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Post No.: 0466heritability

 

Furrywisepuppy says:

 

The interaction of both genetics and environmental factors – such as the amount of nutrition received when young or even when still in the womb, humans preferring to hunt larger prey to extinction, or the amount of oxygen or gravity present – play a role in determining a creature’s size. If people keep poaching the biggest and ‘strongest’ elephants for their tusks then unfortunately that’s selection at work too. (Post No.: 0447 explored the pros and cons of being big or small.) Our conceptions of what it means to be the ‘fittest’ aren’t necessarily nature’s. (Humans should definitely stop poaching rare animals in my opinion though!) Malnourishment, leading to a stunted development, both mentally and physically, is certainly unfortunate for any offspring.

 

Genetic studies tell us that human height has a heritability of ~80%, but while over 40 genes have been identified as contributing to differences in height – taking all these together still accounts for only 5% of the differences between people, and not the maybe 30% or more range of difference of height amongst the world’s population that is evident in reality. This may be down to the problem of ‘missing heritability’?

 

Well it’s certainly evident that diet plays a significant role – within the human species overall, people have grown taller over the past century as a result of greater access to nutrition. We find that when oriental children adopt a diet with increased meat consumption and calories, like diets in more western cultures in general, they grow taller on average.

 

However, shorter people appear to have longer average life spans – albeit gender might explain much of this since women are shorter than men on average. Over-consumption can also lead to obesity and obesity-related diseases. For example, the traditional Okinawa diet is incredibly healthful, with lots of vegetables and soy products, and has been credited as the main reason for the exceptional longevity of its locals; but cultural imports and influences from the USA after WWII have dramatically reduced the life expectancies of the more recent generations of people there, starting with the males. Why women live longer than men on average is likely to be a combination of biological and behavioural factors. But research for firmer answers continues in this area.

 

On average within the population, a person’s level of intelligence is primarily genetically inherited, but someone who experiences a serious brain trauma or abusive event could have an intellectual level that’s far more heavily down to his/her environmental influences. Therefore heritability is not a fixed estimate. Heritability will vary from individual-to-individual within a population, and from time-to-time and culture-to-culture too (e.g. due to contextual events such as public spending boosts/cuts on education, war or famine). It will even vary as people age. For instance, the heritability of general intelligence (g-factor) actually increases as people age – IQ seems to become more influenced by a person’s genetics than their environment once they grow into an adult and leave the home i.e. the effects of parenting and other environmental factors appear to diminish as children age. Although the reasons for this are unclear, these are important things to note.

 

It’s also important to note that we cannot reliably determine what genes, or therefore genetic traits, we’ve inherited from our biological parents just by looking at the traits that our own parents exhibit. This is not just because of receiving different environmental factors to our parents, but we only inherit 50% of our genes from each of our parents and we don’t know which 50% from whom and how our unique combination of these genes interact with each other and with the environment (e.g. we cannot just say that both parents are athletes so their child will definitely become athletic too, or that even if their child does become an athlete too then whether this was more down to his/her genetic inheritances or the support and expertise that both his/her parents granted to him/her on his/her own sporting career?) A 50% chance of inheriting a direct copy of a gene from a particular parent is like flipping a coin i.e. hardly guaranteed. And then genetics in total only roughly overall accounts for half of a person’s predicted traits and outcomes.

 

I digress. Heritability can change over time, which sounds incredibly strange, as if people’s genes change over time – but many people misunderstand how the science of heritability is conducted and therefore misunderstand the nature of these results. We can only currently tell the differences in concordance rates between monozygotic (identical) twins, dizygotic (non-identical) twins and non-twin siblings and then try to infer that these differences are likely down to genetics or environmental factors. For complex constructs such as intelligence or sporting ability that involve multiple genes and environmental factors interacting in complex ways, we cannot easily tell whether the traits of a particular person is x% down to genetics and is or was y% down to his/her environment. How much the environment plays or played a role – and therefore by inference how much genetics plays a role – varies depending on what specific environmental factors are present, hence the absolute impact of nature versus nurture cannot be pinned down. It can only be inferred relatively and dependently upon the specific environmental conditions compared.

 

Perhaps heritability estimates are metaphorically like trying to estimate the build value of a house based on its market value – the market value is affected by things like flight paths and current demand, hence isn’t fixed, and therefore what’s inferred to be the build value, which is believed to be (broadly, or at least relatively more in this case) fixed, won’t actually be fixed when it comes to trying to estimate it.

