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I don't believe this question is suited for Biology.SE, Medicine.SE, or any other stack, since it involves a real mechanism which is not applied in real life. And not every question has to be about magic.

I would like to know if, by modifying their genes, do humans have a 100% chance of receiving those traits, or does it just increase the chance of getting them?

Say we modify a child with the genes to be really smart, will the child 100% be smarter than people without the gene, or will he just have a higher chance to be smarter?

Physically, does giving somebody the blue-eyes gene makes them automatically have blue eyes, or do they just get an increased chance of having blue eyes?

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    $\begingroup$ Look up "dominant and recessive genes." $\endgroup$
    – Daron
    Commented Jul 2, 2020 at 20:09
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    $\begingroup$ "The genes to be really smart": (1) We have no idea which genes are the genes to be really smart. (2) Are you certain that those genes don't do anything else besides smartness? Most genes have multiple effects, and there are well-known cases where the combination to get the maximum of one effect is severely detrimental for another effect, which could be rather important. $\endgroup$
    – AlexP
    Commented Jul 2, 2020 at 20:24
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    $\begingroup$ Intelligence is consisted of multiple traits govt by many diff genes, also excess neurons & synapses doesn't necessarily translate high IQ ;D $\endgroup$
    – user6760
    Commented Jul 2, 2020 at 23:37

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Chances

What you are looking for is suited for biology.se as well as here. That said: just because you have a gene does not mean you express it.

Wikipedia has an article on the regulation of gene expression:

Regulation of gene expression, or gene regulation, includes a wide range of mechanisms that are used by cells to increase or decrease the production of specific gene products (protein or RNA). Sophisticated programs of gene expression are widely observed in biology, for example to trigger developmental pathways, respond to environmental stimuli, or adapt to new food sources. Virtually any step of gene expression can be modulated, from transcriptional initiation, to RNA processing, and to the post-translational modification of a protein. Often, one gene regulator controls another, and so on, in a gene regulatory network.

Think of it: all your cells have practically the same genome, yet different cells express different genes. For example, your brain does not produce insulin.

Gene expression is also regulated by age (think of the hormonal changes in adolescents, and how lactose intolerance usually starts at least after weaning), and by the environment (you tend to produce more melanin in the skin when exposed to UV radiation).

Sometimes a gene will block another one - albinism and porphyry happen when a whole chain of genetic expression is blocked by one faulty gene.

Giving a child the genes for blue eyes may not necessarily work because those are recessive. So you need to make sure the child has all the blue eye alleles and that no other mutations are present. Genetic therapy on someone who has already been born may or may not work, but the eye color change would take time (I don't know exactly how long, though).

As for intelligence, that is a complicated one. Intelligence is not purely genetic - genes is a factor, but environment is a huge one too. And playing with just the genetic part might produce kids that are able to get high IQ scores, but which have little social intelligence.

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Depends

Blue eyes, yes. Intelligence, no. You can broadly divide traits into two categories - nature vs nurture, although a better term here would be internal traits versus external traits - that is, traits which come about through nothing more than the expression of genes versus traits that come about through interactions with external causes.

Eye color, for instance, is a 'nature' trait - it's inherently caused by developing and controlled by a strict set of genes. If you replaced all the eye color genes with the requisite color, you'll receive basically a 100% chance that the subject will have blue eyes. (Not 100% - phenotypical expression is a topic for another time, though.)

Something like intelligence is a bit different, because part of intelligence is learned. The same thing goes for, say, athleticism. If you insert the same genes as a fantastic baseball player into a embryo, but never teach the resulting human any kind of skills, they will not be good as baseball, because it's partially reliant on an external factor.

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  • $\begingroup$ My answer would have been very close to yours. I would have elaborated more on how the difference of how certain qualities are defined and how complex they actually are to achieve through genes. Prime examples would be simple genes like Mendel's peas and the traits he explored in his research to human height and how several different genes contribute to the overall result. The further explored how the environment can further effect the result, such as malnutrition during growth or pregnancy can result in the overall result, etc.. overall you summed it up that it's not always simple. $\endgroup$
    – V. Sim
    Commented Jul 2, 2020 at 20:17
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Genes are an ecosystem: Other than a few dominant genes with straightforward and unambiguous expression, your body can be thought of as an ecosystem for your genes. Each merely codes for a protein that may or may not be expressed (depending on other regulatory genes) and may or may not affect anything significantly (a gene for a light color in a person expressing genes for a dark color). Each gene is only relevant based on how its protein interacts with dozens, if not hundreds of other proteins. These effects also are highly variable dependent on when they are expressed. There are albinos with black hair and brown eyes, but because when they are babies they don't express melanin, they have defects in foveal development associated with albinos. A gene that would make a human 10% smarter might kill an animal due to brain pressure, or might be just as likely to make someone 10% less intelligent due to retardation associated with abnormal brain development, based on other genes and proteins. Sorry, evolution is a crapshoot.

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It does if it does.

I don't think there are particular "genes to be really smart". Certainly we haven't found them yet. Most likely it is a combination of a huge number of different genes, paired with upbringing.

That said there are certainly some combinations of genes that help. For example the combination of all human genes usually makes you smarter than having all dog genes.

We could genetically modify a dog to be a human and make them smarter. Then the human has human children who are also smart. So the answer is yes. But the question then be comes what else do you need to change.

On the subject of eye color, you want to look into dominant and recessive genes. Everyone has two sets of "eye color"* genes. If you have Blue|Blue then you have blue eyes. If you have Blue|Brown or Brown|Brown you have brown eyes. Every blue eyed person is Blue|Blue but it's impossible to tell by looking whether a brown eyed person is Blue|Brown or Brown|Brown. They child takes one at random from each parent. So two Blue eyed parents will have a blue eyed child but otherwise there is no way to tell.

*But this is not the only thing that determines eye color!

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    $\begingroup$ Eye color in humans is polygenic... As easily seen in the large spectrum of possible colors -- dark blue, watery blue like English people, green, gray, hazel, brown, dark brown; and in the existence of people with party-colored eyes. That is, the are multiple genes involved in determining eye color. Two people with blue eyes can have child with blue, green, gray, or even (shudder) brown eyes. $\endgroup$
    – AlexP
    Commented Jul 2, 2020 at 20:20

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