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I've read a little about hybrid animals, and have been amazed by the fact that the sex of the parent species determine the species of the offspring, in some hybrids. A male horse and a female donkey will give birth to a Hinny, while a female horse and a male donkey will give birth to a Mule.

The same goes for tigers and lions, male lion and a female tigress will give birth to a Liger, while a male tiger and a female lioness mating will result in a Tiglon

I thought that the only difference between the male and female reproductive cell's DNA code is the X-Y chromosome that determines gender, not differences that can define the phenotypical result of the offspring. Why does this happen?

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    $\begingroup$ Excellent question. Unfortunately, I am guessing that the answer is simply “because we defined it that way”, rather than a biological insight. Species are usually defined in terms of the possibility of interbreeding, which isn’t relevant here (hybrid offspring is usually sterile). $\endgroup$
    – Konrad Rudolph
    Jul 31, 2012 at 11:38
  • $\begingroup$ @KonradRudolph, but species are not defined just for the fun of it (I guess, I don't really know), they are defined because there are significant differences between two animals, and in the case of those two interbreeding, those differences are consistent with the gender of the parent species. In other cases, like the different types of Zebroids (Zebra and donkey/horse/pony hybrid) that doesn't seem to be differentiated by parent species gender. (en.wikipedia.org/wiki/Zebroid) $\endgroup$
    – SIMEL
    Jul 31, 2012 at 13:33
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    $\begingroup$ @Ilya Melamed: there is no strict definition of species... and anyway hybrids are NOT defined as separate species, but as the cross of two different ones. $\endgroup$
    – nico
    Jul 31, 2012 at 13:36
  • $\begingroup$ @nico, maybe I'm using the word "species" wrong, I use it in its layman definition, as "different types of animals". The meaning is, as I said, that significantly different animals are born from the same parent animals, and that the change is consistent with the gender of the different parent animals. If you can think of a better term than "specie" please edit the question to bee more accurate. $\endgroup$
    – SIMEL
    Jul 31, 2012 at 14:03

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I thought that the only difference between the male and female reproductive cell's DNA code is the X-Y chromosome that determines gender, not differences that can define the species of the offspring. Why does this happen?

Well that is not the only difference, there are certain characters that are only transmitted by one of the parents.

First of all you have to think that the zygote (the cell deriving from the fusion of two gametes) contains the nucleus of the egg and of the sperm but the cytoplasm of the egg only (or at least in great part). One thing that is present in the cytoplasm are mithocondria, which contain DNA, called mtDNA. mtDNA is therefore transmitted only by the mother (at least, in most species). Therefore a hinny will have donkey mtDNA, while a mule will have horse mtDNA.

Furthermore, cytoplasm contains transcription factors and various proteins that will influence the subsequent development of the zygote into an embryo and, again, these are maternally derived.

Another thing that changes is that there are certain genes on nuclear DNA which are expressed in a single-parent manner. That is, you have 2 copies of these genes, called imprinted genes, one from the mother and one from the father but only one of these copies -that of the mother or of the father, depending on the gene- will be expressed.

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