| Date | May 2009 | Marks available | 1 | Reference code | 09M.2.HL.TZ1.2 |
| Level | Higher level | Paper | Paper 2 | Time zone | Time zone 1 |
| Command term | State | Question number | 2 | Adapted from | N/A |
Question
In some maize plants the seed is enclosed in a green sheath called a tunica. The allele (T) for this is dominant to the allele (t) for normal, unenclosed seeds. The endosperm of the seed can be starchy (allele E) or sugary (allele e). The genes for these two characteristics are linked. The table below shows the outcome of crosses between a plant heterozygous for both characteristics and one that is homozygous recessive for both characteristics.
State the genotype of the heterozygous parent using the correct notation.
Identify which individuals are recombinants in this cross.
Explain what has occurred to cause these results.
Maize belongs to the group of plants known as angiospermophyta. Distinguish between angiospermophytes and bryophytes.
Markscheme
unenclosed seeds, starchy and tunica present, sugary /
crossing over;
between non-sister chromatids (in prophase I);
results in exchange of alleles / change in linkage groups;
so some gametes are T e or t E; (linkage notation not expected)
test cross expect ratio of two phenotypes / correct Punnett Square showing test cross;
but instead get four phenotypes with smaller percentage of recombinants;
Above points can be shown in diagrams.
Examiners report
Most candidates gave the heterozygous genotype but could not express it using the correct notation.
Candidates were generally able to identify the recombinants, but explanation of the cause was often incorrectly attributed to independent assortment.
Some mentioned crossing over, but could not accurately describe what this involves.
The structure of answers was not always what was expected in response to the command term; i.e., a list of features of one group was followed by a list of features of a second group. Some candidates accurately described the characteristics of one group but did not distinguish them from the other group. Lack of familiarity with terminology such as rhizoids etc. was common. Bryophytes were commonly equated with gymnosperms and pteridophytes.
