Distinguish between autosomes and sex chromosomes in humans.

Describe the inheritance of hemophilia including an example using a Punnett grid.

Explain how meiosis results in an effectively infinite genetic variety of gametes.

X and Y chromosomes determine sex;
females XX and males XY;
X chromosome is larger than / carries more genes than the Y chromosome;
22 types/pairs of autosomes;
males and females have same types of autosomes;

sex-linked / due to gene on the X chromosome;
more common in males who only receive one X chromosome;
female is hemophilic if homozygous recessive / homozygous recessive normally fatal;
X^H for dominant/normal allele and X^h for recessive/ hemophilia allele; (accept in Punnett grid/square)
example in Punnett grid/square with correct parental genotype and gametes;
correct genotypes of offspring;
correct phenotype ratio or percentage;

half the males are hemophilic and half of the females are carriers / OWTTE;

Allow marks for correct genotypes if the alleles are not shown superscript on an X, as long as the Y chromosome is indicated.
Do allow marking point d. if the letters for the dominant and recessive allele are not upper and lower case versions of the same letter.

one (homologous) chromosome is from the mother and one from the father;
homologous chromosomes pair (in prophase I);
crossing over/chiasma formation in prophase I;
recombination of linked genes / alleles/genes swapped;
many possible points of crossing over;
crossing over occurs at random positions;
due to crossing over the two chromatids of metaphase I chromosomes are not identical;
random orientation (of bivalents) in metaphase I;
in anaphase/at end of metaphase I chromosomes move to opposite poles;
independent assortment of chromosomes/genes;
2ⁿ/2²³ combinations (without considering crossing over);
four genetically different nuclei/gametes from each meiosis;
Accept any of the above points in a clearly annotated diagram.

A usual guideline for examiners is to have 50% more points on the mark scheme than raw marks in Section B questions. There were fewer points than that for part (a) of this question and only the strongest candidates found enough to say to reach a total of four. A point that was almost always missed was that males and females do not differ in the autosomes that they possess. This is a significant distinction between the sex chromosomes and autosomes.

For part (b), a small proportion of candidates forgot or did not know that hemophilia is a sex linked condition and so scored few marks here. Most candidates who did know that sex-linkage is involved used the expected notation of an upper case X to represent the X chromosome with superscript upper case and lower case letters to show the alleles. If an upper case Y is also shown, even though it does not carry a copy of the gene, it makes mistakes much less likely when working out possible outcomes from a cross between two parents. The most significant cross is one between an unaffected male and a carrier female as this is how almost all cases of hemophilia are derived. Most candidates showed this. Parental genotypes were often missing and gametes on the Punnett grid were usually shown but not labelled as gametes. The best answers showed the phenotypes of each possible type of offspring, together with the genotype on the Punnett grid. It was also useful to add a ratio or percentages below the grid. Candidates who showed a series of different crosses rarely scored any more marks after the first cross.

Part (c) is a standard question but even so, answers were very variable, probably because meiosis is complicated and there are multiple causes of genetic variety, which some candidates struggle to understand. Terminology was sometimes used rather loosely. The best candidates distinguished between random orientation of bivalents in metaphase I and independent assortment of genes due to random orientation or crossing over, depending on whether pairs of genes are on different or the same type of chromosome.