Genetic inheritance: phenotype.

The idea

The phenotype is the observable characteristic an organism shows, produced by the interaction between its genotype (the alleles it carries) and its environment. For example, a mouse with genotype BB or Bb for fur colour may appear black, while bb gives brown fur; the black or brown coat is the phenotype. You work out phenotype by identifying the alleles present, deciding which are dominant or recessive, then stating the visible feature. Environmental factors such as diet or sunlight can also affect phenotype, so identical genotypes need not always look identical. In exams, you may be asked to state a phenotype from a genetic cross, or to explain why two organisms with the same genotype can differ.

What earns the marks

• Phenotype is the observable or expressed characteristic of an organism, such as fur colour, eye colour or flower colour.
• Phenotype results from the genotype interacting with the environment, not from genotype alone.
• Dominant alleles are expressed in the phenotype even when only one copy is present; recessive alleles are expressed only when two copies are present.
• A phenotype can be predicted from a genetic cross by combining parental gametes and interpreting the resulting allele pairs.
• Organisms with the same phenotype may have different genotypes, for example BB and Bb both giving black fur.
• Environmental effects such as nutrition, temperature or light can modify the phenotype without changing the genotype.

Common mistakes and how to put them right

• Confusing phenotype with genotype.

  Put it right: State the characteristic, such as black fur, not Bb.

• Assuming a dominant phenotype always means two dominant alleles.

  Put it right: A dominant phenotype can arise from a homozygous dominant or a heterozygous genotype.

• Ignoring environmental influence: the correction is to recognise that phenotype is genotype plus environment, so identical twins or clones can differ in appearance.

Exam tips

• Underline the command word: state means give the feature, explain means link genotype and environment to the feature.
• When interpreting a Punnett square, write the phenotype beside each genotype so you do not lose the visible-characteristic mark.
• Use the phrase observable characteristic when defining phenotype, and give one named example to secure the definition.