Allele, genotype and phenotype
An allele is a variant at a genetic locus. Most autosomal positions have two alleles, one on each chromosome of the pair; together they form the genotype. Sex chromosomes, mitochondrial DNA and copy-number changes follow different patterns. Phenotype reflects genetic and other influences.
Key points
- An allele is a variant of a gene; the genotype is the two alleles together.
- Homozygous means two equal alleles, heterozygous two different ones.
- The phenotype is the trait; the genotype is only one part of what shapes it.
Allele and genotype
At a typical autosomal position, the base may be C or T, for example. Each possible form is an allele. Because autosomes occur in pairs, there are usually two alleles at that position: C/C and T/T are homozygous, while C/T is heterozygous. The number can differ on sex chromosomes or in copy-number changes.
From genotype to phenotype
The phenotype is what you ultimately see or measure, from eye colour to a lab value. It does not follow one to one from the genotype. Environment, lifestyle, chance and the interplay of many genes shift the picture. The same genotype can turn out differently in two people.
How Genome presents this
Genome shows the technical genotype at a position, that is which two alleles you carry. What this genotype means is given in the respective article and depends on whether an allele acts dominantly or recessively and how fully it prevails. The genotype is the starting point of interpretation, not its end.
What Genome measures. Genome reports the genotype at a position, for example C/T. Whether that becomes a visible trait depends on dominance, environment and other genes.
Related topics
Sources
- 1Johannsen, 1911 The genotype conception of heredity. The American Naturalist 45:129-159. doi.org/10.1086/279202
- 2Hardy, 1908 Mendelian proportions in a mixed population. Science 28:49-50. doi.org/10.1126/science.28.706.49