Inheritance Sex Linkage

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.

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

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.

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

Showing genetic pedigree Unaffected female: Affected female: Unaffected male: Affected male: I II III

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

This powerpoint was kindly donated to is home to over a thousand powerpoints submitted by teachers. This is a completely free site and requires no registration. Please visit and I hope it will help in your teaching.