Genes are the blueprint for proteins. An Allele is a variation in the blueprint for a given gene, and while humans and dogs (and all diploid organisms) carry two alleles for each gene [one from mom, one from dad], it is possible for many variations to exist. Variations in alleles within a population is what we’re talking about when we speak of genetic diversity.
The broadest definition of a recessive allele is “one of a pair of alternative alleles whose effect is masked by the activity of the second when both are present in the same cell or organism.” While this explains the resulting phenotype, it doesn’t address the underlying mechanism. What about an allele makes it recessive?
Often, being entirely broken.
Working genes produce proteins that do functional things like make antibodies for the immune system, create enzymes that guide chemical reactions in cells (making pigments, breaking down food), form structures that give shape and strength to all the body’s structures from cells to bones, generate hormones that transmit signals between organs to coordinate biological processes, and construct packaging and transportation mechanisms that move molecules and elements within cells and around the body.
Mutations cause a change in the formula for the protein which can prevent it from doing its job entirely. When this happens, the gene becomes “recessive” because it no longer performs the function that working alleles do. There’s no effect for the other copy of the gene to cover up or compete with, so that allele is dominant by default.
One working copy of the allele is sometimes enough for normal operation or even partial expression in the phenotype as some protein output is maintained. But when two broken copies are present the full phenotype of a broken “recessive” allele is in effect and zero protein is produced.
Because the body reuses the same complex compounds for multiple different purposes, bringing out broken recessives–often for aesthetic or conformation purposes–leaves other essential processes broken as well. This often results in disease or death as doubling up on recessives circumvents the safety provided by redundancy in having two copies of each gene.
Breeding practices (inbreeding, linebreeding) designed to reveal the observable, shallow, superficial recessives like long coats and funky coat colors are also doubling up on the invisible, broken, and lethal recessive alleles that aren’t desired or even known about by the breeder until it’s too late.
Bringing out recessives is a dangerous game and not without serious risks that can’t be accounted for by a few DNA tests and a little pedigree research.
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