Genes and their Alleles

Every gene has two alternative forms for a character producing different effects. These alternative forms are called the alleles.

The kind of allele which expresses itself regardless of the presence of another allele/s for a given gene. e.g., The dominant allele for height in garden pea is tall. 

The kind of allele whose expression is suppressed in presence of a dominant allele for a given gene. A recessive allele can only express itself in presence of the same recessive allele. e.g. The recessive allele for height in garden pea is dwarf.

1. Alleles are alternative forms of a gene.
2. One allele is dominant, the other is recessive.
3. Dominant allele expresses even if single (e.g., tall pea = "T").
4. Recessive allele expresses only if both are recessive (e.g., dwarf = "tt").
5. Homozygous = same alleles (AA/aa); Heterozygous = different alleles (Aa).