Alleles (Edexcel GCSE Biology): Revision Notes

Alleles

• All genes exist in versions called alleles (represented by letters in genetic diagrams)

• There is always 2 versions (alleles) of every gene – one of each chromosome in your body

  
  • If both alleles are the same, the individual is homozygous for that gene.
  • If the two alleles are different, the individual is heterozygous.

Genotype and Phenotype

• The genotype is the combination of alleles an individual has for a particular gene. It determines the phenotype, which is the physical expression of the trait (such as eye colour). Examples:

  • Individual A: One allele for blue eyes and one allele for brown eyes (heterozygous). Since brown is dominant, this person has brown eyes.
  • Individual B: Two brown eye alleles (homozygous dominant). This person has brown eyes.
  • Individual C: Two blue eye alleles (homozygous recessive). This person has blue eyes.

Dominant and Recessive Alleles

• A dominant allele is always expressed, even if only one copy is present. Dominant alleles are represented by a capital letter (e.g., A for brown eyes). If a person has one or two dominant alleles, they will show the dominant trait (e.g., brown eyes).
• A recessive allele is only expressed if two copies are present, meaning there is no dominant allele. Recessive alleles are represented by a lowercase letter (e.g., a for blue eyes). For a person to have blue eyes, they must inherit two recessive alleles (aa).
• Your genotype is the combination of alleles
• Your phenotype is determined by what alleles make up for your characteristic

Dominant characteristic (CAPITAL):

2 dominant alleles (CC) Or one recessive and one dominant (Cc)

Recessive characteristic (LOWER CASE):

2 recessive alleles (cc)

Inheritance of Characteristics

Most traits are not determined by a single gene but by the interaction of multiple genes. These interactions can result in a wide variety of phenotypes beyond simple dominant and recessive patterns.