In horses, the genotype little-B little-B causes early embryonic lethality. The A locus behaves as a classic Mendelian gene. If two horses with identical genotypes AaBb [big-A, little-A, big-B, little-B] mate, what is the expected phenotypic ratio of their offspring?

W) 9:3:3:1 [read as nine to three to one]

X) 3:1:1 [read as three to one to one]

Y) 9:3:1 [read as nine to three to one]

Z) 3:1 [read as three to one]
Z) 3:1