Guinea pigs, Cavia porcellus, vary in the length and colour of their fur. Fig. 1.1 shows a guinea pig with short black fur. [Figure 1.1] Two genes that determine the length and colour of the fur occur at the A/a locus and the B/b locus. These two gene loci are on separate autosomal chromosomes. • The allele A results in short fur. • The allele a results in long fur. • A is dominant to a. • The allele B results in black fur. • The allele b results in chocolate fur. • B is dominant to b.

1. (a)(i)List all the possible genotypes of a guinea pig with short black fur.[2]
2. (a)(ii)A test cross could be used to determine the genotype of a female guinea pig with short black fur. Describe the phenotype of the male guinea pig that could be used to carry out this test cross.[1]
3. (b)A black guinea pig with long fur that was homozygous at both loci was crossed with a chocolate guinea pig with short fur that was homozygous at both loci. The F1 offspring of this cross had short black fur. F1 offspring were mated together to produce the F2 offspring. Complete the Punnett square to: : show the cross between the F1 offspring predict the F2 offspring genotypes. You should include the gametes in your answer. State the ratio of F2 offspring phenotypes. You should include a key to link phenotypes to genotypes.[4]
4. (c)Some genes in guinea pigs are structural genes and some are regulatory genes. Describe the difference between a structural gene and a regulatory gene.[2]

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