December 14, 2008

Genetic predisposition for fathering sons

A good explanation from BBC News:

Newcastle University researchers found men were more likely to have sons if they had more brothers and vice versa if they had more sisters.

They looked at 927 family trees, with details on 556,387 people from North America and Europe, going back to 1600.

The same link between sibling sex and offspring sex was not found for women.

...

In the years after World War I, there was an upsurge in boy births, and Dr Gellatly said that a genetic shift could explain this.

The odds, he said, would favour fathers with more sons - each carrying the "boy" gene - having a son return from war alive, compared with fathers who had more daughters, who might see their only son killed in action.

However, this would mean that more boys would be fathered in the following generation, he said.

Evolutionary Biology doi:10.1007/s11692-008-9046-3

Trends in Population Sex Ratios May be Explained by Changes in the Frequencies of Polymorphic Alleles of a Sex Ratio Gene

Corry Gellatly

Abstract

A test for heritability of the sex ratio in human genealogical data is reported here, with the finding that there is significant heritability of the parental sex ratio by male, but not female offspring. A population genetic model was used to examine the hypothesis that this is the result of an autosomal gene with polymorphic alleles, which affects the sex ratio of offspring through the male reproductive system. The model simulations show that an equilibrium sex ratio may be maintained by frequency dependent selection acting on the heritable variation provided by the gene. It is also shown that increased mortality of pre-reproductive males causes an increase in male births in following generations, which explains why increases in the sex ratio have been seen after wars, also why higher infant and juvenile mortality of males may be the cause of the male-bias typically seen in the human primary sex ratio. It is concluded that various trends seen in population sex ratios are the result of changes in the relative frequencies of the polymorphic alleles of the proposed gene. It is argued that this occurs by common inheritance and that parental resource expenditure per sex of offspring is not a factor in the heritability of sex ratio variation.

Link

16 comments:

Thras said...

"The odds, he said, would favour fathers with more sons - each carrying the "boy" gene - having a son return from war alive, compared with fathers who had more daughters, who might see their only son killed in action."

The point being that genetics researchers can't do even basic math?

Dienekes said...

Please elaborate.

Thras said...

Imagine that 5% of the male population have some gene that causes them to have more sons than average. Kill off 50% of males at random. Now what percentage of the male population has the gene?

It's still 5%.

Maju said...

I fear Thras may be right: maybe the odds for an individual father are of (a) all or nothing or (b) one among several. But the overall odds are that men carrying either version of the gene will die at similar proportions in the war, so the result should be the same.

Anyhow the important part of the finding is more just that some men are much more prone to have only sons, while others to have only daughters - and the majority being neutral). While women also transmit both genes, these are not active in them, so they play no direct role in this peculiaraity (only via their sons, or other male descendants).

But I can't see any reason why this curiosity would have implcations in the statistical whole, increasing or decreasing the total number of boys born, but it seems to affect only the individual families those men father.

Now, it certainly seems to have implications for the field of population genetics, specially in regards to Y-DNA transmission in the mid and long run, providing an extra reason for male lineages to suffer much larger drift than female ones. Or in other words: it was not just male promiscuous tendencies after all... but also this curious in-built mechanism.

Actually this mechanism basically says that if, in an ideal population, there are four fathers (MM, MF, MF and FF), each one with a different clade and all with the same (again ideal) ammount of offspring... in the next generation, one of the lineages (FF) is out and another one (MM) has doubled its proportion. The accumulative effect of this phenomenon should be brutal.

In fact, on second thought, it seems way too brutal. And makes me wonder what apportion of men actually have the MM and FF genes, and if it's not something much more minoritary than what simple mendelian genetics have made me (and the media) suppose. If so, this would mean that there are more than just two alleles or one single locus acting here.

Does anyone know or has a good idea?

eurologist said...

Seems to me this drives wars and drives men away from home in cultures with smallish groups.

If it were not for war (which has dominantly affected men both in historic times and in prehistory), and if it were not for the added risk of life and travel alone or in a tiny group of men, no matter how small the initial percentage of this gene, it would soon dominate.

However, in societies with small groups at large distances, and in times of intermittent famine, this strategy is counter-productive, since it soon leads to a lack of women, and thus offspring. Seems to me it can also be advantageous to your number of (even male) offspring if your genes make sure there are enough women around to reproduce, in the future...

Maju said...

However, in societies with small groups at large distances, and in times of intermittent famine, this strategy is counter-productive, since it soon leads to a lack of women, and thus offspring.

Why? It could also mean lack of men.

Actually I find is that this phenomenon seems raher against partilocality, because you may end up with a clan (all syblings or cousins by male side) that only produces women. Instead in matrilocality the effect of this anomaly would be null or nearly so.

coraldo said...

As far as I am aware, the author of the paper addresses the point that Thras makes. The models assume that mortality of males is not completely random, but evenly distributed among families. I noticed this quote: "removing a single son from a family with two sons removes 50% of their sons, whilst from a family with five sons, it removes 20%. Across a population, this translates to a greater loss of males from families with less sons".

pconroy said...

I suggest reading:
http://en.wikipedia.org/wiki/Trivers-Willard_hypothesis

As being a better explanation of the phenomenon!

Maju said...

As far as I am aware, the author of the paper addresses the point that Thras makes. The models assume that mortality of males is not completely random, but evenly distributed among families. I noticed this quote: "removing a single son from a family with two sons removes 50% of their sons, whilst from a family with five sons, it removes 20%. Across a population, this translates to a greater loss of males from families with less sons".

