Sex-linked Traits
X and Y Chromosomes
The X and Y-chromosomes are called sex chromosomes and are responsible for
determining gender (females are XX and males are XY). In humans, Xchromosomes are much longer than Y-chromosomes and contain many more
genes.
Gender
What are the chances of having a male child versus a
female child?
Use the Punnet square to illustrate your answer.
Sex-linked traits
Traits caused by genes located on X-chromosomes are called sex-linked traits.
Examples of sex-linked disorders include colorblindness, hemophilia and a type
of muscular dystrophy. These are all recessive disorders.
Sex-linked traits are much more common in men than women. Since men only
have one X- chromosome, inheriting just one allele for a sex-linked disorder
means they will get the disorder. Women, on the other hand, because they contain
two X-chromosomes, need two of these alleles to get a disorder, and thus are less
likely to do so than men. Women are often carriers for sex-linked disorders,
meaning they carry an allele but do not have the disorder (are heterozygous). Men
cannot be carriers for sex-linked traits, since they can’t be heterozygous.
Example: With sex-linked traits we use X and Y’s to represent the chromosomes
and superscripts to represent the alleles. Notice, the Y-chromosome does not
contain an allele since Y-chromosomes do not contain the genes.
Normal = D
Disorder = d
XD Xd
Xd Y
Phenotype:
Phenotype:
normal
disorder
The female in this example will be a carrier for the disorder, but will not get the
disorder because she has a normal allele, which is dominant. The male in this
example, although he only has one copy of the allele, will get the disorder because
he does not have a normal, dominant allele to mask the recessive allele.
Colorblindness
Colorblindness is a sex-linked disorder. It affects about one in every 20 men, and
a much lower percentage of women. If a woman who is a carrier for
colorblindness has children with a man who is not colorblind, what percentage of
their children do you predict will be colorblind?
1. First, write the genotypes and phenotypes of the parents.
2. Now, use a Punnet square to predict the genotypes and phenotypes of
the offspring. What percent of girls will get the disorder? What percent
of boys?
3. If this couple had ten children, would it be possible for none of them to
be colorblind?
4. Imagine another set of parents, a woman who is a carrier for
colorblindness, and a man who is colorblind. What predictions can you
make about their male and female children?
5. Imagine another set of parents where the mother is neither colorblind,
nor a carrier, while the father is colorblind. What are the chances of
having a colorblind son? A colorblind daughter?
Hemophilia
Hemophilia is a disease that prevents blood from clotting properly.
It is much more common in males than females because it is a sexlinked trait. Queen Victoria was a carrier for this disorder and, as a
result, more than ten of her royal male descendants developed
hemophilia.
6. Imagine a carrier for hemophilia had children with a healthy male. Use a
Punnet square to make predictions about their male and female children.
7. Now imagine that a woman who had hemophilia had children with a healthy
male. What percent of their male and female children would be likely to
have hemophilia? Use a Punnet square and show your work.
8. Now imagine a healthy, non-carrier woman has children with a man who has
hemophilia. What percent of their male and female children do you predict
would have hemophilia? Use a Punnet square and show your work.