Allele

An allele is one of the different forms of a particular gene. For any given gene, an individual normally has two alleles (one inherited from each parent). If both alleles are the same, the person is homozygous for that gene; if the alleles are different, the person is heterozygous.

• We use letters to symbolize alleles in genetics problems. For example, for a gene where "A" = dominant allele and "a" = recessive allele, possible genotypes include AA (homozygous dominant), Aa (heterozygous), or aa (homozygous recessive).
• Different alleles can produce different variations of a trait. For instance, a gene for flower color might have a "red" allele and a "white" allele. Which alleles an organism has (its genotype) will determine the observable trait (phenotype).
• Some genes have more than two alleles in the population (multiple alleles), although each individual still only inherits two. A common example is the ABO blood group gene: it has three main alleles (IA, IB, i), of which any person can have two (leading to blood types A, B, AB, or O).

• Genetics questions often use the term "allele" when referring to gene variants. For example, a problem may say "the allele for tall plant height" vs "the allele for dwarf height." Recognize that alleles are just versions of the same gene.
• Be prepared for exam prompts about allele frequencies or Punnett squares. If a question asks about the chance of a child inheriting a certain allele, remember each parent contributes one allele per gene. Also recall that dominant/recessive interactions are properties of alleles.
• Sometimes exam questions test understanding that alleles are not separate genes but different forms of the same gene. For instance, a classic misconception they may check: blue vs brown eye color are due to alleles of the same gene, not two different genes.