It ‘s a confusing area.

Some times you see "Dominant" used as a catch all for any non-recesive type until we figure it out. For example, we know that spider, pinstripe, and spot nose aren't recessive but I don't think a homozygous version of any of those three has been proven out yet so we don't know which type of dominant they are.

You also sometimes see "Dominant" used for what I like to call "completely dominant" to avoid confusion. This is a version of a gene that completely dominates another version of the same gene. Even one copy of the dominant gene overpowers the gene it's being compared to. For example, if the spider gene turns out to be completely dominant over the normal version of the same gene then the homozygous spiders (with two copies of the spider gene) will look the same as the heterozygous spiders (with only one copy of the spider gene). If the current heterozygous spiders are as “spider” as they get (i.e. the homozygous exists and there is no visibly different “super spider”) then even one mutant spider gene is enough to completely cover the remaining normal version of the spider gene.

Then you get into the co-dominant (which some argue should technically be called "incomplete dominant" type. In this type of gene having only one copy of the mutant gene has an effect but apparently having one copy of the gene you are comparing it to (usually the normal version) also has an effect because the homozygous mutant version looks different (and usually more extreme) than the heterozygous version. An example of this is pastel. The heterozygous for the pastel mutant gene animals are visible mutants so even one copy of the gene is enough to show. However, two copies of the pastel gene (homozygous pastel) make the super pastel phenotype so apparently the one normal copy of the pastel gene in the het pastel had some effect to prevent it from being the super pastel phenotype.

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