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In most societies marriages between cousin are viewed negatively, due to a fear of "bad genes". But lets say I am a girl and I marry the son of my mother's brother (i.e. my uncle), than there is no gene similarity. Then I have two X-chromosomes, where one came from my father and the other one from my mother. My cousin (the son of my uncle) will have one Y-chromosome from his father and one X-chromosome from his mother (wh o is unrelated to me). My mother will also not be a carrier of my cousins Y-chromosome (since she is a female).

Does this all mean that this particular cousin marriage is unproblematic, and no different than a marriage between completely unrelated people?

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  • $\begingroup$ I tried to clarify your question, so that there are no confusion. If I have misunderstood you, please feel free to roll-back the edit. Your question is however based on a misunderstanding of human genetics (autosomal vs sex chromosomes), which @mattDMo has explained clearly. $\endgroup$ – fileunderwater Dec 9 '16 at 18:19
  • $\begingroup$ I think I am confused with X and Y chromosome and how gene overlap happen, according to my understanding X and Y has completely different genes , which is wrong $\endgroup$ – murmansk Dec 9 '16 at 19:18
  • $\begingroup$ It depends on the physical fitness of the parents. A village in italy has been studied because all 400 of its inhabitants descend from the same two people, and it has very low disease results and high longevity, whereas on average, places like SA and Pakistan have above median levels of club foot and genetic disorders from cousin marriage. pak has 55 percent marriage between cousins and 10-15 percent genetic recessive disorders more than non cousin marraiges. 4.4 percent higher mortality rates also from cousin marriages, which is on par with mothers over the age of 40. $\endgroup$ – com.prehensible Dec 27 '16 at 18:03
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The problem is that humans have 22 other pairs of chromosomes than just the sex chromosomes X and Y. Reproducing with your cousin (the son of your mother's brother is the son of your uncle which means your cousin) is generally looked down upon because of inbreeding.

Here's how it breaks down. Your mother and her brother (your uncle) each received 50% of their genes from your grandmother, and 50% from your grandfather. You received 50% of your genes from your mother and the other 50% from your father, who presumably is unrelated to the family. Therefore, you received 25% (50% of 50%) of your genes from your grandmother, and 25% from your grandfather. The same is true of your cousin - 25% from grandmother, and 25% from grandfather. The chances of some of your 25% and his 25% overlapping are rather high (for any one allele of a gene, it has a 1 in 4 chance of being the same as the cousin's). Combine that with 19-20,000-something genes, and that's a lot of overlap.

The problem with overlap/inbreeding is that any "hidden" (recessive) alleles may become homozygous. This may not be a problem for some genes, but a rather big problem for others (see my previous answer about Hunter Syndrome for an example.) There is a real potential of serious genetic diseases in any children resulting from such a relationship, which of course isn't fair to them.

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Simply put, if first cousins have children, there is an increased risk of birth defects in those children (as compared to children of unrelated couples). This has been shown by studies such as this one reported in a major medical journal.

There is a genetic basis for this, which is known as inbreeding depression. Basically, everybody has some genetic defects, but they generally occur in different places among the genes. However, in two closely related people, such as cousins, the defects are more likely to fall in the same genes because those people share some genes from the same source (e.g. the common grandparents).

It's true that certain kinds of cousins do not share either X or Y genes from a recent ancestor, but that has only a small impact. That's because almost all genes are on chromosomes other than X or Y, and are inherited by all descendents regardless of the sexes of the people involved. Therefore, the problem of birth defects (and miscarriages) occur for all kinds of first cousins.

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Welcome to Biology.SE!

In most of society cousins marriage [..]

Who cares about marriage in biology? We only care about reproduction. Marriage is a social construct that is mainly irrelevant here.

[..] is seen bad due to fear of bad genes

The term "bad" is poorly used for 2 reasons. 1. There are no bad genes but only a selective differential between two alleles of this gene (or at any other sequence for that matter). 2. It relationship of dominance and epistasis that causes inbreeding depression and not bad alleles.

I have "X" and "X" chromosome, X came from my father and X from mother son of my mothers brother will have Y chromosome and X from his mother which is not family and since my mother has nothing to do with "Y" chromosome.

That's one long sentence :) Who is "mother son of my mothers brother"? You might have a typo here.

Why do you talk only about "X | Y" chromosomes? "X | Y" is one of the 23 pairs of chromosomes we have (not mentioning mtDNA).

Does this all mean that this marriage is as good as between unrelated people?

Yes.

The whole question is to know whether the two parents are related (in biological terms). What I mean by related is not is they know each or if their respective siblings got a baby together but if they share a common grandmother for example. If you draw the genealogical tree of each parent and you see a recent shared ancestor between the two parents, then inbreeding depression can be an issue. Otherwise, it is fine. The reproduction events occurring between their respective siblings is irrelevant.

However, be careful if you intend to jump into societal consequence. Science will not tell you what is acceptable or not in a society. It won't tell you what is good and what is bad. Knowing that a couple have no risk of inbreeding depression may affect your view about this couple but it does not mean that science has demonstrated that the marriage should or should not be allowed (assuming the society has anything to say on that matter).

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    $\begingroup$ The OP seems to be asking if a marriage between cousins (or rather, reproduction) could be problematic from a genetic point of view (restated: what if I would marry the son of by mother's brother? see: "...marry son of my mother's brother...[...] Does this all mean that this marriage is as good as between unrelated people?"). Therefore, your second to last paragraph is misleading or can cause confusion. $\endgroup$ – fileunderwater Dec 9 '16 at 10:27
  • $\begingroup$ @fileunderwater i think you got my question, but answer I am not clear? $\endgroup$ – murmansk Dec 9 '16 at 14:30
  • $\begingroup$ @fileunderwater Can you please expand to why it is unclear. I edited it but I don't quite understand what may have been unclear to the OP. Feel free to edit yourself as well. Thanks $\endgroup$ – Remi.b Dec 9 '16 at 16:13
  • $\begingroup$ @remi.b The question is asking whether reproduction between cousins (the "this marriage" part you are quoting refers to a cousin marriage), and the two persons are therefore definitely biologically related. Your initial answer was saying that this wasn't a problem - probably due to misunderstandings caused by unclear language in the question - which is incorrect. $\endgroup$ – fileunderwater Dec 9 '16 at 18:01

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