Many different genes are involved in the production of pigments in mammals. One example is the TYR gene.

1. (a)(i)Homologous chromosomes have the same genes located at the same loci. State one other feature shared by homologous chromosomes.[1]
2. (a)(ii)Albinism in humans can be caused by recessive mutations of the TYR gene. Explain why a person who is homozygous recessive for the TYR gene shows albinism.[3]
3. (b)Fur colour in rabbits involves a number of different genes. Some of these genes interact. Gene 1 has two alleles, B and b. • The dominant allele, B, results in black fur. • The recessive allele, b, results in brown fur. Gene 2 has two alleles, F and f. • The dominant allele, F, codes for a protein that allows the expression of gene 1. • The recessive allele, f, codes for a protein that does not allow the expression of gene 1, resulting in white fur. The two genes are on different pairs of autosomes. Complete the genetic diagram for a cross between two black rabbits that are heterozygous for both genes and show the ratio of possible offspring phenotypes. parental phenotypes black X black parental genotypes BbFf X BbFf Punnett square ratio of offspring phenotypes:[4]

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