The alleles present, or genotype, operate at a molecular level to develop…

The idea

Alleles are versions of a gene, and the combination an organism carries is its genotype. At a molecular level, a gene is a length of DNA whose base sequence codes for a polypeptide; the polypeptide may be an enzyme or a structural protein. The allele present determines which polypeptide is made, and that protein produces the characteristic. For example, a dominant allele may code for a functional enzyme that makes a pigment, while a recessive allele may code for a non-functional enzyme, so no pigment is made. The observable feature, such as eye colour or flower colour, is the phenotype. Phenotype results from the proteins produced, and the environment can also influence it. So genotype determines the proteins made, and those proteins develop the phenotype.

What earns the marks

• Define genotype as the combination of alleles an organism carries for a gene or genes.
• State that a gene is a section of DNA that codes for a polypeptide, and that different alleles have different base sequences.
• Explain that the allele present determines which polypeptide is synthesised, for example a functional enzyme or a structural protein.
• Link the protein produced to the characteristic, such as an enzyme making a pigment that gives flower colour.
• Distinguish phenotype as the observable characteristic, and note that it develops from the proteins coded for by the genotype.
• Recognise that the environment can also affect the phenotype, so identical genotypes may not always look identical.

Common mistakes and how to put them right

• Writing that alleles directly 'are' the characteristic, rather than coding for proteins that produce it.

  Put it right by stating that alleles code for polypeptides, and proteins produce the characteristic.

• Confusing genotype with phenotype.

  Put it right by defining genotype as the alleles present and phenotype as the observable feature.

• Claiming that a gene codes for a whole characteristic.

  Put it right by saying a gene codes for a polypeptide, and many characteristics involve several genes and the environment.

Exam tips

• Use the chain 'allele → polypeptide → characteristic' in explanations to show the molecular link clearly.
• When asked to explain a phenotype, name the protein involved, such as an enzyme or pigment, rather than only naming the allele.
• If a question gives a genotype, work out the proteins likely to be made before predicting the phenotype.