4.3 Theoretical genetics – summary of mark schemes
4.3.4
Describe ABO blood groups as an example of codominance and multiple alleles.
Mark Scheme
A.
Codominance is when alleles of gene / pairs of alleles which both affect the phenotype / both expressed
(when present together in an individual / in the heterozygote);
B. multiple alleles means a gene has three or more alleles / more than two alleles;
C. ABO blood groups / other named example of multiple alleles;
D. ABO gene has three alleles / equivalent for other example;
E. IA IB and i shown (at some point in the answer) / equivalent for other example;
F. any two of these alleles are present in an individual;
G. all six genotypes with phenotypes given (shown somewhere);
H. example / diagram of a cross involving all three alleles;