GCSE Combined Science (AQA)

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Genetic Crosses & Probability

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Inheritance, Variation and Evolution Reproduction

Genetic Crosses & Probability

5:24 Genetic Crosses and Family Trees
Spec 4.6.1.6
Previous Homozygous & Heterozygous Next Sex Determination
  • Genetic crosses occur through sexual reproduction, creating new combinations of alleles.
  • Crossing chromosomes can result in different combinations of alleles.
  • Dominant alleles are represented by uppercase letters, while recessive alleles are shown with lowercase letters.
  • Punnet square diagrams are used to predict possible genotypes of offspring from two parents.
  • Example given with both parents heterozygous for a single gene (Aa x Aa).
  • Four possible combinations for offspring: AA, Aa, aA, aa.
  • Three possible genotypes for offspring: homozygous dominant (AA), heterozygous (Aa), and homozygous recessive (aa).
  • Each child has a probability of having one of these three genotypes.
  • Genotype ratios from a Punnet square can be expressed as 1:2:1 for AA:Aa:aa.
  • Probabilities can also be expressed as fractions or percentages: 25% AA, 50% Aa, 25% aa.
  • Understanding and interpreting Punnet squares is crucial for genetics exams.
  • Real-life example: calculating probabilities of eye color (brown dominant, blue recessive) using Punnet squares.
  • Parents with genotypes Bb (brown eyes, heterozygous) and bb (blue eyes, homozygous recessive) can produce offspring with a 50% chance of having blue eyes.

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