Inheritance
Sex Linkage
What is Sex Linkage?
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In addition to their role in determining sex, the
sex chromosomes have genes for many
characters.
Genes located on a sex chromosome are
called sex linked genes.
In humans the term usually refers to X-linked
characters: genes located only on X
chromosomes.
Fathers can pass X-linked alleles to their
daughters, but not sons.
Mothers can pass sex-linked alleles to both
sons and daughters.
Recessive alleles
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If a sex linked trait is due to a recessive allele:
– A female will express the phenotype only if she is
homozygous recessive.
– If a male receives the recessive allele from his
mother he will express the phenotype.
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Far more males have disorders that are
inherited as sex linked recessives than
females.
 Examples:
Colour blindness
Haemophilia
Red-green colour blindness
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X chromosome has a locus for colour
vision with two alleles:
– XN = Normal colour vision
– Xn = Red-green colour blindness
Y chromosome does not have a colour
vision locus.
 If a male receives the Xn allele he will
have impaired colour vision, whereas a
female with XNXn will not.

Red-green colour blindness
Parental
Phenotypes
Genotypes
Gametes
Carrier Female x Normal Male
XNXn
XNY
XN
Xn
XN
Y
Female Gametes
Offspring 1
Genotypes
Male
Gametes
XN
Xn
XN
XN XN
XN Xn
Y
XN Y
Xn Y
Phenotypes
Normal Female : Carrier Female : Normal Male : Colour blind Male
1
:
1
:
1
:
1
Showing genetic pedigree
Unaffected female:
 Affected female:
 Unaffected male:
 Affected male:
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I
1
2
II
1
2
4
3
5
III
1
2
3
4
5
What now?
Complete WS5: Sex-Linked Genes
 Complete WS6: Extension questions
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