Question 21M.1A.HL.TZ1.36
| Date | May 2021 | Marks available | [Maximum mark: 1] | Reference code | 21M.1A.HL.TZ1.36 |
| Level | HL | Paper | 1A | Time zone | TZ1 |
| Command term | Question number | 36 | Adapted from | N/A |
In fruit flies (Drosophila melanogaster), grey bodies (b+) are dominant to black bodies (b) and normal wings (vg+) are dominant to vestigial wings (vg). Homozygous vestigial winged, black bodied flies were crossed with individuals that were heterozygous for both traits. 2300 individuals were counted and the phenotypes observed were recorded as shown.
965 normal wings, grey bodies
944 vestigial wings, black bodies
206 vestigial wings, grey bodies
185 normal wings, black bodies
Which statement is valid?
A. The predicted phenotypic ratio was 9:3:3: 1.
B. There is independent assortment of wings but not body colour.
C. The expected number of vestigial winged, grey bodied flies was 575.
D. The traits are on different chromosomes.
[1]
C
This proved to be the third most difficult question, with all of the four choices attracting some candidates. Perhaps the 1:1:1:1 ratio from a dihybrid test cross was unfamiliar. It was not a quick question to answer and was close to the end of the exam, but the number of questions left unanswered by candidates (blanks) did not rise with the last few questions, suggesting that lack of time was not the problem.
