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unbalanced gender ratios of offspring

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unbalanced gender ratios of offspring

Postby TYLS » Thu Jun 13, 2013 11:11 am

hi,

i've read that the natural sex ratio at birth is 105 boys to 100 girls. this is counter-intuitive since, assuming no other factors are in play that determine sex ratios besides the random sorting of our chromosomes, we would expect a 100:100 boy girl ratio.

is there a variation of kin selection that affects the sex ratio? it makes good sense for there to be more boys than girls since the boys are more prone to early death as a result of their recklessness. But how can this translate, biologically, to the higher number of boys being born?

thanks for your help!
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Postby JackBean » Thu Jun 13, 2013 7:01 pm

I think I've heard once this is because the sperms with Y chromosome are little lighter and thus faster. But I don't know whether is it really true.
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Re: unbalanced gender ratios of offspring

Postby TYLS » Sun Jun 16, 2013 2:27 pm

hi thanks for your reply jackbean. i've never considered that before! it is plausible but then again if that were the reason, there isn't really much of a link between the necessity to produce more males and the ability to do so. it would be more of a naturally occurring phenomenon.

i'm still pretty puzzled over the relationship between the necessity to produce more males and the natural ability to produce more males.

in my opinion it's safe to assume that our natural sex ratios favouring males is derived, directly or indirectly, from the necessity to produce more males. but the biological mechanism behind this whole process of integrating the necessity into our genes is puzzling. this is not an adaptation that would benefit individuals alone, but is something which benefits the population instead. hence it should not be due to natural selection.

i've read about kin selection and how that worked for colonies of bees and ants. is there a similar mechanism in play?
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Postby JackBean » Mon Jun 17, 2013 7:09 am

and why do you think there is necessity to produce more males?
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Postby biohazard » Tue Jun 18, 2013 11:51 am

All in all, it seems paradoxical that (in most mammals) the sex ratio is close to 50:50, since only a fraction of the males would suffice to impregnate all the females - and this is actually what happens in many mammals that collect harems of females to themselves and where most of the males never get to copulate at all. In mainly monogamous mammals, such as humans, it makes more sense. And if we consider that human males are much more prone to accidents and violent death and certain hereditary diseases, maybe that extra few males per 100 women is justified from the evolutionary point of few.

But apparently the ~50:50 ratio is there to cause competition; to really ensure that there is a fight over the females and only the fittest ones get to breed. Then again, why not make the females even fewer in order to heat up the competition even more? Surely 1 female per 10 males, for example, would ensure that only super-fit males manage to breed? I do not know what is the exact reason for even sex rations, but reliance solely on the meiotic division sounds like a suspiciously simple answer, since nature would have surely found a means to alter the sex rations if there was a need for that - so why is the ~50:50 ratio maintained? This has puzzled me many times, so if there is any evolutionist well versed in this matter do not hesitate to step forward! ;)
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Re: unbalanced gender ratios of offspring

Postby Amphibitile » Sun Jun 23, 2013 8:01 pm

The mechanism of sperm fertilisation, with the y chromosome being lighter I don't know the validity of, but it seems a reasonable presumption.

I personally think that the ratio is perhaps due to the preservation of genetic diversity. Most mammals tend to be polygynous, we can be either of polygamous or monogamous depending on the society and culture. I think the majority of the human population is monogamous like you say though, however I am not sure on this.

Intraspecies comparison is quite difficult as each species tend to have their own unique selection technique, as you said Harems being one of the methods. Homo sapiens have been an extremely successful species so far, proving over the last 6000 years or so great developmental and survival methods. One male impregnating several females reduces genetic diversity in comparison to several males producing with several females. If we take a genetic example:

One male - TT (homozygous dominant) Breeding with several females lets say they're all Tt.
Production: TT x Tt - 2 possibilities

However an array of males TT, Tt, tt all breeding with these females increases the chance of variability within a population, hence preserving genetical diversity rather than hindering it. My thought becomes more apparent the more genes are added to the example.
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Postby wildfunguy » Sat Sep 07, 2013 10:47 pm

If I want my chromosomes to get intermixed, I'll want offspring of the sex of which there are fewer. Suppose there is currently one female and ninety-nine males. If I have a male offspring, his chromosomes can only mix with the chromosomes of that one female, assuming she's even interested in him.
If I have a female offspring, she'll have up to 99 options. With any single partner, several matings is probably enough to get most of his chromosomes. After that, she'll want to get some different chromosomes from someone else.

Amphibitile wrote:I personally think that the ratio is perhaps due to the preservation of genetic diversity. Most mammals tend to be polygynous, we can be either of polygamous or monogamous depending on the society and culture. I think the majority of the human population is monogamous like you say though, however I am not sure on this.


Even if primates organize into polygynous groups, the females probably won't remain faithful.
Furthermore, nobody is alpha male for life. They get old, weak, and frail, and some young guy kicks their groin.

Amphibitile wrote:One male impregnating several females reduces genetic diversity in comparison to several males producing with several females. If we take a genetic example:

One male - TT (homozygous dominant) Breeding with several females lets say they're all Tt.
Production: TT x Tt - 2 possibilities

However an array of males TT, Tt, tt all breeding with these females increases the chance of variability within a population, hence preserving genetical diversity rather than hindering it. My thought becomes more apparent the more genes are added to the example.


How about this proof? Imagine that we have a total of 100 individuals in the population.
1 females, 99 males; 1 · 99 = 99 genetic permutations in the next generation
2 females, 98 males; 2 · 98 = 196 genetic permutations
3 · 97 = 291
4 · 96 = 384
5 · 95 = 475
Let's skip ahead.
45 · 55 = 2475
46 · 54 = 2484
47 · 53 = 2491
48 · 52 = 2496
49 · 51 = 2499 - approximately our ratio
50 · 50 = 2500
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