Define codominant allele, recessive allele, locus and sex linkage.

ABO blood groups are inherited from parents, but it is possible for a child to have a different blood group from either parent. Outline how this can happen using a Punnett grid.

Explain how males inherit hemophilia and how females can become carriers for the condition.

a. codominant allele: (pair of) alleles that both affect the phenotype when present in a heterozygote / both alleles are expressed;
b. recessive allele: an allele that produces its characteristic phenotype only when present in homozygous state / is expressed when dominant allele not present;
c. locus: the (particular) position of a gene on a chromosome/homologous chromosomes;
d. sex linkage: a gene located on a sex chromosome/X/Y/X or Y chromosome;

Example / annotated Punnett grid showing a cross between blood groups showing:
a. parental genotype (for example I^Ai and I^Bi therefore A and B phenotypes);
b. gametes of one parent; (shown in Punnett grid)
c. gametes of other parent; (shown in Punnett grid)
d. genotypes of offspring; (shown in Punnett grid)
e. phenotypes of offspring expressed as a ratio or possibly in the Punnett grid;
f. blood group different to parent shown and identified (ie ii is blood group O);
Award [4 max] if correct notation not used.
Award [2 max] if Punnett grid is not used.

NB Other possible crosses could be used as long as the offspring include one or more offspring of a different phenotype from either of the parents.

a. hemophilia is due to a recessive allele/is a recessive trait / X^H is normal allele and X^h is hemophilia allele;
b. hemophilia is sex linked;
c. allele/gene is on the X chromosome;
Reject disease/hemophilia carried on X chromosome.
d. (sex chromosomes in) females are XX while males are XY;
e. Y chromosomes do not have the allele/hemophiliac males are X^hY;
f. males inherit their X chromosome from their mother/do not pass the allele to sons;
g. males have only one copy so recessive trait/allele is not masked;
h. males have a 50 % chance of hemophilia/receiving the allele if mother is a carrier;
i. carrier is heterozygous for the gene/is X^HX^h;
j. dominant/normal allele masks the recessive allele (so clotting is normal);
k. females inherit one X chromosome from father and one from mother;
l. affected/hemophiliac males have carrier daughters;
m. hemophilia allele could have been inherited from either parent;
Accept the points above explained either in text or clearly using a Punnett grid or genetic diagram but not for simply reproducing an unlabelled Punnett grid or diagram without explanation.

Question 7 was by far the most popular questions and it was answered well by many.

Codominant allele, recessive allele and locus are all specifically defined in the guide so answers were only awarded credit when they closely matched the guide. For example, in locus allele was not accepted for gene. However, for sex linkage the guide offers no definition so trait and gene were equally accepted as being located on a sex chromosome.

Question 7 was by far the most popular questions and it was answered well by many.

ABO blood group inheritance seemed to be well understood. Through the use of Punnett grids with different examples, candidates were usually able to show how a child could have a different blood group from either parent. However answers were often penalized because of notation errors. This meant that in some cases Punnet grids seemed to show phenotypes rather than genotypes. 

Question 7 was by far the most popular questions and it was answered well by many.

This hemophilia question presented an organizational challenge to candidates. Often candidates had the knowledge to answer the question but struggled to connect meaningful statements to produce a coherent passage. Instead of splitting their answer into two sections (how males inherit hemophilia and how females can become carriers) candidates just tended to write on and on about hemophilia. Many carelessly said that hemophilia is carried on the X chromosome rather than the allele/gene for hemophilia.