Transcript Genetics * Sex Linkage

Genetics – Sex Linkage
• Autosomal genes = genes found on all
chromosomes except X and Y.
• Sex-linked genes = genes found on X and Y
chromosomes.
• In mammals, females are XX (homogametic)
and males are XY (heterogametic)
• (In birds, butterflies and moths it is the other
way around!)
• Sex-linked genes are responsible for
conditions such as red-green colour blindness;
haemophilia; some types of baldness and
hairy ears!
• In most species the Y chromosome carries few
genes (‘genetically empty’).
• Why are males more likely to show one of
these sex-linked conditions?
e.g. Haemophilia A
• Factor VIII is a protein crucial for blood to clot.
• The gene for it is on the X chromosome.
• The recessive allele produces a nonfunctioning protein (i.e. it doesn’t work!)
• The result is that their blood doesn’t clot well
and may be fatal.
• If a male has the recessive allele he will suffer
from haemophilia
• Show the results of a cross from:
– A carrier mother x normal father.
– A normal mother x haemophiliac father.
• Female haemophiliacs are rare. Show a cross
which produces a haemophiliac girl.
• Have a go at qu. 2 – 5 on p.125