 

If we compared humans with other animals, we’d say that genetics plays an overwhelming role in explaining the differences in intelligence, but when we compare humans with other humans then genetics plays a relatively less significant role, hence estimates of the impact of nature versus nurture depend on what we’re comparing to, and depend on what specific environmental factors we’re comparing between too. The limitations in this field of research are due to it being totally unethical to take babies right from birth (or prenatally) then randomly placing them in tightly-controlled environments until they’re old enough to complete the tests we’re interested in.

 

It could be inferred that twins who come out with concordances, or similarities, later in life – despite being raised apart – must therefore share genetic inheritances for these similarities. But it could still be down to environmental factors because twins have of course shared a womb together (although not necessarily an amniotic sac or placenta) and therefore similar environmental pregnancy conditions as each other that could explain their similarities? We can get around this by comparing dizygotic twins with non-twin siblings (both sets of pairs on average share the same percentage of genes with their twins/siblings), where dizygotic twins have shared the same pregnancy conditions as each other but non-twins have not, hence any differences between these two sets aren’t likely to be genetic (because the differences in genetics on average will be cancelled out). Both monozygotic twins and dizygotic twins share a pregnancy hence any differences between these two sets are likely to be genetic (because the differences in prenatal environmental conditions will be cancelled out).

 

Still, genes do not ever mean non-malleability or a strict destiny when it comes to behaviour – genes need an obligate environment to express. No behavioural trait is 100% determined by genes alone, hence (sometimes quite large) mental and physical differences, as well as life outcomes, between even identical twins. In reality, many genes set up probabilities rather than are absolutely deterministic on their own. The left and right sides of your own face and body are slightly different, such as the fingerprints or palm lines of both your hands not being exact mirror images of each other i.e. the exact same genes even in the exact same person can end up exhibiting different outcomes! Genes and environment both combine to create an outcome.

 

Genetics interact complexly with the environment, such as from the parenting, peers, teachers and culture, to magnify or attenuate certain phenotypes. Epigenetics is a mechanism for how the environment, individual life experiences and age can directly affect the expression (the switching off or on) of genes themselves, possibly even transgenerationally (across generations via the germline) too – so not only do our genes partly shape the environment we personally inhabit but the environment can personally affect our genes too. Epigenetic modifications, such as via DNA methylation, change over time as people with idiosyncratic lives consume different diets, do different things and face different life events – thus environmental influences should really hypothetically increasingly play a more key role over time in affecting which genes become or remain expressed in a person’s life.

 

Well according to what is known so far, both genes/nature and environment/nurture roughly play an overall equally important role as each other. One example is the fact that you were born as a human rather than a slug genetically, and the way no matter how hard anyone tries to teach a slug to read and write it won’t be able to, which means that genetics plays a key role in intelligence (as defined by reading and writing ability at least). But if a human were raised in and lived in an under-stimulating white box environment for all of his/her life, then no matter how genetically gifted this human is, he/she won’t become very intelligent, which means that environment plays a key role in intelligence too. Thus, with few exceptions, it’s actually nature and nurture, not nature versus nurture.

 

On the one paw, people are on average 99.9% genetically identical with each other, so the differences between everyone must be importantly and significantly down to their environments rather than genes; yet on the other paw, that other 0.1% is about 6.4 megabases of potential difference. How big is a ‘big’ difference? Well we judge based on what we’re comparing to, and when compared with other humans, a human may think that humans are highly varied, but when compared with the animal kingdom as a whole, humans are quite similar with other humans; true to a virtual homogeneity with each other. For instance, we don’t get some people who, as a genetic instinct, ‘moo’ or ‘oink’, build nests out of twigs, feed their offspring through regurgitation, kill their own less-strong offspring or sniff other people’s butts to judge them (erm, woof!) Humans have ‘human’ brains and body parts and do ‘human’ things. From my furry perspective, people are far more similar to each other than are different, which confuses me when I see groups of people go to war against each other. If it’s about genes then they’re basically willing to kill what’s on average 99.9% similar to them in order to eradicate that 0.1% that’s different(!) People are all far closer to being family than aliens or outsiders to each other but they don’t always see themselves in this way.

 

Anyway, we’ve found out that heritability doesn’t necessarily mean fixed. For instance, height is highly heritable yet average heights have increased over the past century due to changes in diet and healthcare i.e. changes to the environment. Childhood nutrition, prenatal conditions and infection history can all affect height noticeably, for example. Heritability can also change with place, time and even age.

 

Woof.

 

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