Sibling gender (biological sex at birth)?

[ocwc]

@elam84 This is a topic I wondered about a lot when meeting other moms and seeing so many with 3-boy families. The odds are not exactly as one might expect, and even if the boy/girl split is 51/49, the odds do change slightly based on the sex of previous children.

This article has some interesting stats.

The odds of having a boy seem to increase after having girls, except after 2 girls, when a 3rd girl is more likely.

Odds of Boy with no previous children: 51%

Odds of Boy after 1 Girl: 54.5%

Odds of Boy after 2 Girls: 46%

Odds of Boy after 3 Girls: 52.7%

Similarly, a family is more likely to have a 3rd or 4th boy after two previous boys, rather than having a girl. Like 55/45 instead of 51/49 as the odds would have you expect.

 

[christiangirl4life]

@elam84 I don’t have the source but I read that if a male has more males in the family like brothers and male cousins they’re more likely to have boys and girls if they have more female relatives from their paternal side. The theory suggested they likely have a higher ratio of viable Y chromosome if it’s boys or x ones if it’s girls and that’s why certain families tend to have the same ratios generation after generation.

 

[ooconvert]

@christiangirl4life Ah. My husband is one of 3 brothers, 1 sister, his dad was 2 brothers, 1 sister, and every generation on his dad's we can trace back was all brothers. We've got a son. One of his brothers did have one of each. We'd also been hoping for one of each but sounds like it might be slightly more likely it will be another boy?

 

[katrina2017]

@christiangirl4life I'm not sure I get it. My husband comes from lots of boys (brothers, uncles, male cousins), whereas I have mostly sisters and aunts.

We have a girl. Does that mean our next is more likely to be a boy?

 

[christiangirl4life]

@katrina2017 Yeah based on the theory there maybe a genetic predisposition for males to have more of either X or Y carrying sperm. Since mens sperm is the deciding factor for the offsprings sex. Males that have more males within their immediate family and uncles/cousins from their fathers side are more likely to have boys. It’s likely that they have more sperm with the Y chromosome (X always comes from mom so combined XY would be a male). Men with more female relatives from their immediate family as well as aunts or cousins from their paternal side likely also carry more sperm with the X chromosome resulting in female offspring as XX is girls.

 

[denlhj]

@elam84 Sometimes the vaginal ph is such that one sperm type is more successful than the other. X sperm are hearty but slower; y sperm are faster but more frail. It's also not the "winning" sperm, but a sperm and egg that choose each other, so back to how people match biologically. Further, some drugs used by men can alter the odds: marijuana speeds sperm up, which makes Y sperm more likely to burn out and X sperm more likely to be selected since more of them survive.

 

[worldprayerchain]

@denlhj I thought the first bit of your comment had been disproven? Or at least in regards to attempting to conceive one sex over another

 

[denlhj]

@worldprayerchain Maybe! I did my last research on it almost 4 years ago before I had my own kid.

 

[coas93]

@elam84 Bryan Sykes looks at this a bit in '7 daughters of Eve' and 'Adams curse' . I haven't read either book in a long time, so I might be off base in my memory.

I'm pretty sure he theorised looking at genetic patterns of mitochondrial DNA (passed on female line) and y chromosome (male line) that it appears some mitochondrial groups favour female offspring and some less so. And Vice versa. Honestly, I can't remember the details of his theory or know if they've been backed up or disputed, though.

Anecdotally - in my family what ever you have first is what you'll have more of (either all that sex or more of that sex e.g. 2 or 3 boys and 1 girl). My husband's family is the opposite they all have a boy and a girl, then stop having kids.

 

[christian99]

@elam84 Looking strictly at the way gamete production happens, no, I can’t imagine there’s a pattern. Sex is determined by the sperm, and it’s a 50/50 on the baby’s sex.

