1. Inheritance Sex Linkage

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.

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

4. Red-green colour blindness X chromosome has a locus for colour vision with two alleles: –X N = Normal colour vision –X n = Red-green colour blindness Y chromosome does not have a colour vision locus. If a male receives the X n allele he will have impaired colour vision, whereas a female with X N X n will not.

5. Red-green colour blindness Parental Phenotypes Carrier Female x Normal Male Genotypes X N X n X N Y Gametes Offspring 1 Genotypes Phenotypes Normal Female : Carrier Female : Normal Male : Colour blind Male 1 : 1 : 1 : 1 XNXN XnXn XNXN Y Female Gametes XNXN XnXn Male Gametes XNXN X N X N X n YX N YX n Y

6. Showing genetic pedigree Unaffected female: Affected female: Unaffected male: Affected male: 12 1 2 3 4 5 1 2 3 4 5 I II III

7. What now? Complete WS5: Sex-Linked Genes Complete WS6: Extension questions

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