CAIE A-Level Biology 17 Selection and Evolution Question Bank

1 genetic / genotype / alleles

or

example ; mutation / crossing over / independent assortment / random fertilisation / random mating

2 environmental

or

example ; climate / disease / food availability / soil pH / selection pressure

3 combination / interaction, of genes and environment or

example ; gene (expression) is modified by environment

any three from:

1 formation of new species from, pre-existing species / common ancestor ;

2 change in, characteristics / phenotypes ;

3 over, time / generations ;

4 natural selection ;

5 changes in, gene pools / allele frequencies

or genetic drift ;

any three from:

1 compare sequences (of DNA) ;

2 the more similar the sequences the more, closely related the species / recent the common ancestor ; ora

3 over time mutations accumulate or less time fewer mutations occurred ;

4 ref. to bioinformatics / software / database ;

5 AVP ; e.g. molecular clock / constant rate of mutation

e.g. mtDNA has no crossing over e.g. the percentage similarity can be used to build a phylogenetic tree

Correct answer for question 3(a):

Total number of plants = 660 + 440 = 1100.

The red-flowered plants are homozygous recessive, so:

$$q^2 = \frac{440}{1100} = 0.4$$$$q = \sqrt{0.4} = 0.632455532$$p = 1 - q = 1 - 0.632455532 = 0.367544468

The heterozygote frequency is:

$$2pq = 2 \times 0.632455532 \times 0.367544468 = 0.4649110641$$

Number of heterozygous plants:

$$2pq \times 1100 = 0.4649110641 \times 1100 = 511.4$$

Nearest whole number: 511 heterozygous plants.

Marking guidance: award 3 marks for: calculating q^2 = 440/1100 = 0.4 and q = sqrt(0.4) = 0.632455532; calculating p = 1 - q = 0.367544468 and 2pq = 0.4649110641; multiplying 2pq by 1100 and giving 511 or 512 as the final whole-number answer, as accepted by the official markscheme.

any two from:

1 colours / shades / categories / phenotypes, not distinct / not discrete / overlap / form a (gradual) range / include intermediates;

2>1 / 4 / several / many, genes;

3 environment affects, it / colour / phenotype;