Question 21M.2.SL.TZ2.5a
| Date | May 2021 | Marks available | [Maximum mark: 7] | Reference code | 21M.2.SL.TZ2.5a |
| Level | SL | Paper | 2 | Time zone | TZ2 |
| Command term | Explain | Question number | a | Adapted from | N/A |
Autosomal genes are located in chromosomes that are not sex chromosomes. The inheritance of autosomal genes is affected by whether the genes are linked or unlinked. Explain the two types of inheritance, using the example of parents that are heterozygous for two genes A and B.
[7]
a. unlinked genes are on different chromosomes / vice versa ✔
b. unlinked alleles migrate/segregate/are inherited independently (during meiosis) / vice versa ✔
c. (In unlinked inheritance) there is an equal chance for all 4 options to occur / AB, Ab, aB, ab / vice versa ✔
d. (dihybrid crosses involving) linked genes do not produce Mendelian ratios ✔
e. (excluding recombinants) there is a 1:1 chance of inheriting the different options/AB or ab ✔
f. in linked characteristics alleles might not migrate together if there is crossing over/ recombinants are formed ✔
g. crossing over occurs in prophase I of meiosis ✔
h. when the sister chromatids migrate in meiosis II the characteristics forming gametes are different/Ab, aB ✔
i. formation of recombinants causes changes in ratio/probability of inheritance/genetic variation ✔
j. correct named example of inheritance of linked/unlinked characteristics ✔
k. Punnett/paired diagrams of both unlinked and linked characteristics ✔
l. genes which are linked but are far apart on the chromosome can display independent assortment ✔
Mp a could be awarded from an annotated diagram.
Allow annotated diagram of inheritance / could be shown in a Punnett square.
Allow annotated diagram of linked inheritance for mp f.
For mp K, accept sex linked examples involving two genes.
If the student interprets the question as sex-linked and autosomal inheritance, look for WTTE marks from the scheme.
Question 5 was not a popular question choice. Few candidates performed well on this question overall. Many students wrote vaguely about dominant and recessive alleles or confused gene linkage with "sex-linkage" or even multiple alleles and blood groups.
