A child inherits the allele for attached earlobes from both parents. The allele for attached earlobes is recessive. Explain why the child has attached earlobes.

Eduqas GCSE Biology — 2.3.2 Inheritance · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

In humans, earlobe shape is controlled by a single gene. The allele for free earlobes is dominant. The allele for attached earlobes is recessive.

Model answer (4 marks)

The child has two copies of the recessive allele (one from each parent)
The child is homozygous for the recessive allele
There is no dominant allele present to mask the recessive allele
Therefore the recessive characteristic (attached earlobes) is expressed in the phenotype

Examiner tips

Use the exact terms ‘recessive allele’, ‘homozygous’, ‘dominant allele’, ‘phenotype’
Show the logical sequence from genotype to phenotype
Keep each point concise – 4 marks, 1 point each

Common mistakes

Confusing dominant with recessive terminology
Failing to state that the child is homozygous
Not linking the absence of a dominant allele to expression of the trait

Mark scheme (4 marks)

The child has two copies of the recessive allele (one from each parent)
The child is homozygous for the recessive allele
There is no dominant allele present to mask the recessive allele
Therefore the recessive characteristic (attached earlobes) is expressed in the phenotype

Key terms in this question

allele · recessive