Genetic inheritance: genotype

The idea

Genotype is the combination of alleles an organism carries for a particular gene or set of genes. Alleles are alternative versions of a gene, and because body cells are diploid, each gene is represented by two alleles, one inherited from each parent. The genotype is written as a pair of letters, such as BB, Bb or bb, where a capital letter represents the dominant allele and the matching lower-case letter represents the recessive allele. The genotype determines the phenotype, which is the observable characteristic, although the phenotype also depends on the environment. Students use genotypes in Punnett squares and family trees to predict the proportions of offspring with each characteristic.

What earns the marks

• Defines genotype as the alleles present for a gene, for example BB, Bb or bb.
• Uses the convention of a capital letter for the dominant allele and the matching lower-case letter for the recessive allele.
• Distinguishes genotype from phenotype, where phenotype is the observable characteristic.
• Applies genotypes to genetic crosses, gametes and predicted offspring ratios.
• Recognises that genotype is inherited from both parents, with one allele from each gamete.
• Explains that phenotype results from genotype and environmental influences.

Common mistakes and how to put them right

• Confusing genotype with phenotype.

  Put it right: Genotype is the allele combination, while phenotype is the observable feature.

• Writing only one allele for a diploid organism.

  Put it right: Body cells carry two alleles per gene, so write pairs such as Bb.

• Using two different letters for alleles of one gene.

  Put it right: Use the same letter in upper and lower case, such as B and b.

Exam tips

• Check whether the question asks for genotype or phenotype before writing your answer.
• Show the parental genotypes and gametes clearly before completing a Punnett square.
• Use the same letter consistently for one gene throughout a cross.