The Secrets of Life (4) - Faulty Genes and Close Relations Marriage

An ordinary human has 46 chromosomes in each cell made up of 23 pairs. In each chromosome, there are hundreds and thousands of genes, and it is estimated that there are about 25,000 - 35,000 genes (exact number is not yet known) in a single human cell. Genes may become faulty due various reasons (e.g. inherited from parents, during the formation of egg or sperm or during the lifetime of the individual due aging or radiation etc). As chromosomes come in pairs, genes also come in pairs. A single faulty gene in a chromosome usually will not cause any problems as there is another working copy of the same gene in the other leg of the chromosome. Ordinary human usually has several faulty genes in the body at any time, and the body still functions perfectly normally. Only when both copies of the same gene in both legs of the chromosomes become faulty, then genetic problem occurs which will then affect the health and development of the individual.
During the formation of the egg or sperm, the two legs of each of the 23 pairs of chromosomes separate. The egg or sperm then has only single legs of the 23 pairs of chromosomes. At the time of conception, the egg and the sperm combine to form the first cell of the child. Each leg of the egg will combine with its corresponding counterpart of the sperm forming 23 pairs of chromosomes like the parents. If there is a faulty gene in the chromosome of the egg, AND if there is a SAME faulty gene in the chromosome of the sperm, then the SAME copy of the genes in the chromosome will be faulty resulting in genetic problems in the child.
Given the facts that there are 25,000 - 35,000 genes in human, a healthy human usually has only a few faulty genes and the faulty genes exist randomly, the chance of having a SAME faulty gene during conception (hence causing genetic problems) of two UNRELATED individuals is extremely low. But when the two individuals are close blood relatives, they share a greater proportion of genes in common than unrelated individuals do because they have a common ancestor from whom they inherited their genes. As such, they have a greater chance of having a same faulty gene in common in their chromosomes. When their egg and sperm combine at time of conception, there is a greater risk of having a child with a same copy of faulty genes in the chromosomes and hence a greater risk of having genetic problems.
The table below shows the proportion of genes shared between close blood relatives:

Relationship Type	Relationship	Proportion of Genes in Common
	Twins (雙生兄妹, 姐弟)	All – 100%
First degree relatives	Brother and sister (兄妹, 姐弟)	Half – 50%
Second degree relatives	Half brother and sister (同父異母或同母異父兄妹, 姐弟)	Quarter – 25%
Third degree relatives	Cousins (堂兄妹, 姐弟, 表兄妹, 姐弟)	Eighth – 12.5%

The closer the blood relations (i.e. the lower the degree of relationship), the greater is the proportion in having genes in common, and hence the higher risk of having offspring with genetic problems .........................................