But does that make any sense at all? In fact the odds of death and survival for each young conscript man are the same, no matter if he has all brothers or all sisters. Unless the conscription system is based on families and not individuals I can't make any sense of this logic. If anything I'd expect families with only one son to make a greater effort to get him to avoid being conscripted (and having only one male son they could potentially dedicate more resources to that single effort).

In general the odds of any man in military age of dying in the war are statistically the same and these odds are totally independent of how many brothers or sisters he has.

I suggest reading:
http://en.wikipedia.org/wiki/Trivers-Willard_hypothesis

As being a better explanation of the phenomenon!


It's just a hypothesis and, for what I can gather from many poor populations, it doesn't seem to be present in reality. Overall, at least in patriarchal cultures, sons are seen as a productive investment for anyone (they are expected to be able to work and help the family and also seen as heirs), while women are percieved as a less clear investment (and in cases like India as a mere burden).

In any cases, providing no explanation on how the hypthesis may work, it really doesn't shed any light on the matter.

pconroy said...

Maju,

You are correct that the mechanism by which well nourished mothers have more sons has not been identified yet - to the best of my knowledge - but that doesn't invalidate the hypothesis.

In the so called "Extended Trivers-Williard Hypothesis", it is further hypothesized that if parents have qualities which would lead to more reproductive success in male offspring - then they will have more male offspring. So for instance it is a fact that Engineers tend to have more sons - as a systematizing mind is more beneficial to sons than daughters.

Whereas if parents have qualities which would lead to more reproductive success in female offspring - then they will have more female offspring. It has been noted that beautiful people tend to have more daughters, etc...

Maju said...

So for instance it is a fact that Engineers tend to have more sons...

Is it? Can you document it? In my experience that trend is not real: there are many engineers and technical engineers in my family and while one does have all sons, several others have all daughters instead.

And, btw, women are sometimes much better engineers than men: I'm 100% sure that my sister would have made an excellent engineer would she have been "allowed" to follow that career, the same she is now a very good informatic. Instead my brother, not even remotely as smart as she is, makes just a mediocre engineer.

In any case, such a hypothesis would need much research and hard data even to be considered seriously, IMO. It's not the "how" but wether if the very assumtions of the hypothesis have any connection with reality.

pconroy said...

Maju,

Here is the reference and supporting data for the fact that "engineers have more sons and nurses have more daughters":
http://www.lse.ac.uk/collections/MES/pdf/JTB2005a.pdf

Interesting reading!

pconroy said...

Also check out:
"Violent men have more sons: Further evidence for the generalized Trivers–Willard hypothesis (gTWH)"
doi:10.1016/j.jtbi.2005.08.010

Abstract:
The generalized Trivers–Willard hypothesis (gTWH) [Kanazawa, S., 2005a. Big and tall parents have more sons; further generalizations of the Trivers–Willard hypothesis. J. Theor. Biol. 235, 583–590] proposes that parents who possess any heritable trait which increases the male reproductive success at a greater rate than female reproductive success in a given environment have a higher-than-expected offspring sex ratio, and parents who possess any heritable trait which increases the female reproductive success at a greater rate than male reproductive success in a given environment have a lower-than-expected offspring sex ratio. One heritable trait which increases the reproductive success of sons significantly more than that of daughters in the ancestral environment is the tendency toward violence and aggression. I therefore predict that violent parents have a higher-than-expected offspring sex ratio (more sons). The analysis of both American samples and a British sample demonstrates that battered women, who are mated to violent men, have significantly more sons than daughters.

Maju said...

Sorry but why does that study use "unstandardized regression coefficients" instead of just indicating the raw average numbers/proportions of sons and daughters? It makes it only interpretable by people with advanced mathematical-statistical knowledge.

Personally I can't gather anything from that paper: it's esotheric - what surely means its "eruditism" hides obvious errors.

I tried to find out about the concept and, apart of long mathematical formulas along terms like "assumptions", "estimator" found phrases like "it is never possible to include all possible confounding variables in a study employing regression" or "For example, recent work published in the Journal of Geophysical Research used regression models to identify data contamination, which led to an overstatement of global warming trends over land".

It would be a lot simpler to just add up fractions. For example, if I consider my generation and their fathers in my own extended family, I get that:

Engineers:
-Sons: 0.42
-Daughters: 0.58
Physicians:
-Sons: 0.00
-Daughters: 1.00
Economists:
-Sons: 0.50
-Daughters: 0.50
Unfinished career:
-Sons: 0.60
-Daughters: 0.40

While my sample is way too limited at least my figures add up to 1 (or 100%). Curiously my sample shows that engineers and physicians appear to have more daughters than the rest. Physicians are under-represented but engineers make up about half of the whole sample.

In any case, give me the raw stats, not pseudo-erudite mathematical estimations based on uncertain asumptions. Just tell me please what proportion of sons and daughters each category has (probably all close to .5 anyhow).

pconroy said...

Maju,

Correct me if I'm wrong, but AFAIK scientists use Regression Analysis, to remove "noise" from the data.

In this case being an IT engineer, as I am, would lead me to have more sons, but that is confounded by the nutritional status of my wife - so if she is skinny, which she thankfully isn't - that might bias her in the favor of having daughters, and cancel out my propensity to have sons, and so forth...

BTW, we just had a son, and my wife's sister who is married to an Electrical Engineer, has 2 children, both sons also - so there's a few data points for you.

Maju said...

In this case being an IT engineer, as I am, would lead me to have more sons, but that is confounded by the nutritional status of my wife - so if she is skinny, which she thankfully isn't - that might bias her in the favor of having daughters, and cancel out my propensity to have sons, and so forth...

That random noise actually should be easily removed by mere sheer size of the sample (apparently a whole national census was used for that - though it's not wholly clear).