Using the table, state whether recessive, dominant and codominant alleles are expressed in heterozygous and homozygous genotypes by writing yes, no or both.

State two alleles in blood groups that are codominant.

Clouded leopards live in tropical rainforests of South-East Asia. The normal spots (brown with a black outline) are dominant and black spots are recessive. The trait is sex-linked. A male with black spots was crossed with a female with normal spots. She had four cubs, two males and two females. For each sex, one cub had normal spots and the other cub had black spots.

Deduce the genotype of the mother. Show your work in a Punnett grid.

Award [1] for each correct row.

(alleles for blood groups) A and B / I^A and I^B

a. a correct representation of the alleles eg X^N normal, Xⁿ dark and Y;
b. Punnett grid including four cubs’ correct genotypes, showing sex linkage;
c. female genotype X^N Xⁿ;
Do not allow ECF.
Accept other suitable alternatives for ^Nn on the X.

Some did not receive a mark for mentioning "both" for the codominant allele in the homozygous genotype.

Many of the candidates did not receive the mark for making reference to the blood groups instead of correctly identifying the alleles. Some put the correct letters and symbols but placed them next to each other so implying it was a genotype and lost the mark.

Many candidates drew a correct Punnett Square, for which they scored two marks but failed to show the genotype of the mother clearly. Some failed to use appropriate sex-linkage allele symbols. Many simply used uppercase and lowercase, yet did not use X and Y symbols. Y was also wrongly associated with a gene in many cases.