Mendelian Genetics Problems #2
Honors Biology
Dihybrid Crosses:
DIRECTIONS: On a separate piece of paper answer the following questions. Make sure to label each problem and
support your answers by showing ALL your work!!
1. In rabbits, floppy ears are recessive to straight ears and spotted coat is recessive to solid white coat.
What is the result of a cross between a floppy- eared, spotted- coat female with a heterozygous white coated hybrid straight -eared male? Find the phenotypic ratio and show all your work
2. In rabbits, spotted- coat (S) is dominant to solid color (s), and black (B) is dominant to brown (b). In a
large population, brown- spotted rabbits are mated to solid black ones and all the offspring are black
spotted. Show all your work to the problems below.
a. What are the genotypes of the parents?
b. What would be the appearance of the F2 offspring if two of these F1 offspring, black spotted
rabbits, were mated?
3. In humans, right handedness is dominant to left handedness and curly hair is dominant to straight hair.
Show the phenotypic and genotypic ratios of the F1 generation of a mother who is heterozygous for right
handedness with straight hair with a dad that is left handed with hybrid curly hair. Show all your work.
4. In horses, black coat (B) is dominant over chestnut (b) coat and trotting (T) is dominant over pacing (t). If
a purebred black, pacer is crossed with a horse that is hybrid for both coat color and trotting, what would
the phenotype of the offspring be? Show all your work!
BONUS POINTS (Tri-hybrid Crosses):
5. In pea plants, tall plants (T) are dominant to dwarf (t), yellow color (Y) is dominant to green (y), and
smooth seeds (S) are dominant to wrinkled seeds (s). What would be the phenotypes of the offspring is
one parents genotype is Tt Yy Ss and the other parent is tt yy ss ? Show all your work separately from the
other problems (+3 pts).
6. The weight of the fruit in one variety of squash is determined by three pairs of genes. The homozygous
dominant condition, AABBCC, results in 6-pound squashes, and the homozygous recessive condition,
aabbcc, results in 3-pound squashes. Each dominant gene adds a half pound to the minimum 3 pound
weight. When a plant having 6- pound squashes is crossed with one having 3- pound squashes, all the
offspring have 4 ½ - pound fruit. What would be the weights of the F2 fruit, if two of these F1 plants were
crossed? Show all your work separately from the other problems. (+5 pts)
Incomplete Dominance:
7. Petunia flower color is governed by two alleles, but neither allele is truly dominant over the other.
Petunias with the genotype RR are red flowered, those that are heterozygous (RR’) are pink and those
with the R’R’ genotype are white. This is an example of incomplete dominance. (Note that the
superscripts are used rather than upper and lowercase letters to describe alleles).
a. If a white flowered plant is crossed with a red flowered petunia, what is the genotypic ratio of the
offspring?
b. What is the phenotypic ratio of the F1 offspring?
c. If two of the F1 offspring are crossed, what phenotypes will appear in the F2 generations?
d. What will be the genotypic ratio in the F2 generation?
8. A certain flower produces red and yellow flowers. Red is produced by the homozygous dominant gene,
yellow is produced by the heterozygous genotype, and the homozygous recessive condition is lethal. If
you cross a yellow with a yellow flowering plant:
a. What percentage of the offspring will yellow?
b. What percentage will be red?
c. What percentage will be lethal (die)?
9. In Andalusian fowl, the gene for black plumage (B) is incompletely dominant to the gene for white
plumage (B’). The heterozygous condition results in blue plumage. List the genotypic and phenotypic
ratios expected from the following crosses:
a. Black with blue
b. Blue with blue
c. Blue with white
Co- Dominance:
10. One well known example of co-dominance occurs in the coat color of Shorthorn cattle. Those with
reddish-gray roan coats are heterozygous (RW), and result from a mating between a red (RR) Shorthorn
and one that’s white (WW). Roan cattle don’t have roan-colored hairs, as would be expected with
incomplete dominance, but rather appear roan as a result of having both red and white hairs. Thus, the
roan coloration is not a consequence of pigments blending in each hair. Because R and W are both fully
expressed in the heterozygote, they are co-dominant.
a. If a roan Shorthorn cow is mated with a white bull, what will be the genotypic and phenotypic
ratios in the F1 generation?
b. List the parental genotypes of crosses that could produce at least some:
i. White offspring
ii. Roan offspring
11. When blue flowers are crossed with white flowers in some plants the result will produce pale blue
flowers.
a. Demonstrate with this with a Punnett Square and explain your results.
b. If a cross between two of these flowering plants produces 27 blue, 49 pale-blue, and 24 white
plants, then what would be the genotype of the parents.
12. Outline a breeding procedure whereby a true breeding strain of red cattle could be established from a
roan bull and a white cow. Refer to questions #10. Show all your work.
Multiple Alleles:
13. Mrs. Doe and Mrs. Roe had babies at the same hospital at the same time. Mrs. Roe brought home a baby
girl and named her Nancy. Mrs. Doe received a baby boy and named him Richard. However, she was sure
she had a girl and brought suit against the hospital. Blood tests showed that Mr. Doe was type O, Mrs.
Doe was type AB, and Mr. and Mrs. Roe had both type B. Nancy was type A and Richard type O. Had an
exchange occurred?
14. A woman with blood type O has a child with blood type O. She claims that a friend of hers is the child’s
father. In the ABO system, IA and IB are both dominant to i and are co-dominant to each other. ABO
genotypes are summarized below:
IAIA and IAi ------------------------ A Blood
IBIB and IBi ------------------------- B Blood
IAIB
------------------------- AB Blood
I
------------------------ O Blood
a. Her friend’s blood type is A. Can he be excluded as the father on this evidence alone?
b. Does the fact that the accused man’s mother has type A and his father has type AB exclude him
from being the parent?
c. Does the additional information that his mother’s parents are both AB permit him to be
excluded?
15. A woman has a child with type O blood. The woman has type A blood and has filed a paternity suit
against a man with type B blood, claiming that he is the father of her child. As her attorney, would you
proceed with litigation? Why or why not? Show all your work.
Epistasis:
16. As in Labrador retrievers, fur color in mice is governed by genes concerned with producing and
distributing melanin. At one gene location (locus), a dominant allele (B), specifies dark brown and a
recessive allele (b) specifies tan. At another locus, a dominant allele (C) permits melanin production and a
recessive allele (c) shuts it down and results in albinism. A mouse that is homozygous recessive for both
traits is crossed with a mouse that is homozygous dominant for both traits. State the probable genotypic
and phenotypic ratios for the F1 and F2 offspring. Show all your work!