What is an alternative form to a gene?

Answer 1

an allele

For example, the gene for hair color might have an allele for blonde, an allele for brown, and an allele for black.

Answer 2

The different forms of a gene are called alleles
of the gene. Examples include one of the
genes that affect eye color in humans, one
allele is dominant and produces brown eye color
in either homozygous or heterozygous state, the
other produces blue eye color but only when it
is homozygous, that is, it is recessive.

There actually are several different pairs of
genes that affect human eye color. The brown
and blue will occur as described above only
when a particular array of other eye color genes
is present. This is a common situation, many
traits are affected by multiple pairs of genes.
In the fruit fly, Drosophila. there are dozens of
gene pairs that affect the eye color as well as
its shape and even whether there are eyes or not.

As an example of the human eye color genes I can use myself. My father's eyes were
blue, my mother's brown. My eyes are of no
particular color - the irises are blue around the
edges, they have yellow toward the center and
a couple of small red spots. When asked to indicate my eye color on a form I have never known what to call it.

Answer 3

the different forms of a gene is called alleles. for eg. the MHC genes(major histocompatibily complex) responsible for specific immunity has 200-250 allelic forms. skin colour is a manifestation of 2 different allelic forms of 3 different genes as in polygenic inheritence.