Punnett Square Practice
1. Albinos are individuals who are unable to produce
melanin, a pigment which is found in skin, eye and hair
colour. This disorder is caused by a recessive allele.
A normal female, whose mother is albino, marries a
homozygous normal male
a) What is the probability that their children will have the
disorder?
b) Of the children that are normal, what fraction will be
heterozygous?
Achondroplasia is a type of dwarfism caused by a
dominant allele. Two heterozygous individuals marry and
decide to have children.
a) What is the probability that the first child will have
achondroplasia
b) If the first child has achondroplasia, what is the chance
that the second child will have it?
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3. Huntington's Disease is a fatal disorder
characterized by deterioration of brain tissue over
time. Symptoms do not generally occur until middle age.
Huntington's is caused by a dominant allele.
A man who is heterozygous for the Huntington's Disease
marries a woman who is homozygous recessive for the
disorder. What is the probability that their children will
develop the disease?
4. Tay-Sachs disease is a hereditary disorder that
causes an abnormal build-up of fat in the brain tissue,
eventually resulting in death at around age 4. The
disease is caused by a recessive allele.
Two heterozygous individuals decide to have children.
What is the chance that they will have a child with
Tay-Sachs Disease?
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5. Xeroderma pigmentosum is a condition in which
individuals are extremely sensitive to sunlight,
resulting in excessive pigmentation and possible skin
cancer. This condition is caused by a recessive allele.
Two heterozygous individuals plan to have children.
What are the chances that they will have a child with
this condition?
6. Deafness in one ear is caused by a dominant allele.
The ability to curl the tongue is also caused by a
dominant allele. A woman who is heterozygous for
both traits marries a man with normal hearing who
cannot curl his tongue.
What is the probability if them having a child who can
curl their tongue and has the deafness trait?
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7. Galactosemia is a recessive disorder. Individuals with
this disease cannot break down galactose sugar.
Albinism is also caused by a recessive allele.
A man heterozygous for both traits marries a woman
who is homozygous for both traits. What is the
probability that their child will be galactosemic and
albino?
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