Chapter Text
The Genetics of Sex:
In our universe, there are two human sex chromosomes, named X and Y, and the particular combination a person has of these chromosomes determines their sex. Most people have a pair. You are probably familiar with the combinations XX and XY, which are the most common. Other combinations, though, are possible. There are some people who are XXY, XXX, or even just X. There are also people who end up with a partial X or partial Y, and there are also people who can have some cells in their body be XX and some XY.
For fertilization to occur, a person with a Y chromosome must contribute sperm and a person with eggs must contribute an egg. For the fertilized egg to end up with a Y chromosome, that chromosome must come from the sperm. Any X chromosomes can come from either the egg or the sperm.
The possibility of the offspring being XX or XY is generally considered to be 50/50, though that is only an approximation and does not take into account the other possible combinations discussed above. You can see this approximation represented in this handy-dandy Punnett Square:
X Y
X | XX XY
X | XX XY
As you can see, there is an equal chance of XX and XY offspring, which is why the human population is roughly half XX and half XY.
The Equilibrium Verse has a triploid chromosomal system for human sex chromosomes. The three chromosomes are X, Y, and O. As in our universe, most people have a pair of chromosomes, though other combinations can and do occur with roughly the same frequency as they do in our own. Here are the six most common configurations of human sex chromosomes in the Equlibrium Verse:
XX - Alpha with a penis, knot, testes, vagina, ovaries, uterus, sperm, and eggs; goes into rut
YY - Alpha with a penis, knot, testes, and sperm; goes into rut
XO - Beta with a vagina, ovaries, uterus, and eggs
XY - Beta with a penis, testes, and sperm
OO - Omega with a vagina, ovaries, uterus, and eggs; goes into heat
OY - Omega with a penis, testes, ovaries, uterus, sperm, and eggs; goes into heat
I’ll get into more of the theory behind how this triploid system might have evolved later on. For now we’ll look at the frequency with which the different configurations occur by making some more of those handy Punnett Squares:
Parents: XX Alpha/XO Beta Parents: YY Alpha/XO Beta
Offspring: 50% XX Alpha, 50% XO Beta Offspring: 50% XY Beta, 50% OY Omega
X X Y Y
X | XX XX X | XY XY
O | XO XO O | OY OY
Parents: XY Beta/XO Beta Parents: XX Alpha/OO Omega
Offspring: 25% XX Alpha, 25% XY Beta, Offspring: 100% XO Beta
25% XO Beta, 25% OY Omega
X Y X X
X | XX XY O | XO XO
O | XO OY O | XO XO
Parents: YY Alpha/OO Omega Parents: XY Beta/OO Omega
Offspring: 100% OY Omega Offspring: 50% XO Beta, 50% OY Omega
Y Y X Y
O | OY OY O | XO OY
O | OY OY O | XO OY
Parents: XX Alpha/OY Omega Parents: YY Alpha/OY Omega
Offspring: 50% XO Beta, 50% XY Beta Offspring: 50% OY Omega, 50% YY Alpha
X X Y Y
O | XO XO O | OY OY
Y | XY XY Y | YY YY
Parents: XY Beta/OY Omega
Offspring: 25% XO Beta, 25% OY Omega, 25% XY Beta,
25% YY Alpha
X Y
O | XO OY
Y | XY YY
XX Alphas have eggs and/or uteruses and are therefore capable of two different types of fertility. Similarly, OY Omegas have sperm and are capable of impregnation. This allows for several more possible combinations:
Parents: YY Alpha/XX Alpha Parents: XY Beta/XX Alpha
Offspring: 100% XY Beta Offspring: 50% XX Alpha, 50% XY Beta
Y Y X Y
X | XY XY X | XX XY
X | XY XY X | XX XY
Parents: XX Alpha/XX Alpha Parents: OY Omega/XX Alpha
Offspring: 100% XX Alpha Offspring: 50% XO Beta, 50% XY Beta
X X O Y
X | XX XX X | XO XY
X | XX XX X | XO XY
Parents: OY Omega/XO Beta Parents: OY Omega/OO Omega
Offspring: 25% XO Beta, 25% OO Omega, Offspring: 50% OO Omega,
25% XY Beta, 25% OY Omega 50% OY Omega
O Y O Y
X | XO XY O | OO OY
O | OO OY O | OO OY
Parents: OY Omega/OY Omega