Any patterns that are specific to certain couples can probably be explained by the timing of sex or something rather than something biological. Some studies suggest that female sperm are larger and move slower and stick around longer to await ovulation. So if a couple has sex shortly after the menstrual cycle is over and not very often otherwise, they may be more likely to have girls. But a couple who has sex every day probably levels the playing field for boys or girls. Editing to add the opposite example: if a couple only has sex on ovulation day, they may be more likely to have boys.

There’s a lot of chromosomal abnormalities and incompatibilities that could be at play too. Overall it reminds me of the gambler’s fallacy where they get several numbers in a row and either think they’re on a streak or that there’s no way they could get one more, but the reality is it has nothing to do with the previous number and it’s a fresh roll each time with new odds.

 

[bsniloy]

@elam84 I'd say there's no evidence of a pattern of boy after girl, girl after boy, etc. They say every child is independent on the prior and not effect the next- but I do wonder if some men have more x or more y sperm, to cause families of more heavily weighted genders to one side.

 

[hedowns]

@bsniloy The way sperm (and eggs) are made is that a cell with the full set of chromosome splits in half as part of normal cell replication but doesn’t end up completing the normal replication process. From a purely how-it’s-done perspective, men would end up with the same number of X and Y sperm. I suppose some men could have something going on that causes more of their Xs or Ys to be viable though.

 

[hedowns]

@drita Oh yeah, for sure. I was mostly trying to point out that a man wouldn’t “make” more X or Y.

 

[cutin]

@hedowns Yes, this. You can't just produce more of one type of sperm.

Some genetic defects are sex-linked and only affect one chromosome. The ones that we know about tend to be the survivable ones - it's likely there are others that we don't know about because they cause failed implantation/early miscarriage, both of which are common enough they are not usually investigated.

But in any case it's a rare/edge case, most cases of a man seeming to produce only one sex of offspring is simply a case of small sample size.

 

[heloise23]

@elam84 My entire family is incredibly girl-heavy and I always wonder what the scientific explanation is. My mom is one of 5 girls, there was one single boy born to my aunt 42 years ago but all the other cousins/every baby since him has been a girl. I have a daughter and all my (female) cousins have only daughters. I’d like one more and I will go into shock if it’s a boy

My husband is one of 4 boys with more boys being born to his grandparents as well, but his genes didn’t win

 

[gwc7223]

@heloise23 His genes always win on deciding the biological sex and yours technically have nothing to do with it.

Women always contribute the X because they only have an X to give to their offspring as their DNA is XX whereas men can contribute an X or Y as their DNA is XY.

Male babies are harder for a female to carry than a female because of the difference in sex hormones, etc. Secondly, a species needs less males because males can fertilize multiples females whereas a female can only procreate so many times in a lifespan.

I’d be curious from a knowledge standpoint if because the Y gene is getting smaller especially in comparison to the X if that’s why the Y gene is given out less.

 

[heloise23]

@gwc7223 Someone down thread posted an interesting study that had found that men have either more X, more Y, or balanced X/Y ratios in their sperm. Men in the study with more Y sperm in their samples were found to have more boys born in their family.

So, I’m assuming my husband has more Y sperm based on family history, but it wasn’t enough to defeat whatever is happening with the women in my family where we only give birth to girls.

This research also speculated that this explains the higher rates of baby boys born after a major war. Families with more sons (who are likely to create more Y sperm) are more likely to have a surviving son return from war and procreate. With a huge loss of the adult male population such as in WWII, it can have a big statistical affect of a higher ratio of boys born immediately after, and quickly rebalance the population.

There are 51 human boys born for every 49 girls, most of the time across all populations. So even though we need less men to perpetuate the species, more of them are born.

 

[worldprayerchain]

@gwc7223 This is true but we also know that the egg chooses the sperm. So, then, wouldn’t it be possible that the egg (mother) is actually the one deciding the biological sex? Or that it’s closer to 50/50 determinant?

 

[iamkingsamuel]

@elam84 I read that you are somewhat more likely to have the same sex child for your second born as you did for your first and that the third child goes back to 50:50 or close to it.

 

[emilylou97]

@iamkingsamuel This makes no sense. How and why?