An allele is a different variation of a gene. Genes are the subunits on chromosomes that cause the expression of a particular trait, or the absence of a particular trait. An Austrian monk named Gregor Mendel pioneered the work in genetics by working with his pea plants in his garden. Mendel kept careful notes on his pea plant experiments. He noticed, for example, that purebred tall pea plants always produced tall offspring, and purebred short pea plants always produced short offspring. One day he decided to cross a tall with a short; the result was all the offspring were tall. Then, he took two of those offspring and crossed them. Much to his surprise, three were tall, while one was short! Today, we know this is the result of mixing different alleles for plant height. In this experiment, the tall plants were dominant, we would label the allele for tallness with a capital "T", while the short plants were recessive, so we would label the allele for shortness with a lower-case "t". The first cross would look like this, for a tall versus a short:
T Tt Tt
T Tt Tt
100% of the offspring had the allele for tallness, so they were all tall. The second cross would look like this:
T TT Tt
t Tt tt
In this cross, we see the reemergence of the two recessive alleles, which would produce a short plant.