A recessive allele is only expressed if two copies are present (therefore…

The idea

A recessive allele is one whose effect on the phenotype is seen only when two copies are present, so no dominant allele is present. In a heterozygous organism, the dominant allele masks the recessive allele, and the recessive characteristic does not appear. The recessive phenotype therefore requires a homozygous recessive genotype, such as tt for short stems in pea plants. This happens because the recessive allele often codes for a non-functional protein, and one working dominant allele is enough to produce the dominant characteristic. Recessive alleles can be passed on by heterozygous carriers who do not show the characteristic, which is important when interpreting family trees and predicting the chance of offspring showing a recessive condition.

What earns the marks

• Define a recessive allele as one whose effect is expressed only when two copies are present.
• State that a recessive characteristic appears only in a homozygous recessive organism, with no dominant allele present.
• Explain that in a heterozygote the dominant allele masks the recessive allele, so the recessive characteristic is not seen.
• Use a genetic diagram to show that two heterozygous parents can produce homozygous recessive offspring.
• Apply the idea to a named example, such as short stems in pea plants or a recessive genetic condition.

Common mistakes and how to put them right

• Saying a recessive allele is never expressed.

  Put it right by stating that it is expressed when two copies are present.

• Assuming that if a recessive characteristic is absent the recessive allele is absent.

  Put it right by noting that heterozygous carriers can pass on the allele without showing the characteristic.

• Confusing a recessive allele with a rare allele.

  Put it right by explaining that recessive describes expression in the phenotype, not how common the allele is.

Exam tips

• Use the phrase two copies are needed when defining a recessive allele, and link this to the absence of a dominant allele.
• In a genetic diagram, show the gametes and combine them systematically so that homozygous recessive offspring are not missed.
• When interpreting a family tree, remember that unaffected parents can have an affected child if both are heterozygous carriers.