SD on Babies born of incest

I tried to look this up in Google but all I could find was page after page after page of porn and abortion sites.

Question --> What happens to the children born to say a father who rapes his daughter, a brother and sister or a mother son. I say they can be born normal. They MAY develop a heredity condition but that isn’t ALWAYS so.

So what is the SD?

I believe you’re right - babies born to closely related parents can be born normal, but run an increased chance of having some abnormality. Hence why there is a social taboo on incest.

I think its because there are many genetic diseases carried by recessive genes. So if your parents are not closely related, it is unlikely they are both carrying a copy of a particular defective (recessive) gene. However if they are closely related, it is far more likely that they do both carry the same defective gene, which will then be expressed in their offspring.

Inbreeding results in a higher chance of re-inforcing recessive genes. But this doesn’t mean that inbred individuals must therefore be genetically defective. They just have higher frequency of appearance of both good and bad recessive traits. That’s all. There’s no reason to think that something bad must happen.


My mother was a product of incest. She had no obvious physical defects. She died as a result of a benign brain tumor from a long past head injury.
I’m very healthy, and pretty smart. The only defects I’ve noticed are I have an extra cervical vertebra, my legs are too short, and I have no filter between my brain and my mouth.
My son was healthy and smarter than me. He battled depression much of his life and committed suicide in 2000. He had no physical defects. He even had long legs.
I have no other childern, so any recessive defect that may have arisen from the incest is blocked.

You had it right from the start. It increases the probability, it does not guarantee it.

Thing is, disadvantageous mutations/recessive traits tend to be far more evident than advantageous ones – and often with more immediate harmful results. e.g. the European royalty of the Victorian period, where a high frequency of cousin-marriage created a higher incidence of hemophilia, but did not make every single male heir of the 2nd generation a hemophiliac.

Lets say a healthy normal male has two normal #7 chromosomes (N N for normal). Both contain proper genes. His wife has one normal chromosome (N), and one with a mutation leading to cystic fibrosis (Mutant), but she is otherwise healthy. They have two children. During Meiosis the chormosomes split up resulting in sperm/eggs with one copy of this gene, which combine to give the child 2 copies. The children will either be (and which parent donated the chromosome in parenthesis):

Normal (father) Normal (mother)
Normal (father) Mutant (mother)

So there’s a 50% chance each child inherits the cystic fibrosis gene. Neither child actually has cystic fibrosis since they both have at least one good copy of the gene.

Now the following incest occurs, with results -

Brother + Sister:
Normal Normal + Normal Normal --> Normal Normal (100%) child
Normal Mutant + Normal Normal --> Normal Normal (50%) or Normal Mutant child (50%) (double this line in final stats)
Normal Mutant + Normal Mutant --> Normal Normal (25%), Normal Mutant (50%), or Mutant Mutant (25%)

So overall there is a 1 in 16 chance their child will actually have cystic fibrosis (Mutant Mutant)
Father + Daughter:
Normal Normal + Normal Normal --> Normal Normal child (100%)
Normal Normal + Normal Mutant --> Normal Normal (50%) or Normal Mutant child (50%)

There is no chance the child will have cystic fibrosis but a 1 in 4 chance of being a carrier.
Mother + Son:
Normal Mutant (mother) + Normal Normal --> Normal Normal (50%) or Normal Mutant child (50%)
Normal Mutant + Normal Mutant --> Normal Normal (25%), Normal Mutant (50%), or Mutant Mutant (25%)

There is a 1 in 8 chance this child has cystic fibrosis, much higher than incest between brother/sister.
So it seems incest between the mutant carrier parent with the child results in a much higher chance of mutant children, at least in autosomal recessive diseases.
Quickly on dominant diseases:
Normal Normal father + Normal Mutant mother (has disease)

results in 50% normal normal child and 50% normal mutant child (with disease)

brother + sister --> 56% normal 44% disease
dad + daughter --> 75% normal 25% disease
mom + son --> 62% normal 38% disease

So in this autosomal dominant case brother + sister incest is worst.
I’ll let someone else work out the X-linked diseases :smiley:
I’m unsure how many genetic disorders each of us carry, but multiply these numbers out by the thousands of inherited disorders and you’re more likely to have a few of these diseases pop up through incest than through sex ouside of the family. I’ve heard that animals in the wild will not mate with 2nd cousins or closer, presumably to reduce the occurance of disease.

