A-LevelBiologyInherited change9700/41 · May/June 2021 · Q79 Marks

People vary in which molecules are present on the surface of their red blood cells. For example, in the ABO blood group system, people may be categorised into one of four blood groups: A, B, AB and O. A single gene, I, with three alleles, determines these blood groups. Gene I is an example of a gene with multiple alleles. • Alleles IA and IB are codominant. • Allele I⁰ is recessive to alleles IA and IB.

(a)Explain what is meant by codominant and recessive. codominant recessive[3]
(b)Table 7.1 shows a cross between a person with blood group A and a person with blood group B. Complete Table 7.1. [Table 7.1][3]
(c)Haemophilia is a blood condition where bleeding continues for a long period of time after a blood vessel has been damaged. A person with haemophilia has a mutation in a gene on the X chromosome, which results in the lack of a blood clotting factor. Explain why a man who has haemophilia cannot pass the condition to his male offspring.[3]

Mark scheme

Marking points from the official mark scheme (May/June 2021). The mark or mark type is shown in brackets.

[1] codominant - both (alleles), affect / seen in / expressed in / contribute to (phenotype) ;
[1] codominant - in heterozygote ;
[1] recessive - affects / seen in / expressed / contributes to, (phenotype) (only) when dominant (allele) absent / in homozygote ;
[3] parental phenotype A B parental genotype IA I° IB I° gametes IA I° IB I° offspring genotype IA IB IA IO IB 10 I° I° ; offspring phenotype AB A B O ;
[1] males / men / boys, are, XY / have (only) one X chromosome ;
[1] man/father, passes Y to, male offspring / son;
[1] man / father, does not pass X to, ♂ offspring / son ;
[1] man / father, passes, X / mutation, to, female offspring / daughter;
[1] woman/mother, passes X / mutation, to, male offspring / son ;

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