In horse breeding what is a 3/4 brother or sister and what does it mean when you see numbers like 2x4x3 cross?

Answer 1

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.

To answer the second part of your question, 2x4x3 refers to inbreeding to a certain individual and what generation it occurs in. Looking again at Pleasant Tap's pedigree, he is inbred 5x6 to both Princequillo and Nasrullah (viewed on his pedigree they have blue and green boxes behind their names). Pleasant Tap would be #1 (first generation), Pleasant Colony and Never Knock are #2, His Majesty, Sun Colony, Stage Door Johnny and Never Hula #3 and so on.

Answer 2

Can't give you a better explanation than Greyt-Mom! Well explained!