Tuesday, May 25, 2010
If you have studied a little genetics you may know that each person has 23 pairs of DNA chromosomes. Twenty-two of those pairs---the autosomal DNA---are what give us our unique characteristics. They are inherited differently than are y-chromosomes if one is male or the mitochondria for both sexes. Y-chromosomes are passed directly from fathers to their male off-spring without mixing with the genes of the mother. Likewise, mitochondrial DNA is passed directly from the mother to all her offspring without mixing with the genes of the father. In this direct transmission, both can pass down unaltered through many generations. That is the norm. The rare copying errors in these intergenerational transfers are what allow us to trace different branches of the human family.
The transfer of the 22 pairs of autosomal DNA from one generation to the next is quite a different process. Each of the parents has 22 PAIRS of autosomal DNA. Only about half of the DNA from each pair of parental chromosomes can be passed on to a given offspring. Generally, the DNA contained in each pair of parental chromosomes is shuffled (like shuffling a deck of cards) before it is passed on. Both the father and the mother contribute their DNA. In this process about half of the characteristics of the mother and about half of the father are transmitted. But, which half?
If this couple has another baby, the mixing process is repeated again. Again each parent contributes to the process. However, after this mixing process is completed, the half of the DNA that is passed on to the second child may be quite different. Hence, the difference between siblings.
This mixing and passing down goes on generation after generation. During this process some strands of DNA remain intact and others are jumbled up into new sequences. Autosomal DNA testing, for genealogical purposes, is looking for intact strands of DNA that are similar between individuals. The more such strands that are shared by two individuals, the closer their biological relationship is thought to be.
[To review previous posts on DNA in this series, click on the Label: "DNA Testing" just below this post and then scroll down through the series.]