Greyt-mom:

1/2 siblings are horses who have the same dam (mother) but different sires. A 3/4 sibling would be the same dam and different sire but the two sires would be 1/2 brothers. A good way to understand this is to actually see it laid out. http://www.pedigreequery.com/go+for+gin is the pedigree for Kentucky Derby winner Go For Gin. http://www.pedigreequery.com/pleasant+tap is the pedigree for his 3/4 brother Pleasant Tap. Notice how both horses are out of the mare Never Knock (the lower left hand pink box). Their sires are Cormorant and Pleasant Colony (both the upper left blue box). If you look at the four boxes in the second column, the first blue box in each is His Majesty. That is the sire of both Cormorant and Pleasant Colony. Below that is a pink box with the name of their dams (obviously different mares). The lower two boxes are the sire and dam of Never Knock (so obviously will be the same). Therefore 3 out of 4 of those relatives are common, hence 3/4s sibling.

thinkidiots:

Technically, the answer given in the first reply was correct — but I do not believe mating is done between mothers and sons, even in the bizarre world of horse racing/breeding.

Here’s how you get a "3/4 brother":

MamaHorse and PapaHorse#1 produce BabyHorseX.

MamaHorse and PapaHorse#2 produce BabyHorseY.

*Now*, you’d tell me they were half-brothers — i.e., that BabyHorseX and Y were half brothers, right? And you’d be correct — they have different fathers and the same mother…

But you’d be missing something!

If PapaHorse#1 and PapaHorse#2 are brothers, THEN BabyHorseX and BabyHorseY share the same mother AND their fathers are also related.

Hence, the 3/4 figure.

And, this doesn’t lead to horses that are inbred (to a greater degree) — at least not when practiced once in a population.