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Eastern Regional High School
Honors Biology
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Unit 13 – Advanced Genetics: Exceptions to Mendel’s Rules
Worksheet – X-Linked Inheritance Practice Problems
Introduction:
Some families have a long history of hemophilia among male members of the line. The royal family of Great
Britain is a commonly mentioned example. Families with this kind of history generally want to know when and
where the disease will occur again.
Problems:
Imagine you are a genetic counselor. A man and woman have come to you for advice. They present the
information given in each problem below. Predict the genotypes and phenotypes that would be expected from
each mating. Let XN = the normal allele and Xn = the allele for hemophilia. Create a punnett square for each
problem and show your work.
1. Normal female with no history of hemophilia anywhere in the family and a normal male with no history
of hemophilia anywhere in the family.
2. Normal female whose father had hemophilia and a normal male
3. Normal female whose father had hemophilia and a male with hemophilia.
4. Female with hemophilia and a normal male.
5. Why is the cross in number 4 not likely to occur?
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6. Why are male more likely to inherit X-linked diseases?
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7. A man with normal vision marries a women who also has normal vision. One of their sons is colorblind.
Show the cross that produced this child.
8. A woman who is colorblind is married to a man with normal vision. The couple goes to a genetic
counselor and asks him if any of their children will be colorblind. What would you tell them if you were
the genetic counselor?
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9. In a large family in which all the daughters have normal vision and all the sons are colorblind, what are
the probable genotypes of the parents?
10. Amelogenesis imperfecta is a dental abnormality, which is inherited as an X-linked recessive trait.
Neither of your parents have the disorder but your father’s brother does. (Assume your mother is
homozygous). What is the chance you will receive the recessive gene?
11. In cats, one gene for coat color 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 resulting form the heterozygous genotype. Cross a black furred female with an orange
male. What are the possible phenotypes and genotypes?
12. In cats, one gene for coat color 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 resulting form the heterozygous genotype. Two cats mate and have kittens. Of the
females, ½ are tortoise-shell, ½ are orange. Of the male kittens, ½ are orange and ½ are black. What
are the genotypes and phenotypes of the parent cats?