A-LevelBiologyInherited change9700/43 · May/June 2022 · Q711 Marks

(a) Epistasis occurs when a gene at one locus can affect the expression of a gene at another locus.

(a)Define the terms gene and locus. gene locus[2]
(b)Fur colour in mice, Mus musculus, is determined by a number of genes. One example is the result of epistatic interaction between two genes, A and B. • Allele A codes for the production of pigment in the fur. • Allele a does not code for the production of pigment and results in white fur (albino). • Allele B codes for the production of brown fur. • Allele b codes for the production of black fur. Construct a genetic diagram to show the results, including the ratio, of a cross between two mice heterozygous for both genes. parent genotypes parent phenotypes gametes ratio.........................................................[6]
(c)White fur is due to a mutation of the TYR gene. This is called albinism. Explain how a mutation of the TYR gene can result in albinism.[3]

Mark scheme

Marking points from the official mark scheme (May/June 2022). The mark or mark type is shown in brackets.

gene – a length of DNA / a sequence of DNA nucleotides, that codes for a particular, protein / polypeptide ;
locus – the position of a, gene / allele, on a chromosome ;
parent genotype (AaBb X AaBb)
parent phenotype brown (brown) ;
gametes AB Ab aB ab X (AB Ab aBab) ;
Punnett square (with correct genotypes) ;;
offspring phenotypes linked to genotypes ;
ratio - 9 brown : 3 black : 4 white;
any three from: 1 (base), substitution / deletion / insertion or frame shift;
2 ref. to change in, primary / secondary / tertiary, structure (of polypeptide / protein) or change in, 3D / active site, shape ;
3 ref. to stop codon;
4 (so) no / inactive, tyrosinase produced ;
5 tyrosine not converted to, DOPA / dopaquinone ;
6 melanin not formed;

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