Phenotype

In the syllabus and the exam

Phenotype is the observable, physical expression of an organism's genotype, and candidates are expected to state clearly that phenotype results from genotype interacting with the environment, not from genotype alone, using examples such as identical twins (the same genotype) developing different heights or body weights due to differences in diet or exercise. In monohybrid inheritance questions, candidates must consistently distinguish phenotype (the visible characteristic, described in words, such as 'tall' or 'purple flowers') from genotype (the underlying letter combination, such as Tt), since examiners frequently ask for one when the other has been given, and answers that provide a genotype where a phenotype was requested, or vice versa, receive no credit even if the biological reasoning is otherwise correct. A common error is assuming that phenotype alone reveals genotype with certainty; a dominant phenotype can result from either a homozygous dominant or a heterozygous genotype, so phenotype only narrows down the possibilities unless further evidence, such as the phenotypes of the parents or offspring, is available. Candidates should also be able to calculate and state a phenotype ratio from a completed genetic cross, expressing it in its simplest whole-number form (such as 3:1) rather than leaving it as raw counts out of the total number of offspring predicted by the cross. Candidates should also be careful when a question describes an intermediate phenotype, such as pink flowers from red and white parents, since this indicates incomplete dominance rather than one allele being fully dominant over the other.

Example

Two pea plants with different genotypes, TT and Tt, both show the same tall phenotype, because a single dominant allele is enough to produce the tall characteristic in either case.

Related terms

GenotypeDominant alleleRecessive allele

Source:SRC-DSKP-EN