Different forms of the same gene that occupy the same position on homologous chromosomes are called:

A gene is a segment of DNA that carries the information for a particular trait, such as height, flower colour or blood group. Each gene is found at a specific position, called a locus, on a chromosome. Most organisms that reproduce sexually have two sets of chromosomes, one from each parent. The paired chromosomes are called homologous chromosomes, and so each individual has two copies of each gene.

The different forms (variants) of a gene are called alleles. For example, in Mendel's pea plants, the gene for height has two alleles: T for tall and t for dwarf. A plant can have the combinations TT, Tt or tt. These combinations are called the genotype, while the observable trait (tall or dwarf) is called the phenotype.

When both alleles are the same (TT or tt), the organism is homozygous for that trait. When they are different (Tt), it is heterozygous.

In many cases, one allele (the dominant allele) hides the effect of the other (the recessive allele). In Mendel's peas, a Tt plant is tall, because T is dominant over t.

In some cases, there is incomplete dominance, where the heterozygote shows an intermediate trait, as in the pink flowers of the snapdragon (Antirrhinum) from red and white parents.

In co-dominance, both alleles are expressed fully, as in the human AB blood group.

Some genes have more than two alleles in a population, called multiple alleles. The human ABO blood group is controlled by three alleles, IA, IB and i.

The word "allele" is a short form of "allelomorph", a term introduced by William Bateson, who also coined the word "genetics" in 1905.

Good to know

A genome is the complete set of genetic material of an organism.

A codon is a sequence of three nucleotides in mRNA that codes for an amino acid.

A mutation is a sudden change in the DNA sequence.