Alleles

Chromosomes and Genes

• Humans have 23 pairs of chromosomes → 46 total.

• One chromosome in the pair is from mother, one from father.

• Genes = sections of DNA that control characteristics.

• Most traits = controlled by many genes, but some (e.g. earwax type) by single genes.

Key Definitions

• Allele = version of a gene (e.g. blue, brown or green eyes).

• Genotype = alleles an organism has (e.g. EE, Ee, ee).

• Phenotype = physical characteristic shown (caused by genotype).

• Homozygous = two identical alleles (e.g. EE or ee).

• Heterozygous = two different alleles (e.g. Ee).

Dominant & Recessive Alleles

• Dominant allele = shows in phenotype if one or two copies present (e.g. E for wet earwax).

• Recessive allele = only shows in phenotype if two copies present (e.g. ee for dry earwax).

Example: Earwax Gene

• Alleles:

  • E = wet earwax (dominant).

  • e = dry earwax (recessive).

• Genotype → Phenotype:

  • EE → wet (homozygous dominant).

  • Ee → wet (heterozygous, dominant shows).

  • ee → dry (homozygous recessive).

✅ Exam Tip:

• Be ready to define all key terms (allele, genotype, phenotype, homozygous, heterozygous).

• Always state which allele is dominant when explaining inheritance.