Definition: An allele is an alternative form of a gene (one member of a pair) that is located at a specific position on a specific chromosome. These DNA codings determine distinct traits that can be passed on from parents to offspring through sexual reproduction. The process by which alleles are transmitted was discovered by Gregor Mendel and formulated in what is known as Mendel's law of segregation.
Examples: Organisms have two alleles for each trait. When the alleles of a pair are heterozygous, one may be dominant and the other recessive. The dominant allele is expressed and the recessive allele is masked. This is an example of complete dominance. In heterozygous relationships where neither allele is dominant but both are completely expressed, the alleles are considered to be co-dominant. Co-dominance is exemplified in AB blood type inheritance. When one allele in not completely dominant over the other, the alleles are said to express incomplete dominance. Incomplete dominance is exhibited in pink flower color inheritance in tulips.
