The genotype is the genetic constitution of an organism.

The idea

A genotype is the genetic constitution of an organism, representing the specific alleles it carries. It can be written for a single gene (e.g., Ff), two genes in a dihybrid cross (e.g., FfGg), or a sex-linked gene (e.g., $X^R X^r$). Alleles are different versions of a gene at a specific locus. While diploid organisms typically have two alleles for each autosomal gene, XY males possess only one allele for X-linked genes (e.g., $X^R Y$). A genotype belongs to an individual, whereas a gene pool describes a population, and a genome is the complete set of genes. The genotype is fixed at fertilisation; cellular differences arise from gene expression, not different alleles.

What earns the marks

• Defining genotype as the genetic constitution of an organism or the specific alleles it possesses.
• Distinguishing genotype (individual level) from gene pool (population level) and genome (complete set of genes).
• Identifying that diploid organisms have two alleles for autosomal genes, but XY males have only one allele for X-linked genes.
• Distinguishing genotype from phenotype, where the phenotype is the observable expression of the genetic constitution.

Common mistakes and how to put them right

• Defining a genetic term using population-level wording (like all DNA in a species) when describing an individual's genotype.

• Confusing genotype with genome.

  Put it right: The genome is the complete set of genes, while the genotype refers to the specific alleles present.

• Using unrelated letters for alleles of the same autosomal gene (e.g., G and b), obscuring their relationship as versions of the same gene.

Exam tips

• Read carefully whether the question asks for a genotype (written in allele symbols) or a phenotype (written in descriptive words).
• If the question supplies specific allele symbols, use those exact symbols; inventing your own can lead to ambiguity.