Explain why carriers of sex-linked (X-linked) genes must be heterozygous.

Label the diagram below which shows a basic gene transfer.

I. .............................................................
II. .............................................................
III. .............................................................
IV. .............................................................

State two general types of enzymes used in gene transfer.

carrier has (one copy of) a recessive allele;
must also have a dominant allele to prevent having the condition/disease;
or
cannot be homozygous dominant or they would not carry the recessive allele;
cannot be homozygous recessive or they would have the condition/disease;

Award [1] for every two correct answers.
I. bacterial cell/bacterium/prokaryote;
II. plasmid;
III. inserted/engineered/cloned/desired DNA/gene / DNA/gene from donor cell;
IV. genetically modified/transformed/GM/recombinant organism/cell/ bacterium/host cell containing recombinant plasmid;

restriction enzymes / endonucleases;
ligases;
reverse transcriptase;
Award [1] for two correct answers.

There was a significant challenge here in explaining why carriers of sex-linked genes must be heterozygous. The inclusion of sex-linkage in this question was something of a “red herring” to use an English idiom, as the answer to the question would have been the same for a gene that was not sex linked. It was not possible to explain the answer effectively without referring to recessive and dominant alleles.  
Some candidates referred to genes rather than alleles, but as long as the meaning of the answer was clear, marks were awarded. Understandably some candidates took more notice of sex-linkage than was necessary and in some cases did little more than explain gender determination.  

Approximately equal numbers of candidates scored two, one and no marks here. Many could recognise the bacterial cell (I) and the plasmid within it (II) but fewer were able to give acceptable names for the gene transferred from the eukaryotic cell or the genetically modified bacterium.

This generally well answered with most candidates naming two enzymes involved in gene transfer. Restriction enzymes and DNA ligase were the most obvious ones but reverse transcriptase was also accepted as it is used to produce DNA copies of genes from mRNA. DNA polymerase and RNA polymerase were the commonest answers that were not accepted.