Biology 9700 · AS & A Level · Passage of information from parents to offspring

Passage of information from parents to offspring — practice question

Domestic goats, Capra hircus, display a very broad variety of coat patterns and colours. A single gene affecting coat colour and pattern has multiple alleles. Four of these alleles are: A, the allele for white, which is dominant over all the others; $A^{b}$, the allele for badgerface (stripes on face) and $A^{g}$, the allele for grey, which are codominant; and a, the allele for black, which is recessive to all the others.
(a)[2]

State the full range of possible genotypes for: a grey badgerface goat; a white goat.

(b)[4]

A cross involving a black goat and a white goat produced a white offspring. This white offspring was then crossed with a grey goat whose genotype was unknown. Use genetic diagrams to show every possible offspring genotype and phenotype that might arise from crossing the white offspring with the grey goat.

(c)[2]

In goat skin cells, the following events lead to black melanin pigment: melanocyte stimulating hormone (MSH) is released into the bloodstream; MSH attaches to the melanocortin 1 receptor on the cell surface membranes of skin cells; this leads to an increase in cyclic AMP inside the cell; the enzyme tyrosinase is switched on and melanin production begins. The white allele A codes for a protein that binds irreversibly to the melanocortin 1 receptor. Even when MSH is present in the blood, no rise in cyclic AMP occurs. Explain the appearance of goats with genotype $aa$.

Worked solution & mark scheme

This 8-mark question has a full step-by-step worked solution and mark scheme. One marking point: “genotype for grey badgerface goat $A^bA^g$” …

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