(a) In continuous variation, a population shows a range of phenotypes between two extremes with no distinct groups. Height and mass are examples of phenotypic traits that show continuous variation.

Mark scheme

Marking points from the official mark scheme (May/June 2021). The mark or mark type is shown in brackets.

1. [3] 1 different alleles at a single gene locus have small effects (on the phenotype) ; 2 different genes may have an, additive / combined, effect (on the phenotype) ; 3 ref. to multiple genes / polygenes / description ;
2. [2] any two from: 1 quantity / availability, of, food / nutrients; A competition for food 2 quality of food / malnutrition ; 3 disease; 4 temperature;
3. [4] any four from: 1 humans apply selection pressure ; 2 select parents that show desired characteristic ; R gene / allele 3 breed these parents together; R IVF ignore AI 4 select offspring with desired characteristics; 5 breed selected offspring; 6 repeating over many generations ; 7 AVP; e.g. some outbreeding required to avoid, inbreeding depression / combining harmful recessive alleles