Genetics Problem Sets Worksheet With Answer Key Page 7
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14Genetics Problems Set #4
SEX-LINKED TRAITS
Humans have 23 pairs of chromosomes. Twenty-two (22) pairs are called autosomes. The
chromosomes in the 23rd pair are called "sex chromosomes" because some of the genes on
these chromosomes determine if the individual is female or male. Under normal
conditions a human female has the genotype XX (meaning that she has two "X"
chromosomes with a combination of genes on these chromosomes that determine that she
is female.) Under normal conditions the human male has the genotype XY with a
combination of genes on these chromosomes that determine that he is a male.
Female =
XX
Male
=
XY
Just like autosomes, the sex chromosomes carry genes for various traits. Some traits other
than the sex of the individual are carried on the X and Y chromosomes. If you look at the
"Y" chromosome you can see that it is physically smaller than the "X" chromosome. A
current hypothesis suggests that there are some traits for which the "X" chromosome
carries an allele, but for which the Y-chromosome carries NO CORRESPONDING
ALLELE. These combinations result in the same phenotype being expressed by different
genotypes in males and in females. Let's look at an example.
Humans can suffer from a disorder that inhibits the blood from clotting properly. This
disorder, known as hemophilia, is due to a recessive allele (h). Normal blood clotting is
dominant (H). Most people are normal. The allele for this blood clotting trait is found on
the X-chromosome and there is no CORRESPONDING allele for this trait found on the Y-
chromosome. Therefore women and men have different genotypes for blood clotting
phenotypes.
Women (XX)
Men (XY)
Genotype
Phenotype
Genotype
Phenotype
X
X
normal clotting
X
Y
normal clotting
H
H
H
normal clotting
X
X
considered a
X
Y
hemophilia
H
h
h
"carrier")
X
X
hemophilia
h
h
*Notice that the genotypes for "normal" in males and females are different yet they
express the same phenotype. Similarly, the genotypes for "hemophilia" are different in
males and females yet they express the same phenotype.
Genetics Problems
7
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