Offspring: 25% OO Omega, 50% OY Omega, 25% YY Alpha
O Y
O | OO OY
Y | OY YY
This gives us 16 possible pairings of parents, producing 64 possible offspring. Therefore, a reasonable approximation of the distribution of the six most common sex chromosome configurations among the human population would be:
XX Alpha: 14.06% (9 out of 64)
YY Alpha: 6.25% (4 out of 64)
XO Beta: 23.44% (15 out of 64)
XY Beta: 23.44% (15 out of 64)
OO Omega: 6.25% (4 out of 64)
OY Omega: 26.56% (17 out of 64)
However, such an approximation is only reasonable if we assume that all pairings occur with equal frequency. Cultural practices of restricting or even outlawing certain pairings while promoting or even forcing others have drastically skewed the distribution, especially within certain populations. But more on cultural practices later.
From the charts above, we can surmise that Dan in this story is an OY Omega and Phil is an XY Beta. Since Dan’s parents are an Alpha and Omega pair, his Alpha Mum is probably YY, and his Omega mum is probably OY. With Dan’s brother being an Alpha, he is most likely YY. Since Phil’s parents are a fertile Beta couple, one of them is probably XY while the other is XO. Since Phil’s brother is also a Beta, he could be either XY or XO, and the same is true of Cornelia.
The most possible options for Dan and Phil’s child would be XO, XY, OY, and YY. There is, of course, a possibility that any of these could be intersex, as briefly discussed above, leading to them having a configuration of chromosomes that is not one of the six most common types.
Gene Expression:
In our own universe, the various sex chromosome pairings (XX, XY, XXY, etc.) produce certain primary and secondary sexual characteristics. Such things as presence of a penis or clitoris, a vulva or scrotum, sperm or eggs, etc. are determined by the chromosomes a person does or doesn’t have. At the onset of puberty, secondary sexual characteristics typically develop, such as larger breasts, a deeper voice, or the growth of thick facial hair.
In general, the presence of a Y chromosome will lead to the growth of a penis, testicles, production of sperm, production of testosterone, etc. The absence of a Y chromosome leads to the development of a vulva, clitoris, uterus, ovaries, enlarged breasts, etc.
The three chromosomes of the Equilibrium Verse also code for various primary and secondary sexual characteristics. However, some of the genes encoded on each chromosome require activation from another set of genes on another chromosome to determine whether and exactly how they will be expressed. For example, the presence of a Y chromosome will typically lead to the development of a penis, but the genes which lead to the penis developing with a knot are only activated in the presence of another Y chromosome. Otherwise, they remain dormant, leading to the lack of a knot in XY and OY individuals.
Unlike in our own universe, though, an X chromosome also carries the necessary genes to spur growth of a penis and a knot. However, the genes that tell the fetus’s body to grow a penis and a knot are only activated in the presence of a second X chromosome. Otherwise, the genes remain dormant. Thus, XO individuals develop no penis, and XY individuals get their penis-growing instructions from the Y chromosome instead.
The X and O chromosomes both code for the development of a vulva, ovaries, eggs, uterus, etc. However, the X chromosome is dominant, so XO individuals receive the instructions for growing the genitalia and reproductive organs from the X chromosome. So, even though the O chromosome carries the code for heats, those genes are overridden and therefore not expressed in XO individuals. O and Y have shared dominance, as do X and Y. Because of the shared dominance of O and Y, OY individuals receive the instructions for genitalia and reproductive organs from both chromosomes.