In-breeding: the facts please?

Inbreeding as you call it occurs a lot more than you would actually think sadly. As pointed out Eqyptians did quite a lot to maintain the blood line.

In normal unrelated births there is 4% genetic issues and in related its about 6-8%, so apart from rare genetic conditions (generally recessive) that can be carried in a family and generally will never be seen there is a higher proporition,s but as to people being born "handicapped" etc no there is not genetic reason why people would be born different (except in genetic conditions).

Its mainly the social issues that makes it so "taboo" and that we have put words like "retarded", deformed etc in relation to related people having children. There is sadly a lot of cases of incest which produces offspring the vast majority are perfectly normal.

In-breeding can produce handicapped people, but I wouldn't say ugly. People forget that some of the most beautiful people, such as the egyptian royals, were always in-bred. The Pharaoh and the rest of the royal family would usually marry siblings or cousins to keep the blood-line strong. The rest of the world often begged the Pharaoh's for at least one beautiful egyptian princess because of their beauty.

Therefore, I wouldn't say ugly. However it is fully true that inbreeding can cause disabilities or deformities. Usually it takes many generations, but inbreeding can cause genetic deformities. Napoleon, and even King Tut is believed to have had genetic deformities because of inbreeding.

Inbreeding is the reproduction from the mating of two genetically related parents, which can increase the chances of offspring being affected by recessive or deleterious traits. This generally leads to a decreased fitness of a population, which is called inbreeding depression. Deleterious alleles causing inbreeding depression can subsequently be removed through culling, which is also known as genetic purging.

Inbreeding may result in a far higher phenotypic expression of deleterious recessive genes within a population than would normally be expected.[1] As a result, first-generation inbred individuals are more likely to show physical and health defects, including:

Reduced fertility both in litter size and sperm viability

Increased genetic disorders

Fluctuating facial asymmetry

Lower birth rate

Higher infant mortality

Slower growth rate

Smaller adult size

Loss of immune system function

All of the above, for Humans, can be disasterous.

Typical inbreeding percentages are as follows, assuming no previous inbreeding between any parents:

Father/daughter, mother/son or brother/sister → 25%

Grandfather/granddaughter or grandmother/grandson → 12.5%

Half-brother/half-sister → 12.5%

Uncle/niece or aunt/nephew → 12.5%

Great-grandfather/great-granddaughter or great-grandmother/great-grandson → 6.25%

Half-uncle/niece or half-aunt/nephew → 6.25%

First cousins → 6.25%

First cousins once removed or half-first cousins → 3.125%

Second cousins or first cousins twice removed → 1.5625%

Second cousins once removed or half-second cousins → 0.78125%

An inbreeding calculation may be used to determine the general genetic distance among relatives by multiplying by two, because any progeny would have a 1 in 2 risk of actually inheriting the identical alleles from both parents.

For instance, the parent/child or sibling/sibling relationships have 50% identical genetics.

NOTE: For siblings, the degree of genetic relationship is not an automatic 50% as it is with parents and their children, but a range from 100% at one extreme, as in the case of identical twins (who obviously cannot mate as they are the same sex), to an exceedingly unlikely 0%. In other words, siblings share an average of 50% of their genes, but unlike the 50% ratio between parents and children, the actual ratio between siblings in any given case can vary.