What are the disadvantages of marriages between relatives?

[mech-eng]

What are the disadvantages of relative marriages? As far as I know relative marriages might cause damages to children born from relative marriages. Is this a correct and scientific information? I know some people that usually do relative marriages but I have never seen any damages on them, neither physically nor mentally.

Source:Personal observation.

Thank you.

 

[DrClaude]

The Wikipedia article will answer many of your questions: https://en.wikipedia.org/wiki/Inbreeding

I know some people that usually do relative marriages but I have never seen any damages on them, neither physically nor mentally.

And I know some cousins who married and whose natural child is mentally retarded. It is a question of statistics, not individual outcomes.

 

[jim mcnamara]

The biological reason for this: close relative have lots of genes in common. Some genes for genetic diseases do not express themselves unless the child got two identical copies of the gene (allele) - one from each parent. The probability of identical alleles occurring increases the more closely related parents are. Identical twins are not a problem since they are the same sex. Brother - sister share 50% of their DNA with each other, son & daughter share 50% of DNA with either parent. Cousins share less.

 

[mech-eng]

And I know some cousins who married and whose natural child is mentally retarded. It is a question of statistics, not individual outcomes.

What do you mean by "mentally retarded"? Does it mean mentally defective or lesser IQ values than normal or average values?

Thank you.

 

[jim mcnamara]

Mental retardation is a catch-all phrase. Like anybody who has a drippy nose has a 'cold'. It does not point to any specific problem. The drippy nose could be allergies, simple cold, influenza, or a life threatening nasal bacterial infection.

 

[DrClaude]

What do you mean by "mentally retarded"? Does it mean mentally defective or lesser IQ values than normal or average values?

I meant it to say that the child's cognitive abilities were below that of a child of the same age. I think that "intellectual disability" is the more proper, modern way of saying it.

 

[Rive]

It just hit me suddenly, but I think it worth asking: regarding relative marriages the lack of relatives // shallow rooting in the local society can be considered as a negative factor at group selection level?

 

[mech-eng]

I meant it to say that the child's cognitive abilities were below that of a child of the same age. I think that "intellectual disability" is the more proper, modern way of saying it.

From mentally regarded, I understand that there is a significant brain or nervous system, but your above explanation is more close to lower IQ or being less smart. Do every normal people have "intellectual ability"?

Thank you.

 

[pinball1970]

Genetics are the potential and the environment governs how that potential develops.My own view is if the potential is not there no amount of nurturing can change that significantly.Matt Ridley wrote an interesting piece on intelligence/IQ his book Genome, chapter 6. http://bioinformaticsinstitute.ru/sites/default/files/genome_the_autobiography_of_a_species_in_23_chapters_-_matt_ridley.pdf

 

[pinball1970]

Do every normal people have "intellectual ability"?

Apologies, in response to the above.

 

[Bandersnatch]

From mentally regarded, I understand that there is a significant brain or nervous system, but your above explanation is more close to lower IQ or being less smart. Do every normal people have "intellectual ability"?

It's a scale, with intellectual disability being the diagnostic term for a severe case of cognitive impairment (low IQ).
'Intellectual disability' (or 'mental retardation') uses low IQ scores as one of its diagnostic signifiers. The other is the inability to function in the person's environment ('adaptive skills').
So a child with very low (<70) IQ scores who has also shown poor ability to function independently would be considered intellectually disabled.

There seems to be good evidence of inbreeding increasing the incidence of mental impairment in children:
Effect of inbreeding on IQ and mental retardation. N E Morton

These data indicate that the risk for mental retardation in matings of normal parents increases from 0.012 with random mating to 0.062 for first-cousin parentage (...)

Estimating the Inbreeding Depression on Cognitive Behavior: A Population Based Study of Child Cohort Mohd Fareed, Mohammad Afzal

We found significant decline in child cognitive abilities due to inbreeding and high frequency of mental retardation among offspring from inbred families.

I've heard that the increase in the risk of intellectual disability in the offspring resulting from one-off (i.e. not generational) mating between first cousins is comparable to the increase in a similar risk if a woman becomes pregnant when she's in her early 40s vs early 30s.
In any case, it's a risk increase, not a guarantee of being less smart or disabled.

 

[Drakkith]

From mentally regarded, I understand that there is a significant brain or nervous system, but your above explanation is more close to lower IQ or being less smart. Do every normal people have "intellectual ability"?

Yes, but no one really says it that way. Similarly, most people have the ability to walk, but a some people have a physical disability that prevents them from walking. They could be missing their legs, be paralyzed, etc.

Ability
1. power or capacity to do or act physically, mentally, legally, morally,financially, etc.

Disability
1. lack of adequate power, strength, or physical or mental ability; incapacity.
2. a physical or mental handicap, especially one that prevents a person from living a full, normal life or from holding a gainful job.

 

[BillTre]

It just hit me suddenly, but I think it worth asking: regarding relative marriages the lack of relatives // shallow rooting in the local society can be considered as a negative factor at group selection level?

If what you mean is is group selection less likely to arise in a less closely related local population, I would think that answer is yes.
As I understand it group selection relies on local populations being closely related so that benefits accrued by one member of a population (organism A), derived from the actions of another member of a population (organism B), will benefit on the genes A carries which are identical to those of B. In this way everyone gets to be happy.