Inheritance

1. Inheritance Sex Linkage Visit www.worldofteaching.com For 100’s of free powerpoints

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: • 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.

5. Red-green colour blindness Parental Phenotypes Carrier Female x Normal Male Genotypes XNXn XNY Gametes Offspring 1 Genotypes XN Xn XN Y Phenotypes Normal Female : Carrier Female : Normal Male : Colour blind Male 1 : 1 : 1 : 1

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

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