Hey, it's not that difficult. But a screwed up vocabulary screws up everyone. Start off with this:

Homozygous = two copies of the same gene in a gene pair. Examples: AA, aa.

Heterozygous = two different genes in a gene pair. Example: Aa.

Recessive mutant gene = Shows its effect only when homozygous. Symbolized with a lower case letter and fits the following pattern:
AA = homozygous normal, looks normal.
Aa = heterozygous because has a mutant gene paired with a normal gene, looks like an AA individual.
aa = homozygous mutant gene, shows the mutant appearance.

Dominant mutant gene = Shows its effect when either heterozygous or homozygous. Symbolized with an upper case letter and fits the following pattern, which is the mirror image of a recessive mutant gene:
AA = homozygous mutant gene, shows the mutant appearance.
Aa = heterozygous because has a mutant gene paired with a normal gene, looks like an AA individual.
aa = homozygous normal, looks normal.

Codominant mutant gene = Shows a different effect when heterozygous than when homozygous. Symbolized with an upper case letter and fits the following pattern:
AA = homozygous mutant gene, shows the full mutant appearance.
Aa = heterozygous because has a mutant gene paired with a normal gene, does not have the appearance of either an AA or an aa individual.
aa = homozygous normal, looks normal.

The codominant symbolism used here stinks. This forum won't show the plus sign, which is the standard symbol for a wild type or normal gene.

Paul Hollander

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