Modern Genetics

5. Genotype and Phenotype

Genotype and Phenotype

Genotype refers to the combination of alleles that an individual has for a certain gene. For each gene, an organism has two alleles, one on each chromosome of a homologous pair of chromosomes. The genotype is often referred to with the letter combinations that were introduced in the previous lesson, such as TT, Tt, and tt. When an organism has two of the same alleles for a specific gene, it is homozygous for that gene. An organism can be either homozygous dominant (TT) or homozygous recessive (tt). If an organism has two different alleles (Tt) for a certain gene, it is known as heterozygous. Genes have a specific place on a specific chromosome, so in the heterozygous individual these alleles are in the same location on each homologous chromosome.

Phenotype refers to the visible traits or appearance of the organism, as determined by the genotype. For example, the phenotypes of Mendel’s pea plants were either tall or short, or were purple-flowered or white-flowered. Keep in mind that plants with different genotypes can have the same phenotype. For example, both a pea plant that is homozygous dominant for the tall trait (TT) and heterozygous plant (Tt) would have the phenotype of being tall plants. The recessive phenotype only occurs if the dominant allele is absent, which is when an individual is homozygous recessive (tt).