The gene is carried on the x chromosome. The stallion is xy and the mare is xx. If the stallion has the gene, it is never passed to his sons by him (he gives his sons his y) but is always passed to his daughters (he gives his daughters his x where the gene resides). If the dam carries a single copy of the gene, then it only appears on one of her x chromosomes. Since she has two x chromosomes and only passes one on to each foal, there is a 50% chance for each foal to get the x chromosome that carries the gene, and a 50% chance that she will pass on the x chromosome that does NOT carry the gene.
In this scenario, (stallion carries gene on his x and mare carries gene on only one of her two x chromosomes) then all resulting colts from this mating have a 50% chance of carrying the gene (won’t get if from sire but a 50/50 chance from dam) and all resulting fillies are guaranteed to carry the gene (because the sire will definitely give her his x that carries the gene) AND there is a 50/50 chance that the filly will be a double copy (depending on whether or not the dam passes on the x that has the gene or the x that does not).
If the resulting filly IS a double copy, then ALL her foals will get the gene (regardless of the stallion), and anytime a double copy dam is bred to a stallion with the gene, ALL resulting foals will get the gene and ALL resulting fillies will be double copies.
As far as the x-factor being highlighted in the Del Mar Pedigree site, they only highlight it if the horse was in a possition to possibly have carried it down to a queried horse. For example, if you look up my mare, Luminary Princess, she is not highlighted and neither are her dam or sire. BUT it shows that either dam or sire MIGHT have inherited the gene if passed on by the highlighted granddams, and so they MIGHT have passed the gene on to my mare. In fact, there are a bunch of horses in her pedigree that are not highlighted but have a 50% chance of carrying the gene: Miss Citation MAY have been a double copy, and even if only a single copy (she was at least that) then Proper Notice MAY have carried the gene (if Miss Citation WAS a double copy then Proper Notice DID carry the gene, then Duran’s single copy MAY have been a double copy then Alittlebitearly would definitely have carried then gene instead of MAY have carried the gene). Island Sultan MAY have it, Tuetra MAY have a double copy if she got Courtesy’s gene through Continue. In fact, my mare may not carry it at all, she may have one copy, or she may have a double copy. I can trace several routes that may have brought the gene down to her.
More thoughts: since the x chromosome is significantly bigger than the y, then actually the dam is passing on about 70% of the genetic material to colts (she gives the x and stallion gives the smaller y) and both dam and sire each give the same amount (through x chromosomes) to fillies. HOWEVER, the mare also gives a bit more physical components because it is her physical system, plasma, etc. that is supporting the foal and transferring through the placenta and coursing through the foal. There is an argument that if you trace the tail female line (the horses that appear on the bottom branch of the pedigree: dam, granddam, great granddam, etc.) that your filly carries some physical/blood/plasma that has been passed/shared all the way down through those generations of mares.
The tail male line (all the sires along the top branch of the pedigree) are the most important males in the pedigree because they all share the exact same Y chromosome. NONE of the other males in the entire pedigree ever pass a Y down to your horse. All those other Ys have “deadended” as they were not passed on in the pedigree: If you look, all the other Ys end somewhere as a mare appears in the pedigree and that sire line’s Y isn’t carried farther in the pedigree. It is the tail male who’s Y makes it all the way down to the resulting male foal.
Hope this is not too confusing! It’s early.
He is a racing bred QH that was injured before he was 2 and never made it to the track. He has what appears to be a double copy mom – that’s what got me interested in this research in the beginning.