O-LevelBiologyGenetics & Variation5090/21 · Oct/Nov 2019 · Q49 Marks
Some people find that certain vegetables, such as Brussels sprouts, taste bitter and are unpleasant to eat. Scientists believe that one dominant allele (T) of a particular gene gives people the ability to detect the bitter taste. Two parents find that Brussels sprouts taste bitter. In the space below, draw a genetic diagram to show how these parents can have a child who does not find that Brussels sprouts taste bitter.
- (a)In the space below, draw a genetic diagram to show how these parents can have a child who does not find that Brussels sprouts taste bitter.[5]
- (b)Genes are also important in the clotting of blood. Blood will clot normally if a person possesses at least one dominant allele (B). This gene is carried only on the X chromosome. Suggest why more males than females are likely to suffer from a disease in which blood does not clot normally.[4]
Mark scheme
Marking points from the official mark scheme (Oct/Nov 2019). The mark or mark type is shown in brackets.
- [1] Tt x Tt;
- [1] all four gametes correctly shown (T, t, T, t,);
- [1] diagram labelled with parents / gametes / offspring;
- [1] four genotypes shown correctly (TT, Tt, Tt, tt);
- [1] identification of tt as child who doesn't find sprouts bitter ;
- [1] males are XY;
- [1] females are XX ;
- [1] idea that males only one allele / B / b AW ;
- [1] idea that females have two alleles/ B / b AW;
- [1] greater chance that males will not have the dominant allele (B) ORA ;
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