What causes a corgi to be a fluffy
Fluffy is a standard Mendelian recessive allele (as is tricolor). It
won’t be expressed unless you have two copies, one from the dam and one
from the sire.
Different variants of the same gene are called “alleles”. Let’s call the
dominant regular-coat allele “N”, and the recessive fluffy-coat allele
“f” [recessives are usually denoted in lower-case letters]. You get one
allele from each parent (unless it’s on the X chromosome; males get only
1 maternal allele because the Y is largely empty).
NN = regular coat [“homozygous dominant” for biology majors]
Nf = regular coat (fluffy carrier) [“heterozygous”]
ff = fluffy coat [“homozygous recessive”]
NN x NN –> 100% regular-coat pups, no carriers (this result is CERTAIN; all following percentages are PROBABILITIES only)
NN x Nf –> 100% regular-coat pups, 50% carriers
Nf x Nf –> 75% regular coat pups (NN and Nf), 25% ff Fluffies. Medelian 3:1 ratio. It’s actually a 1:2:1 ratio: 25% NN regular coat (“clear”) 50% Nf regular coat (“carriers”), 25% ff fluffies
Nf x ff –> 50% Nf carriers and 50% ff fluffies
ff X ff –> 100% ff fluffies (this result is CERTAIN).
In real life, it’s more complicated because many alleles are imperfectly dominant or recessive. They talk about “incomplete penetrance”. F'rinstance, the sable allele is thought to be an incompletely dominant red-color allele that lets a little black show in sable/tricolor heterozygotes. The blackheaded tri allele is thought to be recessive to the regular tri allele, which itself is recessive to red.
But the 100% true and real answer is that FLUFFIES ARE AMAZEBALLS GIFTS FROM THE HEAVENS HERE TO BLESS US WITH EPIC MOMOS AND FLOOFY LOOOOOOOOOOOOOOOOVEEEEEEEEEEE!!! Also, they don’t shed but a fraction of what non-fluff corgs do. Just sayin’.