeds (r).
a. A heterozygous plant is crossed with a homozygous dominant plant.
What percentage of the offspring will be homozygous dominant? ____________
b. A homozygous round-seeded plant is crossed with a homozygous wrinkled-seeded plant.
What are the genotypes of the parents? __________ x __________
What percentage of the offspring will also be homozygous? ______________
8.
In pea plants purple flowers (P) are dominant to white flowers.
a. If two white flowered plants are crossed, what percentage of their offspring will be white? _____
b. A white flowered plant is crossed with a plant that is heterozygous for the trait.
What percentage of the offspring will have purple flowers? _____________
c. Two plants, both heterozygous for the gene that controls flower color are crossed.
What percentage of their offspring will have purple flowers? ______________
What percentage will have white flowers? ___________
9. In guinea pigs, the allele for short hair (H) is dominant to long hair (h).
a. What genotype would a heterozygous short haired guinea pig have? _______
b. What genotype would a pure-breeding short haired guinea pig have? _______
c. What genotype would a long haired guinea pig have? ________
d. Show the cross for a pure breeding short haired guinea pig and a long haired guinea pig.
What percentage of the offspring will have short hair? __________
e. Show the cross for two heterozygous guinea pigs.
What percentage of the offspring will have short hair? ________
What percentage of the offspring will have long hair? _________
f. Two short haired guinea pigs are mated several times. Out of 100
offspring, 25 of them have long hair.
What are the probable genotypes of the parents? ________ x ___________
10. In dogs, there is a hereditary deafness caused by a recessive gene, “d.” A kennel owner has a male dog
(Gilbert) that she wants to use for breeding purposes if possible. The dog can hear.
a. What are the two possible genotypes of Gilbert? __________
&
__________
b. If the dog’s genotype is Dd, the owner does not wish to use him for breeding so that the deafness
gene will not be passed on. Gilbert’s genotype can be tested by breeding him to a deaf female (dd).
Draw two Punnett squares to illustrate these two possible crosses.
c. In each case, what percentage of the offspring would be expected to be hearing? Deaf?
d. How could you tell the genotype of this male dog?
11. The breeder decided to cross Gilbert with a deaf female to see if he had the deaf gene. There were three
puppies in the litter, two could hear but one could not. What is Gilbert’s genotype? How do you know?
12. If Gilbert mated with a heterozygous female what would the probability that the resulting litter of 5 puppies
are all hearing dogs? (HINT: Litters are composed of individuals that are conceived and born AT THE SAME TIME)
Part IV Introduction to Dihybrid Crosses:
1.
What do the letters at the top and to the sides of a Punnett Square represent? ______________________
2. How many copies of each gene (alleles) are in a gamete? ______________________
3. How many traits (genes) are studied in a dihybrid cross? ______________________
4. Based on the previous answers; how many letters should each gamete have in a dihybrid cross? _________
Part V Practice Dihybrid Crosses:
1.
A tall (T) phenotype is dominant to short (t) phenotype in plants. Purple flowers (P) are also dominant to
white flowers (p).
a. Cross a plant that is homozygous dominate for BOTH traits,
with a plant that is homozygous recessive for BOTH traits.
i.
What is the probability of producing tall plants with
purple flowers? Possible genotype(s)?
ii.
What is the probability of producing dwarf plants with
white flowers? Possible genotype(s)?
iii.
What is the probability of producing tall plants with white flowers? Possible genotype(s)?
iv.
What is the probability of producing dwarf plants with purple flowers? Possible genotype(s)?
2. In corn purple kernels (K) are dominant to yellow kernels (k). Starchy
kernels (S) are dominant to sweet kernels (s)
a. Cross a corn plant that is homozygous dominant for BOTH
traits, with a corn plant that is homozygous recessive for
BOTH traits.
i. What is the probability of producing purple, starchy corn
kernels? Possible genotype(s)?
ii. What is the probability of producing yellow, starchy corn
kernels? Possible genotype(s)?
iii. What is the probability of producing purple, sweet corn kernels? Possible genotype(s)?
iv. What is the probability of producing yellow, sweet corn kernels? Possible genotype(s)?
3. In guinea pigs black fur (B) is dominant to tan fur (b). Rough fur (R) is dominant to smooth fur (r).
a. Cross two guinea pigs that are heterozygous for BOTH traits.
i. What is the probability of producing guinea pigs with black,
rough fur? Possible genotype(s)?
ii. What is the probability of producing guinea pigs with black,
smooth fur? Possible genotype(s)?
iii. What is the probability of producing guinea pigs with tan,
rough fur? Possible genotype(s)?
iv. What is the probability of producing guinea pigs with tan, smooth fur? Possible genotype(s)?
4. In wolves grey fur (G) is dominant to black fur (g). Brown eyes (B) are dominant to blue eyes (b).
a. Cross two wolves that are heterozygous for BOTH traits.
i. What is the probability of producing a wolf with grey fur and
brown eyes? Possible genotype(s)?
ii. What is the probability of producing a wolf with grey fur and
blue eyes? Possible genotype(s)?
iii. What is the probability of producing a wolf with black fur and
brown eyes? Possible genotype(s)?
iv. What is the probability of producing a wolf with black fur and blue eyes? Possible genotype(s)?