Thanks to some information from Marilyn Thomas Hanson, I have been reading some about genes which are the basis of genealogy. So we should know more than we probably do know, especially in the future where DNA will be increasing important in genealogical studies.
The Y chromosome is one of the two sex chromosomes in humans, and in most other mammals. The other sex chromosome is called X. Males have a Y chromosome and an X chromosome while females have two X chromosomes. A male inherits his Y chromosome from his father and his mother contributes little to this. Thus, the same Y chromosome gets handed down from generation to generation in the male line. In theory, the Y chromosome goes back to Adam for all men. Mutations have developed along the way and once a mutation has occurred it is passed on to succeeding sons if the mutation is not life threatening. These mutations develop slowly over time and make it easy to trace paternal lineage. The Y chromosome contains the lowest number of number of genes and is responsible for the fewest number of genetic disorders. Many of the genes in the Y and X chromosomes are in common but one of the genes on the Y chromosome determines the male sex. Some parts of a new Y chromosome may come from the mother, but the male gene comes only from the father. Sometimes things go wrong on the recombination at the start of a new human life and the resultant individual has an indistinct sex. Such recombinations were not beneficial and as a result 95% of the Y chromosome is unable to recombine. This protection mechanism helped to insure that males and females were distinct.
There are few Y chromosome-linked diseases, in part due to the small number of genes. Sometimes there will be more than one Y chromosome which tends to produce taller men who have learning problems. More than two Y chromosomes are rare, but are known to exist. The individuals are poorly defined sexually.
All chromosomes, except Y, are robust and have accurate repair mechanisms. The primary repair mechanism for a mutation or abnormal chromosome comes from the receipt of two copies of the chromosomes, one from the mother and one from the father. Damaged genes are swapped out and replaced by good genes in the recombination process which generates new chromosomes from the two parts donated by the father and mother. In some cases the error is corrected simply by the fact that the new chromosomes are not viable and the inheritor dies. This cannot occur for the Y chromosome since the source for it comes only from the father. As a result, errors build up on the Y chromosome. One result is that the Y chromosome has lost much of its original DNA and become smaller and smaller as only the essential genes have remained. Another repair mechanism has evolved, but the next Note will take that up.
(14 Apr 06)
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