Why does having a child with a a close relative increase risk of retardation?

It takes two recessive genes to manifest a multitude of genetically based diseases and syndromes. People who are genetically close are more likely to share these recessive genes and have children who inherit one from each parent for the completed manifestation of the disease or gentic defect.

If neither have these recessive genes, then they won't have any problems. But unless you have a thorough gentic profile, the odds are stacked a bit against close relatives having super healthy kids, although it's certainly possible.

having a child with a close relative increases the passing down of the same genes,because the information in the gene code is very similar in the 2 subjects,as they are related.thus,there is not much variability for the dna to make new recombinations.for example,the dna of 2 identical twins is...identical.if they were to produce an offspring,they'd actually make a clone of themselves.the point in reproduction is variability,to make a new individual with the best possible genes from 2 different individuals,so 2 people that are related will have a high number of genes that are exactly the same,and variability will have a smaller chance of happening...

Because the possibility for both parents being a carrier of a genetic deffect multiplies a great deal... If bob and jan are 1st cousins and both carriers of a genetic form of autism (which has many types), they will most likely have the same form of the gene... verses 2 people who are un-related who may also be carriers of a autism gene, but chances are that the 2 forms are not the same, giving their child a less likely chance of contracting the disorder.

You're close relative is just as likely as you are to carry a genetic defect. Where if your mate is from a different gene pool the chances are less likely that you would both carry the defect.

If you mate with someone closely related to you, it is likely that two defective genes will match up. It is less likely that someone unrelated would have the same defective gene.

Too many genes alike. Bad gene plus bad gene would be greater than one bad gene and no bad gene.

because the genes are closely matched and that is where mutations are likely to occur

the genetics are too simular.. not good