Take the 2-minute tour ×
Biology Stack Exchange is a question and answer site for biology researchers, academics, and students. It's 100% free, no registration required.

It is said that genes define how we look physically and psychologically; so for example if a male human sperm carries a certain number of genes, say x, and a female carries y, then mathematically there would be m genes from the male and n from the female, and a finite total available. Could there be people identical in appearance if the same genetic combinations recur?

Also we find similar-looking people with perhaps similar behaviour accross diferent geographic locations; does this means that they have similar genetic make-up, up to some percentage?

Please correct my biology and share the possiblity of having genetically identical people across different geographic locations.

share|improve this question

closed as too broad by Daniel Standage, biogirl, Amory, jonsca, fileunderwater May 19 at 13:58

There are either too many possible answers, or good answers would be too long for this format. Please add details to narrow the answer set or to isolate an issue that can be answered in a few paragraphs.If this question can be reworded to fit the rules in the help center, please edit the question.

1  
Please see this question and answer (biology.stackexchange.com/questions/10474/…) –  Bez May 17 at 0:21

2 Answers 2

up vote 2 down vote accepted

For ease of explanation let me first tell you that the "genetic makeup" is called "genotype" and that "how we look physically and psychologically" is known as "phenotype". These are not complete definitions of the terms but this should help you understand what we're talking about. I also apologise for the lack of detail or inaccuracies which many fellow contributors may find aggravating, but the question calls for keeping things down to the most important aspects :)

Do genes define how we look?

Genotype defines to some extent the human phenotype, but we don't know exactly to what extent regarding most aspects of the phenotype. This is called the "nature vs. nurture" debate and is concerned with the effects of genotype itself on phenotype vs. the effects of non-genetic influences after fertilisation (such as chemical surroundings during embryonal growth, experiences at young age etc).

Numbers of genes from males and females

Another misunderstanding is that a sperm carries a certain number of genes which it combines with a number of genes from the egg. While this is strictly true, the number isn't what matters because we all carry the same number of genes (*). The genotype doesn't depend on the number of genes, but rather, almost all genes exist in several variants within the human population. So the genotype is actually about which variants an individual carries, not about the number of genes. In biology, we use the term "allele" to refer to variants in this context. So in other words, each gene can have different alleles.

People with identical appearance

If two people have identical alleles for all genes (i.e. identical genotype), then they can indeed look almost identical. However, because of the vast number of genes and alleles present in the human population, it is practically impossible for the same alleles to come together in two humans. With only one exception: if two individuals result from the same combination of sperm and egg cell, they will be identical, and this is exactly what happens in monozygotic twins.

Similarities by geographical region

However, the total number of genes and alleles is limited (**). This concept is what people call the "gene pool" and is the reason why humans are still always humans and share certain similarities. In particular, for most of human history our geographical movement has been limited (most of the time), creating geographical "sub-gene pools" (to keep it simple). This is why people from the same geographical region are more likely to look similar; they have a higher chance to share the same alleles of genes that determine appearance. In terms of behaviour - to whatever degree genotype influences behaviour, peopel from the same geographical region will be more likely to be similar in these regards than people from different geographical regions. However, you can now hopefully understand why this "degree of similarity" is next to impossible to quantify (lack of knowledge: which genes determine what, how many alleles exist of them, how many people have which allele, how much and how far do people travel,...).


(*) Ok, not quite - it is possible for an individual to have the odd gene actually completely missing, but for the sake of simplicity we all have the same number.

(**) Again, not strictly, because new alleles and possibly even entire genes can result from mutation, but this is relatively rare and on the large scale you can work on the assumption that they are limited in number.

share|improve this answer

It is said that genes define how we look physically and psychologically

I would be skeptical of anyone saying this. Nature vs. nurture is far from settled, especially in psychology. The trend seems to be that many behavioral traits have a large heritable component, but are far from being determined only by heritable traits.

if a male human sperm carries a certain number of genes, say x, and a female carries y, then mathematically there would be m genes from the male and n from the female

I think you are confused. If a sperm carries $x$ genes and an egg carries $y$ genes, the resulting zygote will have $x+y$ genes. If the male had $m$ and the female had $n$ genes, $x+y\approx m+n$ and $m \approx 2x$, $n \approx 2y$.

Could there be people identical in appearance if they end up with identical combinations of genes?

Yes, we call these people identical twins. It could also occur by chance in unrelated individuals (well, everyone is somewhat related...) but this is extremely rare. There is just that too much variation in the human genome. There are literally tens of millions of points in the genome where some people have one version and others have another, and this person would have to happen to have the exact same version at each one as you do. This is an order of $10^{-8}$, if we consider humanity completely homogenous (and it isn't, so the probability is even lower).

we find similar-looking people with perhaps similar behaviour accross diferent geographic locations; does this means that they have similar genetic make-up, up to some percentage?

The genes may be different and still lead to the same phenotype. Say one person has the set of genes that result in red hair, another one has the set that results in brown hair. They are both albinos. They'd look the same, but have different genes.

Also, genes are not the only thing that determines how people look or behave.

So the fact that two people are phenotypically identical does not imply they are genetically identical. You can come up with a Bayesian probability that says they are more likely to have similar genes if they look similar, but how do you calculate the priors? Worse, how are you even going to measure how "similar" people look, or behave?

share|improve this answer

Not the answer you're looking for? Browse other questions tagged or ask your own question.