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mad_scientist

Inbreeding

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Hypothetically if you marry your cousin's daughter to your son, are the chances of them procreating to produce offspring with genetic disorders/birth defects relatively small? Should it be a concern at all?

Is it just when first cousins marry which are a problem for healthy offspring production or are 2nd/3rd cousin marriages a problem as well if they want to produce children one day?

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No, not a huge concern, repeated inbreeding is the main source of problems.

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7 hours ago, Endy0816 said:

No, not a huge concern, repeated inbreeding is the main source of problems.

What proportion of DNA between 1st cousins is the same, compared to 2nd cousins and 3rd cousins, respectively?

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~12.5% for first cousins.

0.5^3

That's assuming all the sets of parents are not themselves related though.

 

6 hours ago, Strange said:

Did you miss the word "repeated" ?

Yeah, it is the blind repetition that causes the problems.

 

Has there been any hard numbers released as to what extent this is actually an issue in the UK?

Edited by Endy0816

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I have seen some cohort studies that have showed elevated risk due to marriages among cousins. While the risk for a variety of defects have ~doubled, the absolute risk was overall not terribly high. I.e. in the low percentage points, IIRC (e.g. 3% base chance to 6% with cousin marriages, on the high end and ~0.6% to ~1.8% in some others).

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2 hours ago, Endy0816 said:

~12.5% for first cousins.

0.5^3

Isn't this the chance PER PAIR of chromosomes to be identical to each other ?

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I have wondered about island populations that began with small numbers - human and animal. Starting with single pairs may not be an absolute guarantee they will die out; perhaps depending on which specific minor mutations they carry?

 

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4 hours ago, CharonY said:

I have seen some cohort studies that have showed elevated risk due to marriages among cousins. While the risk for a variety of defects have ~doubled, the absolute risk was overall not terribly high. I.e. in the low percentage points, IIRC (e.g. 3% base chance to 6% with cousin marriages, on the high end and ~0.6% to ~1.8% in some others).

Thanks. Was curious and the topic hasn't been covered any here.

1 hour ago, Roamer said:

Isn't this the chance PER PAIR of chromosomes to be identical to each other ?

Pretty sure lose about half each time.

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On 07/10/2017 at 8:12 AM, Endy0816 said:

~12.5% for first cousins.

0.5^3

 

What about for between two 2nd cousins and two 3rd cousins, respectively?

Edited by mad_scientist

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5 hours ago, mad_scientist said:

What about for between two 2nd cousins and two 3rd cousins, respectively?

You can calculate the inbreeding coefficient of any individual, assuming pedigree is known. 

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On ‎10‎/‎6‎/‎2017 at 8:16 PM, Ken Fabian said:

I have wondered about island populations that began with small numbers - human and animal. Starting with single pairs may not be an absolute guarantee they will die out; perhaps depending on which specific minor mutations they carry?

 

With most animals that make island-fall there is the benefit of multiple offspring per birth. This enlarges the gene-pool very quickly, thereby reducing the loss of genetic diversity in the new population. A human pair would have a harder time building a healthy population. Humans just do not breed rapidly enough to that. In-breeding would mount the defects up pretty quickly if there are any. 

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On 7-10-2017 at 4:04 AM, Endy0816 said:

Pretty sure lose about half each time.

yeah precisely.

As i undertand inbreeding causes problems because a pair of chromosomes is identical.

however with having multiple pairs of chromosomes it would mean that this effect/danger would occur per chromosome-oair, right ?

 

Also, at OP, i ve heard of an islandic study of (reproductive) offspring getting best results from 2nd(?) degree cousings - i cant find it myself right(bit drunk and i dunno if you  realyy care) now but if you re interested you should be able to find it.

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23 hours ago, Roamer said:

yeah precisely.

As i undertand inbreeding causes problems because a pair of chromosomes is identical.

however with having multiple pairs of chromosomes it would mean that this effect/danger would occur per chromosome-oair, right ?

 

Also, at OP, i ve heard of an islandic study of (reproductive) offspring getting best results from 2nd(?) degree cousings - i cant find it myself right(bit drunk and i dunno if you  realyy care) now but if you re interested you should be able to find it.

The danger of inbreeding comes from the fact that it increases the likelihood of the pairing of recessive genes for harmful traits. 

I'll give you an example.  There are two types of Poly-cystic kidney disease. Both are eventually fatal.  One, which is carried by a dominant gene doesn't manifest itself until well into adulthood.  If you get the gene from either parent, you will get the disease. But since this doesn't occur until you have likely already had offspring, it doesn't get culled from the gene pool.  The other type is carried by a recessive gene, and you need to get it from both parents. This version is fatal within a few months of birth.  If it is only passed on by one parent, you are just a carrier and you aren't born with the disease.   This genetic disorder, while it does prevent the person born with it from furthering the genetic line, remains in the gene pool because it is recessive and can be passed on from generation to generation with the disease itself ever manifesting.  This is true of a good many genetic diseases, they hang around because they are recessive traits.

Now let's say that thae chance of any given person in the general populace is a carrier of a particular one of these recessive diseases is 1/100,000. Then is is pretty slim odds that two mated people will both have it and low odds that any of their offspring will suffer from the disease.  But what if you have a lot of inbreeding within a given family.  If that recessive trait exists and is being passed on from generation to generation, the odds of these any two people in this family having the gene is higher than for the general populace and there are higher odds of offspring from a mating between family members of exhibiting the disease.

It is not just the paring of  identical chromosomes, its the increased likelihood of the pairing of the wrong  identical chromosones.

 

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