Sex Linkage

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Slide 1

Inheritance

Sex Linkage

Slide 2

What is Sex Linkage?

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.

Slide 3

Recessive alleles

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.

Far more males have disorders that are inherited as sex linked recessives than females.

Examples: Colour blindness

Haemophilia

Slide 4

Red-green colour blindness

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.

Slide 5

Red-green colour blindness

Parental

Phenotypes Carrier Female x Normal Male

Genotypes XNXn XNY

Gametes

Offspring 1

Genotypes

Phenotypes

Normal Female : Carrier Female : Normal Male : Colour blind Male

1 : 1 : 1 : 1

XN

Xn

XN

Y

Slide 6

Showing genetic pedigree

Unaffected female:

Affected female:

Unaffected male:

Affected male:

1

2

1

2

3

4

5

1

2

3

4

5

I

II

III

Slide 7

What now?

Complete WS5: Sex-Linked Genes

Complete WS6: Extension questions