點解基因相近會導致生育問題？

Sorry answer will be in English because the computer I am using now only support English input.

Usually if a closely related couple (on the family tree) give birth to a baby, the baby tends to be very likely to be retarded and its due to the genetics. Usually, the genes that are controlling for severe disease are working on a double recessive mechanism, meaning you will have to get two problem gene to get the disease. If you onlu have one problem gene in your body, either from your mum or dad, you will only be a carrier but having the disease. However, if two relatives get married, because they are from the same family, the chance of getting the problem genes in them will be much higher than marrying a random guy, and this give a higher chances of putting two diseased gene together and get the offspring inherited the sickness. Thus, the closer the couple on the family tree, the higher the chance of getting diseased or retarted children. If the carrier marry only a random guy, the problem gene can be fixed by the other healthy gene from the partner since most of the gene can work well even there is only one that is working properly. However, this does not apply to X-linked disease such as haemophilla. This kind of disease affect man far more serious than women because the disease is carried on a X chromosome. So if the mother is the carrier, even her partner is healthy, her son will still have the disease because he does not has another healthy X chromosome to replace the unhealthy one. But since girl has 2 X chromosomes, they usually will be only a carrier unless the mother marries a diseased guy.

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【明報專訊】避孕丸除了可令婦女主動控制生育時機，原來亦可能令她們移情別戀！英國研究顯示，避孕丸可改變婦女的荷爾蒙，從而改變她們對男性吸引力的看法，選擇「錯誤」伴侶。利物浦大學的研究人員對97名婦女進行研究，其中一部分在參與研究後開始服食避孕丸。所有婦女被要求用鼻子嗅嗅不同男性穿過的T恤，這些男性體內一系列控制免疫系統的基因「MHC」基因都不同。研究人員抽取所有婦女的MHC作比較，發現婦女開始服食避孕丸後，傾向選擇MHC與自己相近的男性。

研究結果意味避孕丸會影響女性對男性氣味吸引力的感知能力。負責帶領研究的羅伯茨（Craig Roberts）說﹕「結果顯示，服食避孕丸可令婦女揀選基因與自己相近的男性作伴侶。基因相近會導致生育問題。由於避孕丸影響婦女判斷伴侶吸引力的能力，當她們停服避孕丸後，亦可能導致情侶關係破裂。」

泰晤士報