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Inheritance and Punnett squares
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Punnett squares let you predict the chance of a child inheriting a characteristic. They are a favourite exam question.
Key words
Alleles are different versions of the same gene.
Dominant alleles (capital letter, B) show if present. Recessive alleles (small letter, b) show only if there are two.
Homozygous: two the same (BB or bb). Heterozygous: two different (Bb).
Genotype is the alleles; phenotype is the characteristic you see.
Dominant alleles (capital letter, B) show if present. Recessive alleles (small letter, b) show only if there are two.
Homozygous: two the same (BB or bb). Heterozygous: two different (Bb).
Genotype is the alleles; phenotype is the characteristic you see.
Punnett squares
Put one parent's alleles across the top and the other's down the side. Fill each box.
Two heterozygous parents, Bb × Bb:
Two heterozygous parents, Bb × Bb:
Sex determination
Females are XX and males are XY.
A Punnett square of XX × XY gives 2 XX and 2 XY, so there is a 50% chance of each sex.
A Punnett square of XX × XY gives 2 XX and 2 XY, so there is a 50% chance of each sex.
Brown eyes (B) are dominant to blue (b). A Bb parent and a bb parent have a child. What is the probability the child has blue eyes?
- Bb across the top, bb down the side.
- Boxes: Bb, Bb, bb, bb.
- 2 out of 4 are bb, so blue eyes.
- 2 ÷ 4 = 0.5, which is 50%.
Answer: 50%
Dominant alleles show with one copy, recessive need two. Use a Punnett square to find the outcomes, then count boxes to find the probability.
The interactive lesson includes the diagrams for this topic.
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