Genetic inheritance: heterozygous

The idea

Heterozygous describes a genotype in which the two alleles of a gene carried by an organism are different. Alleles are alternative versions of a gene, and a diploid organism inherits one allele from each parent. If the alleles differ, the organism is heterozygous for that gene, for example Bb or Tt. The observable characteristic depends on dominance: a dominant allele is expressed even when only one copy is present, so a heterozygote usually shows the dominant phenotype, while the recessive allele remains hidden unless two recessive alleles are inherited.

What earns the marks

• Defines heterozygous as having two different alleles of the same gene, for example Bb.
• Uses correct allele notation: dominant allele as a capital letter and recessive allele as the matching lower-case letter.
• Explains that a heterozygote usually shows the dominant phenotype because the dominant allele is expressed.
• Distinguishes heterozygous from homozygous, where the two alleles are identical, for example BB or bb.
• Applies the term to genetic crosses, family pedigrees or probability calculations involving gametes.
• Links genotype to phenotype, noting that the recessive allele is still present but not expressed.

Common mistakes and how to put them right

• Writing that heterozygous means two different genes.

  Put it right: It means two different alleles of the same gene.

• Confusing heterozygous with homozygous.

  Put it right: Heterozygous has unlike alleles such as Bb, whereas homozygous has identical alleles such as BB or bb.

• Assuming a heterozygote always shows the recessive characteristic.

  Put it right: It usually shows the dominant characteristic because the dominant allele is expressed.

Exam tips

• Underline the key words 'different alleles' in the question before writing your definition.
• When completing a Punnett square, label gametes with single letters and offspring with pairs such as Bb.
• If asked to explain a ratio, state the genotypes of parents and gametes before combining them.