Download In humans, hemophilia is an X

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Sex Linkage Practice Problems
Genes that are carried on either the X or the Y-chromosome are said to be sex-linked. In
humans the small Y-chromosome carries very few genes. The much larger X-chromosome
contains a number of genes that are vital to proper growth and development. In fact, it seems to
be impossible for humans to develop without the genes of the X-chromosome.
It is particularly easy to spot recessive defects in genes located on the X-chromosome because
the genes are expressed more commonly in males than in females. What is the reason for this?
Recall that males have one X-chromosome. Thus all X-linked genes are expressed in males,
even if they are recessive. In order for a recessive gene to be expressed in females, there must
be two copies of it, one on each of the two X-chromosomes. If one of the X-chromosomes
contains a dominant gene, it will mask the expression of the recessive gene. Women who have
one dominant allele and one recessive allele for X-linked genes are called carriers.
1. In humans, hemophilia is an X-linked recessive trait. A woman who is a carrier for
hemophilia marries a man who is normal. Will any of their children have hemophilia? Show
all of your work.
2. Red/Green color blindness is also an X-linked recessive trait. A color-blind woman marries
a man with normal color vision. Will any of their children be color-blind? Show all of your
work.
3. One kind of muscular dystrophy is a genetic disorder. It is due to a recessive X-linked allele.
Usually, its symptoms start in childhood. Over time, a slow loss of muscle function leads to
death, usually by age 20 or so. Unlike color blindness, this disorder is nearly always
restricted to males. In at least 2 complete sentences, suggest why.