Genetics Problems - Set 2
1. In radishes the shape may be long, round or oval. Crosses between round and oval radishes gave
199 round and 203 oval offspring. Crosses between oval and oval radishes gave 121 long, 243 oval,
and 119 round offspring. What type of inheritance pattern is this? Indicate the genotype of each
phenotype.
2. In humans, homozygotes have either straight or curly hair, and only heterozygotes can have wavy
hair. From a mating between 2 wavy-haired individuals, what is the probability that 4 children in
the family will have straight hair?
3. When chickens with white feathers are crossed with black feathered birds their offspring are all
grey. When grey individuals are crossed among themselves, they produce white, grey and black
offspring. What type of inheritance pattern is this? Indicate the genotype of each phenotype.
4. In Shorthorn cattle three coat colors occur: red, roan and white. Mating red with red or white with
white produces only red or white offspring, respectively. Mating roan with roan results in red, roan
and white calves in the proportion 1:2:1. What is the genotype of the roan animals? Mating a roan
bull with only white cows results in 20 calves. Predict the most likely phenotypes and their
proportion of these offspring. What type of inheritance is this?
5. In a court case concerning a paternity dispute, each of two men claimed three children to be his
own. The blood groups of the men, the children and their mother were as follows:
husband
O
M
Rh+
On the basis of this evidence
lover
AB
MN
Rhcan the paternity of each of
wife
A
N
Rh+
the
three
children
be
first child
O
MN
Rh+
established?
Give
the
second child
A
N
Rh+
genotype of the wife.
third child
A
MN
Rh6. In mice, the following are members of a multiple allele series: CA for yellow, CB for wild-type, CC for
black. Individuals homozygous for the yellow allele die before birth. Alleles C A and CB are
dominant to CC, and CA is dominant to CB. What phenotypic ratios would you expect from the
following matings?
a. CA CC x CA CC
b. CA CC x CA CB
c. CA CC x CB CC
7. Tay-Sachs disease is a recessive phenotype (tt); it causes death by age 5. Achondroplastic dwarfism
is caused by the heterozygous state (Aa) and the homozygous recessive genotype (aa) is lethal.
a. Show the genotypes of the possible children resulting from a cross between parents who are
heterozygous for both genes. Consider only live births as possible.
b. In the previous cross, what fraction of the possible children will die before adulthood?
c. What fraction of the possible children will be normal?
d. What fraction of the possible children will have the normal phenotype with respect to the TaySachs locus but will exhibit dwarfism?
e. What fraction of the surviving children will not be carriers of either defective gene?
Practice Genetics Problems
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8. Sickle cell anemia is a hereditary disease in humans. Severe anemia is found in the homozygous
condition, aa, a milder form of anemia is found in heterozygotes, and normal individuals are
homozygous for the dominant allele. In mating between two individuals suffering from mild
anemia, 2/3 of the adult offspring also have mild anemia. What type of inheritance pattern is this?
Explain these results. Provide the genotypes for all individuals described.
9. Consider the following chicken genes. For the "creeper" gene, cc individuals are normal,
heterozygotes have short legs (creepers), and the CC genotype is lethal, preventing chicks from
hatching. For the feather color gene, B, the phenotype, black feathers is dominant to red feathers.
The Wager: A poultry breeder has two chickens: a black-feathered creeper cock (male) of genotype
CcBb and a red-feathered creeper hen (female). A gambler bargains with the poultryman for the adult
offspring produced from mating the two birds according to the following rules. The gambler will
pay the poultryman $20 for each adult offspring with short legs and red feathers or normal legs and
black feathers. However, the poultryman must pay the gambler $30 for each adult offspring with
short legs and black feathers and $10 for each bird with normal legs and red feathers. Should the
poultryman accept the wager? What potential margin of profit/loss does each person face? Show
your work.
10. In pigeons, checkered pattern is dominant to plain pattern, and red color is dominant to brown. An
unknown pigeon, X bred with pigeon Y, (which is plain and brown) and produced checkered, red
chicks. When pigeon X mated with pigeon Z (which is checkered and red) 2 plain brown chicks
were produced. If pigeon X re-mates with pigeon Y, what is the probability that they will have a
plain, brown offspring? Provide the genotypes of pigeons X, Y and Z.
11. In guinea pigs, color pattern and ear shape are controlled by two X-linked pairs of alleles. Three
varieties of color are possible: black, gray and white. The gray color occurs only in heterozygotes.
Ear shape also has three possibilities: pointed, semi-pointed and round. The semi-pointed
phenotype occurs only in heterozygotes. Two guinea pigs are mated and produce the following
offspring: a white pointed male, gray round female, and a white semi-pointed female. What are the
genotypes and phenotypes of the parents? What type of inheritance is involved?
12. In Drosophila, white eyes is a sex-linked factor that is recessive to red eyes. Vestigial wings are
recessive to normal wings, but these alleles are carried on autosomal chromosomes. Two flies when
bred together produced 1600 offspring. Among the males, 300 were red, normal; 300 were white,
normal; 100 were red, vestigial; and 100 were white, vestigial. Similar phenotypes were found
among the female offspring. How many of these (the female) offspring would you expect to be
white, vestigial? What are the genotypes of the parents?
13. The black and yellow pigments in the coats of cats are determined by an X-linked pair of alleles.
Males are either black (cb) or yellow (cy) Females are black, calico (with patches of black and patches
of yellow), or yellow.
a. A female has the following kittens in a litter: 2 calico females, 2 black females, 1 black male
and 1 yellow male. What is the genotype of the parents?
b. If a black female and a yellow male are crossed, what is the probability of getting a calico
female?
14. A child with blood type A, has a mother with blood type O. What blood types could the father have
had?
Practice Genetics Problems
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15. In humans, red-green color blindness is caused by a sex-linked recessive allele.
a. If a normal man marries a normal woman whose father was color blind, what is the
probability that they will have a color-blind son? List the genotypes of and phenotypes of
all possible children.
b. If a normal man mates with a color-blind woman, what is the chance that they will have a
color blind son?
c. If a normal man mates with a color-blind woman, what is the chance that they will have a
color blind daughter?
16. The allele for yellow body color in fruit flies is recessive and sex-linked. The dominant allele
produced wild-type body color. What phenotypic ratios are expected from the following
crosses?
a. yellow male x yellow female, give F1 and F2
b. wild-type male x yellow female; give F1 and F2
17. A narrow, reduced eye called “bar” is controlled by a recessive, X-linked allele in fruit flies, while
the normal, wild-type eye is produced by the dominant allele. A homozygous, normal female is
crossed with a bar-eyed male
a. determine the expected genotype and phenotype ratios in the F1 and F2.
b. from this mating, what is the probability of producing a bar-eyed female in the F2?
c. from this mating, what is the probability of producing a bar-eyed male in the F2?
18. In humans, both color blindness and hemophilia are caused by recessive, sex-linked alleles. A colorblind boy whose clotting time is normal (without hemophilia) has a hemophilic brother with
normal color vision. If the parents of the above children have another boy, what is the
probability that he will be normal for both traits? Give the genotypes of all individuals.
19. In humans, the allele for dark hair is dominant to the allele for red hair and the allele for righthandedness is dominant to the allele for left-handedness. A dark-haired, right-handed man
mates with a red-haired, left-handed woman. Their first child has red hair and is left-handed.
a. What are the genotypes of the parents?
b. If the same couple have a second child, what is the probability that it will have dark hair and
be right-handed?
20. For each of the following pedigrees, determine the possible modes of inheritance.
(a)
Practice Genetics Problems
(b)
(c)
(d)
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