REVIEW OF GENETIC CROSSES
COMPLETE DOMINANCE - MONOHYBRID / ONE-TRAIT CROSSES
1. In pea plants, round seeds (R) are completely dominant to wrinkled seeds (r). In a genetic
cross of two plants that are heterozygous for the seed shape trait, what fraction of the offspring is
expected to have round seeds?
2. In humans, brown eyes are completely dominant over blue. What phenotypes, and in what
ratios, are expected from a cross between a father who is homozygous dominant for eye colour
with a blue-eyed female?
COMPLETE DOMINANCE - DIHYBRID / TWO-TRAIT CROSSES
3. In Martians, the ability to yodel is completely dominant over the inability to yodel. Martians
without antennae are recessive to those with antennae. What phenotypes, and in what ratios,
are expected from a cross between a dihybrid male Martian with a female without antennae that
is heterozygous for yodeling?
4. In humans, a type of blindness is due to a dominant allele; normal vision is the result of a
recessive allele. Migraine headaches are due to a dominant allele, and normal (no headaches) is
recessive. A male who is heterozygous for blindness and does not suffer from headaches marries
a woman who has normal vision and does not suffer from migraines. What proportion of their
offspring is expected suffer blindness and but no headaches? Will there be any offspring that are
expected to experience blindness and migraines?
COMPLETE DOMINANCE - TRIHYBRID / THREE-TRAIT CROSSES
5. In humans, brown hair is completely dominant over blonde. Widow’s peak is completely
dominant over a straight hairline. The inability to roll your tongue is recessive. Two grade 11
students are daydreaming about their future children together. She is trihybrid for all three
traits. He is homozygous dominant for hair colour, cannot roll his tongue, and has a widow’s
peak, which is like his father, but unlike his mother. Could these two students create a tonguerolling blonde with a straight hairline? If so, what is the expected frequency?
6. A fruit fly of genotype AaBbCC is mated to another fly of identical genotype. What proportion
of the offspring will be of AaBbCC genotype?
INCOMPLETE DOMINANCE
7. The colour of snapdragon flowers can be white, red, or pink. The alleles are incompletely
dominant with one another as pink flowers result from a cross of individuals with white and red
flowers. What phenotypes, and in what ratios, are expected in crosses between parents that are:
a) red and pink
b) pink and white
c) pink and pink
CO-DOMINANCE
8. Feather colour in chickens is governed by co-dominance. Black roosters crossed with white
hens result in checkered black-and-white (barred) plumage, where some feathers are white and
some are black. Predict the phenotypes and ratios from crosses between:
a) a barred rooster and a barred hen
b) a black rooster and a barred hen
c) a barred rooster and a white hen
9. The interaction of alleles that cause sickle-cell anemia is an illustration of co-dominance in
humans. One of the homozygous conditions allows the individual to produce normal red blood
cells. Those individuals with the other homozygous condition produce red blood cells that are
“sickled.” These cells are unable to carry oxygen properly and severe anemia generally results.
A heterozygous individual exhibits something called “sickle-cell trait”, where some red blood cells
are normal and some are sickled.
a) Suppose a normal male mates with a heterozygous female. What are the chances that the
children of this match will develop sickle-cell anemia? sickle-cell trait?
b) Now suppose that each of the children from the above mating is matched with a heterozygote.
What will be the probability from each of these matches that the children will be anemic? exhibit
the sickle-cell trait?
MULTIPLE ALLELES
10. The colour of rabbit fur is determined by at least four alleles : C – agouti, ch – Himalayan, cch
– chinchilla, c – albino (order of dominance : C > cch > ch > c).
a) Give the genotypes and phenotypes for the following cross : Cc ch X cchc
b) Now show the genotypes and phenotypes of offspring if each of the rabbits from the F1
generation mates with a rabbit with the genotype chc.
SEX-LINKED (X-LINKED) INHERITANCE
11. One of the best-known examples of an X-linked recessive defect is the disease known as
hemophilia. Predict the genotypes and phenotypes that would be expected from the following:
a) Normal female (no history of hemophilia anywhere in the family) X Normal male (no history of
hemophilia anywhere in the family)
b) Normal female whose father had hemophilia X Normal male
c) Normal female whose father had hemophilia X Male with hemophilia
d) Female with hemophilia X Normal male (Why is this an unlikely match?)
12. A man with hemophilia marries a woman who carries the hemophilia allele but has normal
blood clotting time.
a) What proportion of their sons would you expect to have hemophilia?
b) What proportion of their daughters would you expect to have the disease?
COMBOS
13. Colour blindness is sex-linked and recessive. Brown eyes are dominant over blue. List the
phenotypes of a cross between a colour-blind female heterozygous for eye colour and a normal,
blue-eyed male.
14. Among cattle, coat colour is controlled by two co-dominant alleles, one producing a red coat
and the other producing a white coat. The two alleles together produce a roan coat. Not having
horns is completely dominant over the presence of horns. What phenotypes would be expected in
a cross between a roan, horned goat and a hornless red goat carrying a horned allele?
15. Assume tall is dominant over short, brown eyes are dominant over blue, and hemophilia is a
sex-linked recessive trait. Cross a trihybrid female with a male who is homozygous dominant for
height, and has blue eyes and normal blood clotting to determine the chance the couple will have
a tall, blue-eyed son with hemophilia.
16. In cats, one gene for coat colour is X-linked. Male cats are either black or orange, depending
on which allele they carry. Females are black, orange, or tortoise-shell (patches of black and
orange), with the tortoise-shell phenotype resulting from the heterozygous genotype. Two cats
mate and have kittens. Of the female kittens, ½ are tortoise-shell and ½ are orange. Of the
male kittens, ½ are orange and ½ are black. What are the genotypes of the parent cats?