Through centuries, the royalty, in order to perpetuate itself, retain its wealth, and consolidate its kingdoms, concluded marriage alliances with neighbouring royal families. In the European empires of England, France, Germany, Spain, Russia etc through the 13th to 17th century, royal inbreeding was a most important political expediency. By virtue of such strategic marriages, not only was wealth retained in the hands of a few families, but in case of war and annexation, the rulers, through their various family alliances, became an invincible force. This guaranteed their dynastic rule for generations.
This method of consolidation of family power was commonly practiced in other parts of the world, including in India, where even today, the erstwhile rich and famous still prefer to marry into each other's families. Taken from a social perspective, this system of marrying into equivalent social levels is understandable. After all, society comprises of various groups, with each group pretty much preferring to keep to itself, drawing comfort from equivalent standards, similar backgrounds and shared objectives. This is all very well when it comes to consolidating family values and wealth.
However, there is a downside when it comes to family health. Research on consanguineous marriages (i.e. marriages between blood relatives up to the third degree: brothers and sisters being the first degree, maternal and paternal uncles and aunts being the second degree, and first cousins being the third degree) has established that genetic, congenital and family diseases are also consolidated and perpetuated through marriages between close relatives. It is for this reason, that common custom in most countries frowns upon marriages between relatives even upto the sixth degree of relationship.
Genetics and heredity:
It was only in the 20th century that scientists could explain why children so often resemble their parents. The reason they do, is because parents pass on to their children blue prints of themselves, which are contained in every cell of their bodies, including the sperm and ova. These blue prints are carried in the genes, which exist in the nucleus of every cell. Every cell has a complete set of blue prints which contain a detailed genetic code of skin colour, hair, eyes, body type, inheritable disease etc.
But the sperm and ova contain only 50 percent of the blue prints. So when fertilization takes place, the 50 percent of the father's genes carried in the sperm merges with the 50 percent contained in the ova, making a 100 percent brand new life whose genetic blue print is an equal merger of both parents. Keep in mind that even while the gene contains 50 percent of the characteristics of each parent, it in turns contains 25 percent of each grandparent, 12 and a half percent of each great grandparent and so on. This is the basis of genetic inheritance.
The reason why no two children are exactly alike (except for identical twins - fertilized from the same egg and sperm) is because the 50 percent mix of each parents is random, and the possible different genetic combinations are calculated to be a stupendous 64 million different possibilities! To complicate the picture even further, remember that in each male ejaculate, there are over a million sperms – and the repository of ova in the female exceed eight million in a lifetime, of which only one is released each month, and only a few are fertilized in a lifetime.
Genes do not only carry family genetic codes, several new characteristics - good and bad - may be created within the parents' lifetime prior to conception. These changes in the genetic code are called mutations and could be caused by a good and healthy lifestyle and diet, or by a bad environment and radiation etc. These mutations and changes in the DNA make each individual different and unique.
Genes are either dominant or recessive. For example, the genes for brown eyes and black hair are dominant, whereas the gene for blue eyes and blond hair are recessive. A dominant gene will always find expression over a recessive gene. For a recessive gene to find expression, it will need to be contained in the genetic code of both parents and passed on to the child. What further confounds the situation is the randomness by which dominant or recessive genes will be contained in the 50 percent genetic code passed on by each parent, which also contain in the grandparents' contribution.
Besides inheritance from dominant and recessive genes, certain characteristics are specifically attributed to inheritance from the mother's X chromosome. This remains ineffective or recessive in a daughter who comprises of two XX chromosomes, but is dominant and pronounced in a son who comprises of the XY Chromosomes - of which the X chromosome is inherited from the mother.
Keeping in mind that inbreeding and consanguineous marriages between relatives result in a common gene pool, here are few of the more prominent of the 1800 and more genetic abnormalities that can be passed down as a consequence:
Dominant Gene disorders
Recessive Gene disorders
Source: Adapted from an article by Prof. Kennedy