A-LevelBiologyInherited change9700/41 · May/June 2022 · Q78 Marks

(a) The fruit fly, Drosophila melanogaster, usually has red eyes. A gene for eye colour has four alleles: red, apricot, honey and white. Define the terms gene and allele.

(a)The fruit fly, Drosophila melanogaster, usually has red eyes. A gene for eye colour has four alleles: red, apricot, honey and white. Define the terms gene and allele. gene allele[2]
(b)• The allele for red eyes, CR, is dominant to the other three alleles. • The allele for apricot eyes, CA, is dominant to the allele for honey eyes, CH. • The allele for white eyes, CW, is recessive to the other three alleles. Construct a genetic diagram to show the genotypes, phenotypes and ratio of the offspring from a cross between a fruit fly with red eyes, CRCH, and a fruit fly with apricot eyes, CACW. parents phenotypes red eyes apricot eyes parents genotypes CRCH CACW gametes offspring genotypes offspring phenotypes ratio[3]
(c)Describe how you would carry out a test cross and use it to determine the genotype of a red-eyed fruit fly.[3]

Mark scheme

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

[1] (a) gene length of DNA coding for a polypeptide ;
[1] (a) allele alternative / different, form / version, of a gene;
[3] (b) gametes (CR CH CA CW) offspring genotypes CRCA CRCW CHCA CHCW; offspring phenotypes red eyes (red eyes) apricot eyes honey eyes; ratio 2 : 1 : 1 ;
[3] (c) 1 cross with, white-eyed fly / CWCW ; any two from: 2 if all red-eyed then CRCR ; 3 if (some red-eyed and) some apricot-eyed then CRCA; 4 if (some red-eyed and) some honey-eyed then CRCH; 5 if (some red-eyed and) some white-eyed then CRCW ;

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