Sex linkage
There are some genes that are called sex-linked genes because they are
positioned on the 23rd chromosome – the set chromosomes.
Often genetic traits, which show sex linkage, affect one gender more than the
other. Two examples of genetic traits, which have this particularly, are colour
blindness and hemophilia.
 Colour blindness is the inability to distinguish between certain colours,
often green and red. To people who are colour blind, the two colours look
the same.
 Hemophilia is a disorder in which blood does not clot properly. For most
people a small cut or scrape on there skin stops bleeding after a few mins,
with hemophilia there is a risk of bleeding to death.
The alleles for both colour blindness and hemophilia are found only on the X
chromosome.
Sex linked conditions are represented as XX or XY to show gender and then
superscript letters to show the dominant or recessive genotype.
Alleles on the X chromosome for colour blindness
Xb = recessive allele for colour blindness
Xb = alleles not being colour blind.
Alleles on the X chromosome for hemophilia
Xh = allele for haemophilia
XH= allele for not having haemophilia.
Question
A male without haemophilia reproduces with a female that is a carrier for the
condition. What is the chance that their children will have the condition?
Parent
XHY
XHXh
XH Y Xh XH
XHXH, XHXh, XHY, XhY
3:1 chance of their child having haemophilia.