If the two alleles present are the same the organism is homozygous for…

The idea

Alleles are different versions of the same gene, and a diploid organism carries two alleles for each gene, one inherited from each parent. When both alleles are identical, the genotype is homozygous for that trait, for example AA or aa. When the two alleles differ, the genotype is heterozygous, for example Aa. The combination of alleles present is the genotype, while the visible or detectable characteristic is the phenotype. In a heterozygote, a dominant allele is expressed even though a recessive allele is also present, so Aa usually shows the dominant phenotype. Only when both alleles are recessive, aa, is the recessive phenotype seen. Writing genotypes with a capital letter for the dominant allele and the matching lower-case letter for the recessive allele makes homozygous and heterozygous states easy to identify and use in genetic crosses.

What earns the marks

• Alleles are alternative forms of the same gene occupying the same locus on homologous chromosomes.
• A diploid organism has two alleles per gene, one from each parent, so genotypes are written as pairs such as AA, Aa or aa.
• Homozygous means the two alleles for that trait are the same, giving AA or aa.
• Heterozygous means the two alleles for that trait are different, giving Aa.
• Genotype describes the alleles present, whereas phenotype describes the observable characteristic produced.
• In a heterozygote, the dominant allele is expressed and the recessive allele is masked, so Aa shows the dominant phenotype.

Common mistakes and how to put them right

• Confusing genotype with phenotype.

  Put it right: Aa is heterozygous and shows the dominant phenotype because the dominant allele is expressed.

• Treating heterozygous as meaning two dominant alleles.

  Put it right: Heterozygous requires two different alleles, such as Aa.

• Assuming homozygous always means dominant.

  Put it right: Homozygous can be AA or aa, depending on whether the identical alleles are dominant or recessive.

Exam tips

• Underline the words same and different in the question so you classify the allele pair correctly before writing your answer.
• Use a consistent letter pair, capital for dominant and lower case for recessive, and state the genotype before describing the phenotype.
• When asked to explain, link the allele combination to the protein or characteristic it produces rather than only naming homozygous or heterozygous.