Hi, I would like to marry my niece who is my mother's elder brother's daughter. I would like to know whether is there any problem will occur later if i marry her like getting abnormal childs? My friends inform me that i got high risk for getting this kind of problem.. Please advise? Thank You.
If you've explained the relationship correctly, this woman is your cousin, not your niece. There IS a significantly higher risk of genetic illnesses in children born of consanguineous marriages than in those born of non-related parents. (Belveccio, I hope you're joking, by the way - smoking is legal, but it is far from safe.)
As I guess you know, we all carry two sets of genetic code - one from our mother, and the other from our father. Every individual carries many faulty genes; a few may have arisen spontaneously, but the majority will have come from one of their parents. The reason these faulty genes usually cause no harm is that they are overridden by a healthy copy of the same gene inherited from the other parent. To take a totally arbitrary example, if I inherited a faulty gene causing blindness from my mother, it won't affect me because I also inherited a healthy copy of the same gene from my father, which stops the bad gene from affecting me.
HOWEVER... if my brother ALSO inherited the faulty gene from our mother (a 50/50 chance) and then he and I had a child, there's a one-in-four chance that the child would inherit the faulty gene from both of us with no "good" gene to cancel it out, and therefore our child WOULD be affected by the bad gene and would be blind. Conversely, if I had a child with someone unrelated to me the chances of their having the same faulty gene I have would be almost negligible, and therefore there would be almost no chance that our children would be affected.
The maths for cousins as opposed to siblings is a bit more complicated, but the risks are still very high compared to the risks you encounter in having children with someone you're not related to. Let's say your grandfather was carrying the gene that causes blindness; both your mother and and your uncle (this woman's father) have a 50/50 chance of carrying this gene too. That means that both you AND your cousin stand a one-in-four chance of carrying the gene, although it does not affect you as you both have healthy copies to cancel it out. HOWEVER... if you and your cousin were to have a child together, that child would have a one-in-sixteen chance of inheriting the faulty gene from both of you - that is, a one-in-sixteen chance of being blind.
Now if you consider that every person - everyone - carries dozens if not hundreds of faulty genes, there is a good chance that you and your cousin will have inherited a few of the same ones. If marrying within the family is common in your culture, in fact, and if any of your ancestors were also related to each other, it is likely that you and your cousin will share even more faulty genes. Mathematically, in fact, if the two of you have inherited only eight faulty genes in common (which perfectly possible if not likely)... that's a 50/50 chance for each child that they will have a genetic problem of some sort.
Genetic illnesses can range from almost imperceptible to extremely severe and even fatal, but there's no way to tell in advance what sort your child will inherit from you. If marrying your cousin is legal and would make you both happy then by all means go ahead - but if I were you I don't think I'd be having kids with her. That doesn't mean you can't have kids - you can adopt, or if you'd both be happy with it even use a third party to have children... but there's no doubt that your own biological kids would be a much greater risk of genetic illness than kids born of unrelated parents, and anyone who tries to tell you different or downplay that risk is lying to you.
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