in-breeding and interbreeding of close species

[dichotomy]

In-breeding and closely related species, like lions and tigers interbreeding, both these breeding scenarios produce some infertile offspring. What do both have in common that makes infertile offspring more likely? Or what are the things that are likely to be in-common for sterile offspring to be the result?

 

(I’ve transferred this question here from another thread. Was a wee bit awff topic )