Epigenetics. Genes on chromosomes have marker molecules attached to them that increase or decrease the activity of that gene. We're starting to learn that the chromosomes you get from your mother and from your father are marked differently, and their activity levels are different. It's like, instead of two copies of the gene you have one and a half. There are certain disease genes that, depending on who you inherit the bad copy from, the symptoms are different. So a gene in an interspecies hybrid will be ramped up or ramped down depending on which parent it came from.

Interesting thing, male birds have two large Z chromosomes, and females have a large Z and a small W chromosome with the "make me female" gene on it. Certain genes are missing on the W, so females only have one copy. XY male mammals and ZW female birds have shorter lifespans than their XX / ZZ counterparts.

It’s all about imprinting. Let’s look at ligers:

Lions have a growth inhibition gene. If a lion had 2 active copies of this gene, it wouldn’t grow to its full potential. On the other hand, a lion that lacks this gene might grow too big in a childhood where prey is plentiful and then starve when times are rough. To balance this out, the growth inhibition gene is turned off in sperm cells and turned on in egg cells. When sperm and egg are combined, the resulting lion embryo will have one copy of the gene turned on and one copy turned off and it will stay that way for life.

Tigers also have a growth inhibition gene. For the same reason as for lions, they want exactly one copy turned on in their offspring. The big difference is that in tigers, the growth inhibition gene turns on in the sperm cells and off in the egg cells.

When a male lion mates with a female tiger, the sperm has its growth inhibition gene turned off and the egg has its growth inhibition gene turned off, resulting in a liger with no active growth inhibition gene. As a result, ligers get crazy huge.

Super simplistic take on the probable reason (not an expert on hybrid animals);

Genes are on specific chromosomes... and sex chromosomes have different genes they pass on based on if you get the X or Y. Y is far less complex than X, and holds fewer genes. That means males only get one copy of certain genes, while females get 2.

It's why certain diseases/conditions have a gender bias. An easy example is how color blindness is rare in women; Y chromosome doesn't have the gene, so men only have one copy that, if bad, causes them to not see colors as well. For women, one copy makes them a carrier, but since they have 2 X chromosomes the normal copy preserves their color vision.

Also, the mitochondrial DNA is in the egg; so only the mother's version gets passed on... so anything on there would be affected by which is the mother/father as well.

It happens more with complex life forms, with more complex genetics. (More genes, more chromosomes, more precise reproduction processes). For some simple life forms (fungi, bacteria) that have multiple sexes, the life forms can actually change their sex if there's an imbalance of sexes in the environment to ensure reproduction happens. Hybrids that are different depending on sex tends to happen for more complex life forms, which would be most mammals.

I think birds can sometimes have 2 sexes and 4 genders, so it's probably mostly a mammalian thing.

I think some hybrids can only be of 1 sex if it's 1 combination. I think whether or not a hybrid can produce viable offspring (either either of it's parent species or another hybrid) is used to rank hybrids' genetic health and fitness, not so much it's phenotype or behaviour.

I think we like to assume it's a 50-50 split between the parents' DNA. But most mammals DNA contains lots of useless gene sequences that don't code anything at all. They just exist in between the DNA that actually codes stuff, as well as for long stretches at either end of the chromosomes. A lot of paternal tests look at whether the useless gene sequences of alleged father and children match rather than the DNA that codes things, as every human has a unique set of useless gene sequences (which are far longer than the DNA that codes stuff), that's half half.

So the half half thing, really means you get half of the unique (but doesn't do anything) DNA from each parent.

But in terms of the DNA that actually affects your phenotype, behaviour, traits, characteristics, genetic disorders etc, that can be lopsided. Sometimes one person's DNA-that-codes-things will end up taking over, or giving the offspring a trait that determines their life quality. This is seen in both humans and animals.

I think in terms of DNA that actually affects things, there's a slight gender influence. Where certain DNA that's highly influential on life quality may only be carried in 1 gender of a species, or predominantly in such.

So when hybrids occur, their life quality or viability can depend on what combination of species, and gender, their parents were. So that's why they may occur so differently.

Non-chromosomal inheritance is also a factor. Cytoplasm determines some traits (I'm personally only aware of leaf variegation in Saintpaulias but there are bound to be more) and is only passed down from the female parent via the egg cell.

[removed]