Alleles
Biology: AQA GCSE Higher, topic 6.1.15
Alleles
Alleles are different forms of the same gene. Humans have pairs of every gene and in one gene, each half of the pair may have different alleles. People's characteristics are determined by the alleles that they have. Alleles can either be dominant or recessive:
Dominant Alleles
- A dominant allele is always expressed, regardless of the identity of the other allele.
- It only needs one copy present to be expressed (BB or Bb).
- It is represented by a capital letter, e.g. B.
- If B is the allele for brown eyes:
- When a person has a copy of the B allele, they will have brown eyes, no matter what other allele is present.
Recessive Alleles
- A recessive allele is only expressed if the other allele is also recessive.
- It is represented by a lowercase letter e.g. b. It needs two copies to be present to be expressed (bb).
- If b is the allele for blue eyes:
- A person can only have blue eyes if both of their alleles are b.
Test yourself on Alleles
In a genetic cross, B is the allele for brown eyes and b is the allele for blue eyes. Which allele is dominant?
A person with genotype BB and a person with genotype Bb both have brown eyes. Why do these two different genotypes produce the same phenotype?