| Date | November 2009 | Marks available | 3 | Reference code | 09N.2.HL.TZ0.3 |
| Level | Higher level | Paper | Paper 2 | Time zone | TZ0 |
| Command term | Explain | Question number | 3 | Adapted from | N/A |
Question
In the red squirrel (Tamiasciurus hudsonicus), the allele for grey fur colour (G) is dominant to the allele for red fur colour (g) and the allele for a fluffy tail (F) is dominant to hairless tail (f).
The genes described above form a linkage group. Define linkage group.
A cross is made between squirrels of the following genotypes.
Using a similar format, identify the genotypes of offspring which are recombinants.
Explain how the recombinants are formed during meiosis.
Explain the role of transfer RNA (tRNA) in the process of translation.
Markscheme
genes that are located on the same chromosome (form a linkage group)
Award [1 max] if the candidate does not use the same format, but gives the correct letters Ggff and ggFf.
(recombination) occurs in prophase 1 of meiosis;
homologous chromosomes come together in pairs;
chiasmata form between the (non-sister) chromatids;
chromosomes exchange segments / crossing over takes place;
tRNA attaches to (specific) amino acid;
tRNA (with amino acid) moves to the ribosome;
anticodon of tRNA binds with codon of mRNA;
Examiners report
This was surprisingly poorly known, with only the better candidates being able to state that the genes are located on the same chromosome.
As was shown in part c, the candidates seemed to know the theory of crossing over and recombination. However part b showed that very few really understood the products. In addition, in spite of comments from last year's report, the format used in the syllabus and the instruction of “using a similar format”, most did not.
This was well answered, the theory was known.
Weaker candidates seemed to recall that tRNA had something to do with ribosomes, amino acids and protein synthesis, but were unable to explain any further and gain some marks.
