Basically the Title. How are our body’s so good at regulating how much males and females are born, if they aren’t aware of the Balance at all?
In: Biology
It is just mathematics: the odds are 50/50 (because there is just 2 possibilities) so at the end it is the most probable that there will be almost half of one and half of the other
You mean in India where female babies are killed all the time?
Every normal body cell in a male mammal’s body has an X chromosome and a Y chromosome in it. Meiosis, the process that is used to create both eggs and sperm, first causes a cell to copy all its chromosomes (so it now has XXYY), and then divide twice. That results in two X sperm and two Y sperm. After sex, there’s a roughly 50/50 chance that an X-sperm gets to the egg first, rather than a Y-sperm.
It isn’t always the case. Many species have evolved so one gender, usually female, is entirely over-represented. In fact, there’s a species of lizard that has (recently) evolved to reproduce without males; the species is in a transitional period between sexual reproduction and asexual reproduction, with some females seeking mates and some just doing their own thing. Then there are anglerfish, with which the females are many times larger than the males, and there are many more males than females. That’s because when a male finds a female, he bites her body (about anywhere) and stays latched on like a parasite. Eventually their bodies grow together, and his fins and non-essentials atrophy, leaving only his reproductive system to fertilize the female. Female anglerfish have often been found with multiple males attached.
When our bodies make sperm and eggs, they don’t divide like the rest of the cells in our bodies. They split twice. A body cell has 2 chromosomes in a pair, they split once, so the new cell still has all of our genetic material in it. Splitting twice leaves half of our genetic material in each sperm/egg. Men have 2 different sex chromosomes called X and Y. Womens’ sex chromosomes are the same, called X and X.
In men, when our sex chromosomes split twice, this creates 4 sperm. 2 of them have an X chromosome, and 2 of them have a Y chromosome. When they meet up with the egg (which only has an X chromosome), the embryo has an equal chance of being XX (girl), or XY (boy) because half the sperm is Y and half are X.
Because the options are only 2, xx and xy, it’s like a coin flip, do it enough times and it averages out.
So there are two different way to look at this: first, how does it happen, and second, why does it happen.
In humans (and many species) it happens due to sex chromosomes. A cell has two copies of each chromosome, and for sex chromosomes in people women have XX and men have XY. Each egg or sperm gets _one_, in women that’s an X or an X, in men that’s an X or a Y. There are equal numbers of X and Y sperm produced because each sperm cell originally comes from an original cell that has both X and Y and divides to produce sperm. They get made in sets of four, two with X and two with Y. The X’s produce females, the Y’s males, providing a 50-50 ratio. The ratio at birth is a bit different (because male embryos are more likely to die) but it’s pretty close.
Now, not all animals do things that way, but nearly all have a 50-50 ratio, and for good reason. Imagine a situation where, for whatever reason, the ratio is imbalanced…a species naturally has a surplus of one sex. Any parent producing the usual ratio where most offspring are the dominant sex will have offspring which have a hard time finding a mate. But if any parent happens to have some genetic setup which allows them to produce more of the _rare_ sex, all their offspring will have an easy time finding mates and will therefore reproduce a lot…and will transmit that “have more of the rare sex” trait widely. It’s called balancing selection…basically, any individual with a tendency to produce more offspring of the rarer sex will leave more descendants. Over time this results in a balanced sex ratio.
So a lot of people have answered this question citing the mechanism as to how this occurs but have not actually answered the question as to why this mechanism exists in the first place, or as to why there is no bias in fertilization rates, or miscarriage rates. The answer is because a 50/50 split is the most evolutionary stable ratio.
Imagine a mutation occurs that slightly favours male births over females such that the ratio is 51/49. At first this mutation is completely benign, so it spreads through the population as it is not selected for or against via evolution. Fast forward a few generations and the population starts to have more males than females. Under these conditions, females are more likely to be reproductively successful due to the abundance of males, and males find reproduction harder. Thus the male bias mutation will be selected against, and an opposite female bias mutation will be selected for, thus the birth rates move towards to the 50/50 ratio again. Ergo, the population will inevitably end up at 50/50 regardless of any deviation from this balance.
A phenomenon like this is evolutionary stable. A mutation can’t thrive under these circumstances because it’s own success creates the seeds of it’s failure.
HOWEVER what if a species has a higher infant/juvenile mortality rate for one sex? We would expect the birth rates to be skewed towards that sex such that the species would be balanced 50/50 upon reaching sexual maturity. And lo’ and behold, this is exactly what we see in humans. There are on average 105 males for every 100 females born. So our ratio is NOT actually 50/50.
Your premise is not entirely correct.
The proportion of males to females in humans is slightly skewed. Approximately 51.9% male to 48.1% female.
Random chance says that we should be approximately 50/50 as sperm are 50/50 X vs Y, but real world statistics don’t line up this way.
Different countries have different skews, where some have more males while others have more females. A lot of this is just due to random chance, the rest is social factors.
Females have a longer life expectancy so females are disproportionately represented at greater than age 80.
Another big part of it is the preference for males in certain cultures. The push to have male children to continue the family line results in a lot of selective abortion, particularly in certain asian cultures.
It’s estimated that upwards of 130,000,000 women are ‘missing’ in our population because of it.
The 1 child policy in China alone, is estimated to have cost the lives of millions female embryos.
Well sex determination is different based on species, but for humans, it’s pretty simple. Sex is determined by the 23rd chromosome, Women have an XX and men have an XY. So it’s that second letter that matters. The mother’s egg cell always has an X chromosome, but the father can give either an X or a Y with his sperm cell, with roughly equal probability. A roughly equal number of male and female babies means a roughly equal number of adult men and women.
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