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De novo mutations are changes in the gene sequence not inherited from parents. They are new mutations and have been absent in families in previous generations.

A de novo mutation is a new change in the gene sequence, which hasn鈥檛 been present in any previous generations, and does not 鈥渞un in the family鈥�. In this case both mother and father have normal copies of the gene. De novo mutations听do听not听appear on genetic pedigree diagrams (used to assess the mode of inheritance of a听particular disease听in families).听听

There are听a number of听scenarios in which de novo mutations听can听occur.听

1. The mutation occurs in a sex cell of the parent i.e. the BEST1 mutation is present only in a sperm cell or egg cell of a parent, but all the other cells in that parent are normal.听The听mutation is听then听passed on to the developing embryo and all cells will听contain the mutated copy of the gene. In this scenario,听the听de novo听mutation can听be passed onto the next generation听as an inherited mutation.听
2. Some听disease-causing mutations can occur once the egg is fertilised and begins to divide. If this happens early in development, before the听cells begin to specialise听to produce听the听variety of听different tissues听and organs听in听the body, every听single听cell in the听developing embryo听will听contain听a copy of听the mutation.听In this case,听the听mutation听will then be听expressed听in听the听egg/sperm cells听the individual听goes on to听produce, meaning the听mutation could be passed on to subsequent generations.听
3. De novo mutations听can听occur听in single cells in听later听stages of听embryo听development.听In this case,听as the embryo develops听further, all cells arising from the single mutated cell will contain the mutation.听This means some cells of the body听will听contain the听mutation听whilst others听won鈥檛, a phenomenon termed听mosaicism.听In the case of听BEST1, if the mutation occurs in a cell that contributes to eye development the mutation could cause听disease.听The听mutation may not be present in the egg or sperm cells of the individual,听if this happens听the disease will not be听passed on.听听

De novo mutations are rare, but they can explain how a genetic disease develops when there has been no previous history within the family. De novo mutation can be passed down in future generations as inherited mutations.