AQA Biopsychology Practice Exam — Prep, Practice Test & Study Guide

Recessive alleles express their traits when two copies of the recessive allele are present. This means that for a recessive trait to be observed in an individual, the individual must inherit one recessive allele from each parent. In the absence of a dominant allele, which can mask the effect of the recessive allele, the characteristics associated with the recessive allele can be fully expressed.

In genetics, alleles occur in pairs for each gene, where one allele is inherited from each parent. A dominant allele can overshadow a recessive allele in a heterozygous pair (one dominant and one recessive allele), resulting in the dominant trait being phenotypically expressed instead. Only when an individual has two recessive alleles (homozygous recessive) will the traits dictated by those alleles manifest outwardly.

Understanding this is critical for grasping basic principles of inheritance and the role of alleles in determining phenotypic traits.