Question
Variation in genetically inherited characteristics is the basis for evolution.
a. Outline the inheritance of blood types in the ABO blood system in humans. [4]
b. Explain how genetic variation between the individuals in a species can be generated. [7]
c. Outline the use of analogous and homologous traits in natural classification. [4]
▶️Answer/Explanation
Markscheme
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.
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.
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 ✔
Question
(a) Describe what is shown in a cladogram [2]
(b) Outline how variation in organisms of the same species could lead to natural selection [3]
Answer/Explanation
Ans :
a
a similarities/differences between organisms/species/clades
b «probable» evolutionary relationships/closeness/common ancestry/phylogeny
c divergence/splits/speciation/branches/nodes
d relative similarity/differences between base sequence/amino acid sequence
mpa and mpd concern actual characteristics, not evolutionary relationships. mpb concerns such relationships mpc concerns the structure branching of the cladogram
b
a survival of the better adapted/fittest
b more reproduction of better adapted/fittest/individuals with favorable variations
c genes for favorable variations/adaptations passed on to offspring Accept answers in the converse.
d competition for resources/more offspring produced than the environment can support/a struggle for existence