A-LevelBiologyInherited change9700/41 · Oct/Nov 2022 · Q78 Marks
(a) In some varieties of domestic cats the gene for fur colour is located on the X chromosome. This gene has two alleles. One allele codes for black fur and the other allele codes for ginger fur. The two alleles are codominant so a heterozygous cat will have fur with patches of black and ginger colours. A cat with fur of two colours is known as a tortoiseshell.
- (a)Using appropriate symbols, construct a genetic diagram to show the results of a cross between a female tortoiseshell cat and a male ginger cat. symbols parent phenotypes tortoiseshell female ginger male parent genotypes gametes offspring genotypes offspring phenotypes[5]
- (b)In humans the TYR gene is involved in the production of a dark pigment, melanin, in some cells. Describe how the expression of the TYR gene leads to the production of melanin.[3]
Mark scheme
Marking points from the official mark scheme (Oct/Nov 2022). The mark or mark type is shown in brackets.
- [5] (a) 1 symbols B / XB = (allele for) black (fur) G / XG = (allele for) ginger (fur) ; parents phenotypes tortoiseshell female ginger male 2 parents genotypes XBXG XGY ; 3 gametes XB XG XG Y ; 4 offspring genotypes XBXG XBY XGXG XGY ; 5 offspring phenotypes tortoiseshell female black male ginger female ginger male ;
- [3] (b) any three from: 1 TYR codes for tyrosinase ; 2 converts tyrosine into DOPA ; 3 converts DOPA into dopaquinone ; 4 dopaquinone converted to melanin ;
Write your answer and get it marked
On Oracle Prep you can write or photograph your own answer and have it marked by AI against this mark scheme, with feedback on what you missed.
Sign up free →