Sex-linked Genes
Sex-linked genes are genes which occur on the X-chromosome in humans. Recessive alleles of sexlinked genes are more likely to be expressed in males since males only have one X-chromosome
and without a second X-chromosome there is no opportunity for the recessive (if present) to be
dominated by a second “dominant” allele. Two examples of sex-linked genes are hemophilia and
red-green color-blindness.
Hemophilia
Clotting factors (proteins), platelets and red blood cells work together to repair damage to blood
vessels and prevent blood loss. Coagulation involves both a cellular (platelet) and a protein
(coagulation factor) component. Platelets are small blood cell fragments that form in the bone
marrow—a sponge-like tissue in the bones. Platelets play a major role in blood clotting. When
blood vessels are injured, clotting factors help platelets stick together to plug cuts and breaks on
the vessels and stop bleeding.
There are 13 blood clotting proteins (coagulation factors) found in the blood. They are designated
by Roman Numerals I through XIII. When a blood vessel is damaged, these clotting factors are
switched on in a certain order (Blood Clotting Cascade) and work to form a clot. If one factor is
missing or present at low levels, this causes hemophilia and other blood clotting problems and a
proper clot will not form. The two most common factor deficiencies are: factor 8 (or factor VIII)
deficiency and factor 9 (or factor IX) deficiency. The most common, affecting 80% of the
hemophilia population - those with hemophilia A - is factor VIII. When these blood clotting proteins
aren't present is not easily stopped.
People born with hemophilia have little or none of one or more clotting factors, the proteins needed
for normal blood clotting. The reason is that their DNA does not code for one or more of the
clotting factors. The allele responsible for this is sex-linked (occurs on X-chromosome) and is
recessive. Men either have hemophilia or they do not. Women can be carriers if they are
heterozygous since they will synthesize the blood coagulation factors if they are heterozygous (XH
Xh).
Work Problem. Construct a Punnett Square to answer this question. A man who does not have
hemophilia and a woman whose does not (but had a hemophiliac father) have children. What
percentage of boys might suffer from hemophilia (e.g., 25%, 50% etc)? Remember a woman’s
genotype will be written as X? X? and a man’s X? Y where the question mark subscript (?) is
replaced by the allele H subscript for dominant normal and h subscript for recessive hemophilic.
Red-Green Color Blindness
Red-green color blindness is another sex-linked trait. The genes occur on the X-chromosome (XB is
normal, Xb is red-green color-blind). Red-green color-blindness is recessive so it is more likely to
occur in men since they only have one dose of the gene on their single X chromosome and if their
X-chromosome is carrying the recessive allele Xb then they will be red-green color-blind. Red-green
color blindness is most common (8%) in men of northern European ancestry.
Work Problem – A man who is red-green colorblind and a woman who has normal color vision
have children. The woman’s father was red-green color-blind. What percentage of male children
born to this couple will have red-green color blindness? Remember a woman’s genotype will be
written as X? X? whereas a man’s X? Y where the question mark subscript (?) is replaced by the
allele B subscript for dominant normal and b subscript for recessive red-green color-blind.