Explain monohybrid inheritance using genetic diagrams, Punnett squares and…

The idea

Monohybrid inheritance follows one gene with two alleles. To explain it, set out the parents' genotypes, identify the gametes each can produce, and combine them in a Punnett square. For Bb × Bb, each parent gives B or b, so the offspring genotypes are BB, Bb, Bb and bb — a 1:2:1 ratio, giving a 3:1 dominant-to-recessive phenotype ratio. Family pedigrees show the same logic across generations: shaded symbols show affected individuals, and unaffected parents with an affected child prove both parents carry the recessive allele. Use diagrams to predict ratios and to deduce unknown genotypes.

What earns the marks

• State parental genotypes and the gametes each parent can produce.
• Construct a Punnett square with parental gametes along the top and side and combine alleles in the cells.
• Read offspring genotypes from the square and give the genotype ratio, e.g. 1 BB : 2 Bb : 1 bb.
• Convert genotype ratio to phenotype ratio, e.g. 3 dominant : 1 recessive for Bb × Bb.
• Use a family pedigree to deduce genotypes: unaffected parents with an affected child must both be heterozygous for a recessive allele.
• Explain that a recessive phenotype requires two recessive alleles, while a dominant phenotype needs at least one dominant allele.

Common mistakes and how to put them right

• Putting whole genotypes rather than single gametes along the top and side of a Punnett square.

  Put it right: Each gamete carries one allele.

• Giving the genotype ratio as the phenotype ratio.

  Put it right: Translate 1:2:1 genotypes into 3:1 phenotypes when dominance applies.

• Assuming an unaffected parent in a pedigree is homozygous.

  Put it right: An affected child from unaffected parents shows both parents are carriers.

• Forgetting that a recessive characteristic appears only when both alleles are recessive.

Exam tips

• Always write the parental genotypes and gametes before drawing the square.
• Label the square clearly and state both genotype and phenotype ratios.
• In pedigree questions, work backwards from the affected individual to deduce carrier parents.