Question 21M.2.HL.TZ1.7
| Date | May 2021 | Marks available | [Maximum mark: 15] | Reference code | 21M.2.HL.TZ1.7 |
| Level | HL | Paper | 2 | Time zone | TZ1 |
| Command term | Explain, Outline | Question number | 7 | Adapted from | N/A |
Variation in genetically inherited characteristics is the basis for evolution.
Outline the inheritance of blood types in the ABO blood system in humans.
[4]
a. i and IA and IB are alleles (of the blood group gene) ✔
b. IA is dominant and i is recessive / IB is dominant and i is recessive ✔
c. Group O (only) with ii and Group A with IAi or IAIA / Group B with IBi or IBIB
d. IA and IB are co-dominant so Group AB with IAIB ✔
e. one allele/copy of the gene inherited from each parent/Punnett square showing this ✔
Disallow mpa if allele notation is incorrect (such as using different letters for different alleles) but allow other mps.
The notation Ii can be accepted instead of i.
Reject blood groups or types being dominant/recessive – it should be alleles.
For a well-prepared candidate this was a very routine exercise, but many candidates were not so prepared. A common fault was to confuse blood groups with the alleles that determine the groups, either in notation for alleles, or overall. Commonly, answers did not distinguish between gene and allele. Some candidates focused too much on the consequences of blood groups (including rhesus +ve/−ve) for blood transfusion which was not relevant to the question.
Explain how genetic variation between the individuals in a species can be generated.
[7]
a. mutation (in genes/DNA generates variation)
b. base substitution / change to base sequence of gene / makes single nucleotide polymorphisms /SNPs ✔
c. new alleles formed / different alleles of gene / multiple alleles ✔
d. radiation/mutagenic chemicals/mutagens cause/increase the chance of mutation ✔
e. meiosis (generates variation) ✔
f. recombination/new combinations of genes/alleles produced by crossing over ✔
g. independent assortment/random orientation of (pairs of homologous) chromosomes/bivalents ✔
h. gametes/chromosomes/DNA/genes from two parents combined (in sexual reproduction)
i. random fertilization (increases genetic variation) ✔
j. in reproductively/geographically isolated populations natural selection may differ ✔
k. in small/isolated populations gene pools change/evolution occurs due to natural selection/genetic drift ✔
l. disruptive selection can cause different varieties/variants/types/phenotypes to diverge ✔ transfer of genes (between bacteria) in plasmids ✔
For mpf it is not enough just to state ‘crossing over’.
Do not accept types of mutation other than substitution for mpb.
More marks were scored here, with most candidates at least mentioning mutation and some of the events of meiosis. As this was an explain question it was not enough merely to name a process, such as crossing over, some indication of how it generates variation was expected. A common misunderstanding was to think that geographic isolation in itself generates variation.
Outline the use of analogous and homologous traits in natural classification.
[4]
Analogous
a. different evolutionary origin/do not share (recent) common ancestor (despite similarities of function) ✔
b. arise by convergent evolution ✔
c. classification based on analogous traits brings together dissimilar species/is artificial ✔
Homologous
d. similar (internal) structures/pentadactyl limb/other example of homologous structures due to common ancestry ✔
e. different uses/functions ✔
f. arise by adaptive radiation/divergent evolution ✔
g. (natural) classification is based on homologous traits (not analogous) ✔
h. classification based on homologous traits has predictive values/matches evolutionary history ✔
This was well-known by stronger candidates. Weaker candidates tended to omit significant ideas or get analogous and homologous traits muddled up.