How does inbreeding inherit genetic diseases?

[strategist.com]

http://www.cracked.com/article_18577...or-better.html

It is on number 4. I don't understand how inbreeding can cause genetic related disease? Not that I want to have an incestuous relationship or anything.

[Vizsla]

It doesn’t create the diseased mutations.
We’re all carrying copies of crappy genes. Genetic diseases tend to be recessive so normally people have a healthy copy of a diseased gene as well, so they don’t get ill.
If you start breeding with people who are closely related to you then the chances of producing offspring with 2 copies of diseased genes is high. They will get ill because they don’t have a healthy copy.

Blondness is an example of a recessive mutation that is slowly being bred out of the population. It’s not a disease though – in most respects.

It is possible to get dominant genetic diseases but they tend to kill their sufferers in one generation so they don’t hang around long.

[Phier]

Its estimated that everyone carries about 5-7 lethal genes, normally masked as they are recessive. Dominant lethal genes don't last long unless the lethality doesn't happen until after breeding.

Anyways inbreeding gives these rare lethals a chance to express themselves. Personally I think the estimate of 5-7 is high, but its definitely greater than 0.

Blondness is an example of a recessive mutation that is slowly being bred out of the population. It’s not a disease though – in most respects.

Blonds are an interesting case. Blond hair in youth is common which is then lost. I was a blond until I was about 9 myself. Somewhere in Sweden/Norway a mutation kept the blond hair into older ages, and its believed it spread via sexual selection. Gentlemen really do prefer blonds.

Now its not really being bred out due to 'normal' genetic drift though some of it is with immigration into what where pretty uniform populations of Sweden/Norway, but because its no longer a mating advantage.

Gentlemen still prefer blonds, but....

Spoiler Alert, click show to read: 

Just about anyone can become blond.

Still blonds may become rarer but will never completely disappear, as there is no need to mask them like a lethal recessive.

[Vizsla]

You mean the Blondes becoming extinct thing is a hoax? I've been peddling that in pubs up and down the land for years.

Goddamit

[Phier]

You mean the Blondes becoming extinct thing is a hoax? I've been peddling that in pubs up and down the land for years.

The original news report was most definately a hoax. Typical media not checking sources.

[justicar5]

The original news report was most definately a hoax. Typical media not checking sources.

red heads are fine right? Theres always going to be red headed women right? With blue dreads? Hmm blue dreads...

[Elfdude]

Like they said inbreeding increases the chance of deleterious recessive genes being paired up together. However inbreeding isn't necessarily bad but rather has a ridiculously high (compared to normal) chance of resulting in genetic illness.

For example, a mother/son, father/daughter relationship has a 25% chance of two identical recessive genes expressing themselves. Two siblings may have a chance anywhere from identical twins (100%) to an exceedingly unlikely complete genetic strangers (0%). This is calculated by the chance for the parent to pass on the gene divided by two assuming both parties have the genes in the first place.

Typical inbreeding percentages are as follows, assuming no previous inbreeding between any parents:
Father/daughter – mother/son – brother/sister → 25% (brother/sister may be between 100% and 0%, parents and offspring will always result in a 25% chance)
Half-brother/half-sister → 12.5% (half brother/sister may be between 50 and 0%)
Uncle/niece – aunt/nephew → 12.5% (may vary between 50 and 0%)
Double first cousins → 12.5% (may vary between 50 and 0%)
Half-uncle/niece → 6.25% (may vary between 25 and 0%)
First cousins → 6.25% (may vary between 25 and 0%)
First cousins once removed – half-first cousins → 3.125% (may vary between 12.5% and 0%)
Second cousins – first cousins twice removed → 1.5625% (may vary between 6.25 and 0%)
Second cousins once removed – half-second cousins → .78125% (May vary between 3.125 and 0%)

As you can see inbreeding isn't as cut and dry wrong as people like to pretend nor does it guarantee illness. With modern science the chance of undesirable mutations can be reduced to near 0 between any two persons including identical twins (even of the same gender). Further because of random mutations during fertilization and sex cell formation the percentage can vary even greater. Hypothetically a sex cell could be mutated enough to provide 0 chance of recessive pairing between mother and child but there'd probably be more important issues to worry about at that point.