Inheritance Sex Linkage What is Sex Linkage n

Inheritance Sex Linkage

What is Sex Linkage? n n n 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 n 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. n Examples: Colour blindness Haemophilia n

Red-green colour blindness n. X chromosome has a locus for colour vision with two alleles: – XN = Normal colour vision – Xn = Red-green colour blindness n. Y chromosome does not have a colour vision locus. n 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 Gametes Carrier Female x Normal Male X NX n X NY XN Xn XN Y Female Gametes Offspring 1 Genotypes Male Gametes XN Xn XN XN Xn Y XN Y Xn Y Phenotypes Normal Female : Carrier Female : Normal Male : Colour blind Male 1 : 1 : 1

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

What now? n Complete WS 5: Sex-Linked Genes n Complete WS 6: Extension questions

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