So, this problem is kind of a two-in-one; arguably two different issues but very closely related, so I'm posting them as one question.
I'm designing a lineup of fantasy races for a roleplaying setting, and I think I want to have two single-gender player races. My favorite idea for how is to have Half-Giants for the masculine and Witches for the feminine, but I'm not certain how this works genetically.
EDIT for clarity: More specifically the so-called Giant race has male and female members, with the men being significantly larger than the vast majority of Humans - 8' tall on average, more broadly built, thick-boned and muscular. Giantesses on the other hand are just noticeably larger (6' on average and more robust) than the average Human man. Half Giants, who are intermediary in size, exist only in male form; there are no female Half Giants either as a playable option or in lore.
The Witch race (men and women) mostly resemble normal humans, but the women are more naturally attuned to magic. The men (Warlocks) lack this magical affinity and so are ostensibly identical to Mundane Human men, except in the fact that they can produce Witch daughters. So a player might play as a Warlock for lore reasons, but they have the same stats as Human players.
Since both the races of Giants and Humans are comprised of both males and females, that implies that in the case of Human-Giant mating, female zygotes are formed as often as male, but only the males come to term. I can't remember where I heard this, but supposedly all first-generation sapiens-neanderthal hybrids were female because of some minor genetic incompatibility, and that would seem to imply that the Y chromosome is the cause of the incompatibility; but could the inverse be the case, where the Y chromosome causes compatibility where otherwise hybridization is not feasible? I'm not aware of any real-world precedent for that.
Witches on the other hand are always female, and best I can figure for how this works is that one needs two X chromosomes with the witch gene in order to be a "full-blooded" Witch; females with only one are "Half-Witches" who are functionally identical to Mundane Humans, as are male "Warlocks" who are required to produce more full-blooded Witches. I like the logic of this as it applies to potential plotlines, but I don't think the two-X requirement works that way. From what I understand, only one of the X chromosomes in biological females is active, with the other being dormant; and if all it takes to go full Witch is one Witch gene, that means all Warlocks have the same functional witchy traits as the other Witches, which means Witches would have to be a standard two-gender player race, and I would prefer just one.
Easy solutions I don't prefer:
- Female Half-Giants and male Witches; this is a lot easier to explain, with the Human and Giant genomes being incompatible at the Y chromosome and the Witch genome being entirely on the Y, but it lacks aesthetic appeal for me and I suspect most potential players on several levels. It has its ups too, but not enough compared to the other way.
- Asexual Witches; this is boring. I like the premise of arcane creatures whose bloodline is interwoven with humanity rather than completely separate; there's a lot more story potential there.
- Female Half-Giants do exist but might as well be full-blooded Humans, like Warlocks; I considered this, but I much prefer the premise of Half-Giants being sort of uncanny mulish men produced by two species barely chemically compatible, always male and always sterile, giving Half-Giants more potential conflict and reminding the audience that Giants really are a different type of creature rather than just big dumb cavemen.
- Y chromosome somehow suppresses Witch traits, the same way male hormones suppress the growth of breasts, uterus etc.; this is a clean explanation, but it removes a potential story element of the Lost Bloodline, where characters don't know that they have Witch blood; if there are no dormant female Witches but rather all females with the potential to pass on the Witch gene are proper Witches themselves, it's extremely implausible that any family would not know that they are Witches. If this is the only way all-female Witches makes sense, I'll settle for it, but I'd prefer to have the potential for dormant females.
So, in brief: Is it plausible to have two species to whom hybrids are always or at least overwhelmingly male, and a distinct genetic trait that is only expressed in females who have the trait on both X chromosomes?