alternative forms of a gene are called as alleles..

No they are alternative versions of a gene.

Simply because not all snakes have the mechanism to produce venom. Constrictors developed an alternative method of subduing their prey, since they lack venom and fangs.

Do you mean Don't Cry by Guns n Roses? There are two versions on Use Your Illusions, the original on I and the alternative on II. I don't know about live versions.

The 1st concept describes that alternative versions of genes account for variations in inherited characters.