When in the offspring of two different animal races one sex is absent, rare, or sterile, that sex is the heterozygous (heterogametic) sex.This rule is almost universally true (see table**) and is supported by the observations that among hybrids such as ligers, zonkeys and mules the males (XY) are all sterile while females (XX) may be fertile (though often poorly so, due to mismatched chromosome numbers, discussed briefly here). The reverse observation - sterile females - is true in species where the male is the homogametic sex such as birds and butterflies. Hypotheses explaining the genetic basis of Haldane's Rule and its challenges are explained nicely here.
Recently, a new fertile hybrid was discovered. The grizzly-polar bear hybrid has been seen before, as discussed here, and with changing habitats and more grizzly-polar bear encounters we'll likely see more. The Toronto Star is reporting the first offspring of a hybrid bear.
Researchers in the Northwest Territories say they may have found the first recorded offspring of a hybrid female polar-grizzly bear in the wild. [...] Officials with the territorial government say those test showed that the dead bear was the offspring of another hybrid bear — a female polar-grizzly mix who had mated with a male grizzly.Given Haldane's Rule, that the hybrid parent was female isn't surprising, but it remains to be seen whether the males are sterile or if this becomes the first mammalian exception.
*In addition to being one of the founders of population genetics, Haldane also wrote poetry about rectal cancer, a disease he succumbed to in 1964.
**Table source: Nature 355, 511-515 (1992). Ref. 41 refers to Coyne, J.A. and Orr, H.A. in Speciation and its Consequences (eds Ott, D. and Endler, J.) 180-207 (Sinauer, Sunderland, Massachusetts. 1989)