Sex Inheritance and Sex Linkage

Some basic notes on sex inheritance and sex linkage for A2 Biology (AQA.) These includes application to Haemophilia and referance to pedigree charts. 

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Sex Inheritance and Sex Linkage
Humans have 23 pairs of chromosomes, 22 of these are identical regardless f if
they are in a male or female. However, the last pair determines the sex of the
individual and these are called sex chromosomes. Woman have two x chromosomes
while men have one x and one y.
Sex Linkage ­Haemophilia
Genes carried on an x chromosome are said to be sex linked. However, the x
chromosome is a lot longer than the y chromosome and so the x chromosome
often then has no equivalent on the y chromosome and so many parts of the x
chromosome are expressed in the male as there is no contribution from the y
The x chromosome carries many genes, an example of which is a condition called
haemophilia. Since there has been selective removal of this gene it is now a rare
occurrence, but mainly occurs amongst men, although female haemophiliacs are
One of the causes of haemophilia is a recessive allele that alters DNA nucleotides
so they cannot produce a protein required for the blood clotting process. The
presence of this protein in donated blood means that haemophiliacs can live a
near-normal life. Alleles for the non/production of blood clotting proteins are always
attached to the x chromosome, since the y chromosome provides no alternative
allele, males often inherit haemophilia from their carrier mothers.
If a mother does not suffer the disease she may be a carrier in that she has a
heterozygous pair for the character. Since males only pass the y chromosome to
their sons, they cannot pass haemophilia onto them, but since they pass their x
chromosome to their daughters they can pass haemophilia onto them and they
would become carriers.
Pedigree Charts

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Pedegree charts are a useful way to trace the inheritance of sex linked
characteristics.…read more


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