Heterozygous
Exam-ready definition
Having two different alleles of a particular gene.
How this is marked
The paying point is 'two different alleles' of the same gene. A heterozygous individual carrying a recessive allele is a carrier, and in Paper 4 you are often asked to identify carriers from a pedigree or cross. Do not write 'two different genes', and remember a heterozygote shows the dominant phenotype unless the alleles are codominant.
In an exam answer
Identifying a carrier: 'The genotype Tt is heterozygous because it has two different alleles of the gene, one dominant and one recessive.'
Full topic guide: Inheritance