What Are Alleles?
Geneticists who have understood how the genes work knows perfectly what alleles are, as in pure genetics, alleles are like constituents of the genotype of an organism, mainly in pairs or more.
What Are Alleles – Definition
Allele, also called allelomorph, one of two or more genes that can occur alternatively at a particular location (locus) on a chromosome.
Alleles can occur in pairs, or there can be multiple alleles that influence the expression (phenotype) of a particular trait.
The combination of alleles carried by an organism forms its genotype. If the paired alleles are the same, the organism’s genotype is said to be homozygous for that trait; if they are different, the organism’s genotype is heterozygous.
A dominant allele will cancel out the characteristics of a recessive allele in a heterozygous mating.
However, for some traits, the alleles may be codominant, meaning that neither has a dominant or recessive effect.
One example is the human ABO blood group system; individuals with blood group AB have one allele for A and one for B. (Persons with neither allele are type O.)
Thus, most traits are controlled by more than two alleles. Multiple forms of the allele may exist, although only two attach to the designated gene locus during meiosis.
In addition, some traits are controlled by two or more gene loci. Both possibilities multiply the number of alleles involved.
All genetic traits are the result of the interactions of alleles. Mutation, crossing over and environmental conditions selectively alter the frequency of phenotypes (and thus their alleles) within a population.
For example, alleles carried by individuals with high fitness (meaning that they reproduce successfully and pass on their genes to their offspring) have a higher probability of persisting in a population than alleles carried by less fit individuals, which are lost from the population over time.