The sites I found regarding Cousins say first cousins carry no significant difference for herditary conditions than that of unrelated people. This so?

According to Discover magazine: [url=""Go Ahead, Kiss Your Cousin
Heck, marry her if you want to


Hemophilia is not an example of incest reinforcing recessive genes. That’s because of the way it’s inherited with a recessive on the X chromosome.

For a better example, see the Spanish Hapsburgs, who interbred more than any other royal family of Europe. In that family, the king usually married either a niece or first cousin. The line died out because the incest reinforced recessives generated deformities to an such extent that they could no longer reproduce.

No effects would be expected in you or your children. One generation of outbreeding undoes any past inbreeding. You cannot have received both of your copies of any gene from your mother’s two parents (assuming no other inbreeding).

While it’s true that recessive genes can be good or bad, most of the really bad genetic disorders are carried on recessive genes. This is because if a gene is “hiding” as a recessive, it takes longer for natural selection to weed out that gene.

As an extreme case, consider a defective gene which carries a trait that’s instantly fatal: Any fetus bearing that trait will be stillborn. If the gene responsible for that trait is dominant, then the first time that mutation occurs, it’ll immediatly kill its carrier, and the defective gene will not spread. But suppose instead that the gene is recessive. In that case, the individual in which the mutation occurs can survive and have children. Until that individual’s decendants eventually start intermarrying, or the same mutation shows up somewhere else, the gene will have no effect, and it might spread quite far through the gene pool before that happens.

If you want better Google results, try linebreeding. Most all of the sites I found dealt with domestic animals, but the principle is the same.

Actually, the hemophilia in the royal family didn’t have much to do with incest, if at all, from what I understand.

Some people seem to be referring to a study released about a year ago that claimed that cousin marriage wasn’t a big problem.

I went thru the studies numbers and discovered something interesting. The absolute increase is quite small, but since the value is already small to begin with, this meant a relative doubling of genetic problems.

To me, doubling the odds of something bad happening is not insignificant.

Secondly, the issue of repeated inbreeding wasn’t properly addressed. I don’t see how you can tell people it’s okay to do it once, but not more times. Real Humans don’t understand such subtleties.

It is true that, given identical mutation rates and identical evolutionary fitness costs of disease, a completely recessive allele will have a higher frequency in the population than a completely dominant allele. However, it is also true that at any given allele frequency, the disease frequency will be lower for the recessive allele, since only homozygotes are affected. It turns out that these effects cancel. This can be considered an example of what is called the Haldane-Muller principle.

I found this site (note-it’s about the Kingston clan, a heretical sect of Mormons, whose beliefs and practices do NOT reflect the official LDS doctrine in any way, shape, or form), When incest becomes a religion.

While it goes into the gross abuse and practices of this family, it also shows what happens with frequent interbreeding in a family, over time. It’s not pretty, that’s for damn sure.

According to my old Sociology professor (which is hedge-speak for “I don’t have a cite for this”), the incest taboo had very little to do with fear of three-headed mutations; the taboo predated Mendel for centuries. Instead, it was about maintaining social order within a family, an order that could be seen to crumble when blood relatives saw each other as potential sexual partners.

The taboo is also almost exclusively a middle-class phenomenon; the very rich and very poor have never been under any such constraints.

Anthropologists generally reject the notion that birth defects (or other biological factors) are the basis for the incest taboo. From Wilkipedia’s article on the incest taboo:

It is generally accepted that the prohibition is based on social factors. Basically, the taboo encourages endogamy in order to forge alliances and strengthen social bonds. Those who practice incest are ostracized because they are seen as insular, imbeciles (for not understanding such basic rules) and/or, well, very creepy.