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Q: genetic inheritance in humans ( Answered,   0 Comments )
Subject: genetic inheritance in humans
Category: Science > Biology
Asked by: dnadna-ga
List Price: $25.00
Posted: 15 Oct 2003 10:18 PDT
Expires: 14 Nov 2003 09:18 PST
Question ID: 266526
Are there any research studies in animals or humans which show that
breeding between siblings or cousins produces genetic defects?
Are these studies the basis of legal prohibitions on marriage between
siblings and cousins?
Subject: Re: genetic inheritance in humans
Answered By: alanna-ga on 15 Oct 2003 14:07 PDT
Hi dnadna -

Your question is very interestingly worded. Are there studies which
show that interbreeding is harmful?  Certainly the answer among
animals is yes.  Or maybe, I should say yes, but!


Geneticists, dating back to Gregor Mendel in the 1800's have shown
that if they selectively breed for a certain gene, that gene will
increase in the population.  Perhaps the least esoteric example coming
to mind is dog breeding.  If a breeder keeps selecting smaller parents
to breed, eventually the breeder will have a population of tiny dogs
in his kennels.  The breeder has done this by breeding close relatives
with the needed trait.  Eventually a small (or whatever) dog is


Now imagine a human population with a harmful but hidden (recessive)
trait.  While no one can manipulate the population to achieve high
frequencies of the trait, it will appear if carriers of the recessive
trait mate with each other.  Many studies have shown that a harmful
hidden trait will show itself (by being present in a double recessive
dose) in families formed by cousin marriages.

In fact it is a generic genetic "rule" that consanguinous
marriages--between close relations --increase the chance of previously
hidden genes will come to the fore:

When Parents are Relatives: Consanguinuity

These hidden genes may be benign, and two cousins, say, will have
healthy babies.  But if  the previously hidden gene is harmful, 
developmental defects will show up in the offspring.

Some examples are:

MULTIPLE SCLEROSIS:  first cousin marriages in Sweden

HEARING IMPAIRMENT: first and second cousin marriages in Riyadh, Saudi

HEART DEFECTS: first, second, and third cousin marriages in a Bedouin
tribe in Israel

BRAIN DEFECTS: first cousin marriage in Afghanistan

SCALP AND LIMB DEFECTS:  first cousin marriage in Australia


The prohibition of sibling and cousin marriages antedates even the
19th Century knowledge of genetics.  In Europe the Catholic Church
forbade marriage not only between brothers, sisters, and cousins, but
also with a godparent or even son or daughter of a godparent.

Consanguinity in Canon Law

Today, in the United States, marriage laws are promulgated by the
States, and each differs with respect to marriage between relations.

In all, according to, 16 States permit cousin
marriages with no exceptions; an additional 7 permit them with some
exceptions (for example, a blood test for certain genetic diseases).


Under Australian law, marriage between people who are of "direct blood
line," that is, brothers/ sisters, brothers/ half sisters, sisters/
half brothers, grandparents/ grandchilden are forbidden.

Genetics Fact Sheet

In Canada, parents cannot marry their blood-related children nor can
blood- related siblings marry.

Canada Statutes

I hope this answer has been helpful to you.  Do ask for a
clarification if needed.

All the best,


SEARCH TERMS USED IN GOOGLE SEARCHES:	civil law consanguinity;
inbreeding siblings cousins; "animal breeding" negative traits;
"gregor mendel" experiments;  "dog breeders" weaknesses;  "cousin
marriages" states.

consanguinity siblings cousins
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