A genotype refers to an organism’s genetic composition, specifically the set of alleles it possesses for a particular gene or genes. It represents the underlying genetic blueprint that contributes to an organism’s observable characteristics, known as its phenotype. While the environment can influence how these traits are expressed, the genotype provides the fundamental instructions. Accurately writing genotypes is important for studying inheritance patterns and predicting the genetic makeup of offspring. This article outlines the standardized conventions for denoting genotypes, covering single genes, multiple genes, and sex-linked genes.
Core Rules of Genotype Notation
Representing alleles with letters forms the basis of genotype notation. A dominant allele, which expresses its trait even when only one copy is present, is symbolized by an uppercase letter. Conversely, a recessive allele, whose trait is only expressed when two copies are present, is denoted by the same letter but in lowercase. For instance, if ‘P’ represents the dominant allele for purple flower color, then ‘p’ would represent the recessive allele for white flower color.
Diploid organisms, including humans, carry two alleles for each gene, inheriting one from each parent. When both alleles are identical, the genotype is described as homozygous. A homozygous dominant individual carries two identical dominant alleles (e.g., ‘PP’ for purple flowers), while a homozygous recessive individual possesses two identical recessive alleles (e.g., ‘pp’ for white flowers). If the two alleles are different, the genotype is heterozygous (e.g., ‘Pp’ for purple flowers), and the dominant allele’s trait is expressed.
Representing Genotypes for Multiple Genes
When considering genotypes involving two or more distinct genes, the notation expands by listing the alleles for each gene consecutively. These genes are located on different chromosomes and are inherited independently. For clarity, the alleles for one gene are written together, followed by the alleles for the next gene. A consistent order, such as alphabetical by the gene’s letter, helps maintain readability.
For example, if considering both pea plant height and seed color (where ‘T’ is dominant for tall, ‘t’ for short; ‘Y’ is dominant for yellow seeds, ‘y’ for green seeds), a plant heterozygous for both traits would have the genotype ‘TtYy’. A plant homozygous dominant for height and heterozygous for seed color would be ‘TTYy’. A plant homozygous recessive for both traits would be represented as ‘ttyy’.
Notating Sex-Linked Traits
Sex-linked traits involve genes located on the sex chromosomes, primarily the X chromosome. In humans, females have two X chromosomes (XX), while males have one X and one Y chromosome (XY). The Y chromosome contains fewer genes compared to the X chromosome, making X-linked inheritance more common. The notation for these traits reflects their unique inheritance patterns.
For X-linked traits, the allele is written as a superscript on the X chromosome. For example, if ‘XA‘ represents a dominant allele and ‘Xa‘ represents a recessive allele, a female could have genotypes like ‘XAXA‘, ‘XAXa‘, or ‘XaXa‘. A female with the ‘XAXa‘ genotype is considered a carrier, meaning she carries the allele but does not express the trait herself.
Males, having only one X chromosome, express any allele present on their X chromosome because there is no second X to mask it. Their genotypes are written as ‘XAY’ or ‘XaY’. Males are more frequently affected by recessive X-linked conditions than females, as a single recessive allele on their X chromosome will result in the trait’s expression. Y-linked traits are rare and only affect males, with notation such as ‘YA‘.