GCSE Combined Science (AQA)

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Dominant & Recessive Alleles

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Inheritance, Variation and Evolution Reproduction

Dominant & Recessive Alleles

3:14 Genetic Inheritance Terms
Spec 4.6.1.6
Previous Genotype Next Drawing Punnett Squares
  • Alleles can be either dominant or recessive.
  • The allele for brown eyes is represented by a capital B, and for blue eyes by a lowercase b.
  • Only discussing blue and brown eyes in this context, other eye colors like green are covered beyond GCSEs.
  • Dominant alleles are denoted with uppercase letters, while recessive alleles are with lowercase letters.
  • An organism's phenotype depends on the combination of alleles inherited.
  • Each allele for a gene is inherited from one parent: one from the mother and one from the father.
  • There are three possible combinations of alleles: two uppercase Bs (BB), two lowercase bs (bb), or one of each (Bb).
  • BB and bb are homozygous genotypes as they contain the same type of alleles.
  • Bb is a heterozygous genotype as it contains different alleles.
  • Dominant alleles are always expressed, even if only one copy is present.
  • The phenotype for BB (both uppercase) and Bb (one uppercase, one lowercase) is brown eyes due to the dominance of the B allele.
  • Blue eyes result from the bb genotype, where both alleles are recessive and no dominant allele is present.
  • Individuals with blue eyes are homozygous for the recessive allele.
  • For exams, it's crucial to understand the difference between dominant and recessive alleles and describe them accurately.
  • Homozygous dominant (BB) and homozygous recessive (bb) individuals have two copies of the same allele.
  • Heterozygous individuals (Bb) have one dominant and one recessive allele.
  • Understanding the difference between homozygous and heterozygous genotypes is important for exams when discussing allele combinations.

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