In a past column (Sept. 2014) I explained some basic information about human inheritance: deoxyribonucleic acid (DNA) encodes the genetic instructions for the physical traits we express. Once one understands the different types of DNA, their varying patterns of inheritance can provide information about our genetic ancestry. Evaluating genetic testing will be discussed next month in Part Three of The Genes in Genealogy.

The cells of our bodies contain different kinds of DNA. Most of our genetic information (called our genome) consists of nuclear DNA, which is contained in the nuclei (plural of nucleus) of our cells. The cell nucleus is like the ‘command center’ of the cell. Here, DNA is stored as structures called chromosomes. These chromosomes normally come in pairs; one of each pair is inherited from our mom and one from our dad. Humans have 23 pairs of chromosomes. (Occasionally there will be an error in the inheritance process resulting in too many or too few chromosomes; for example trisomy 21, or three copies of chromosome 21, is expressed as Down Syndrome.) One of the chromosome pairs is called the sex chromosome pair and it is here that the genes governing sex are carried. An XX pair creates a female, while an XY pair creates a male. The Y-chromosome is small because it carries only genes related to maleness (more than 95% of the chromosome is male-specific), while the X-chromosome is larger because, besides genes for femaleness, it also carries the code for other traits that are exhibited in both males and females (remember males also have an X-chromosome).

Outside the nucleus of the cell, in the goop called the cytoplasm, there are various cell structures. One type of cytoplasmic structure is the mitochondria. One cell contains many mitochondria, which function as the ‘power generators’ of the cell. Mitochondria also possess a small amount of DNA (mtDNA).

Having defined various types of DNA, now we can examine what each type can tell us about our ancestry. This may be easier to understand if we start at the point of fertilization. When the first cell, from which each of us is formed, was created from the combination of the sperm and the egg, we each received our full compliment of DNA. We inherited nuclear DNA from our mother and nuclear DNA from our father. If we are male, we got a Y-chromosome from our dad; whereas if we are female, we got an X-chromosome from our dad. (Thus, the determination of sex is dependent on our father’s contribution to our genetic soup.) We also inherited mtDNA; the cytoplasm of the fertilized egg comes from the mother, consequently mtDNA comes from the mother. (In rare cases, there may be just a few mitochondria from the sperm.)

As shown in the illustration, the present generation consists of a brother and sister (bottom line). Both have inherited their nuclear DNA from both parents, so by going back four generations we see that sixteen ancestors (top line) have contributed to their nuclear DNA. Both the brother and sister have mtDNA from their mother, but only females pass this on, so only one female of the sixteen ancestors is the source of that mtDNA. Only the brother has a Y-chromosome and it is passed from father to son, so only one male of the sixteen ancestors is the source of that Y-chromosome. Therefore, by studying different types of DNA, we can gain information about different ancestral lines. This is not as simple as it may seem, however.

 

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Inheritance of Y-chromosomal DNA and mtDNA

During the processes of replicating DNA in the body, changes or mutations can happen. Most mutations are so small they have little or no effect on the body. If mutations occur during the formation of sperm and eggs, these mutations are passed on into the next generation and, assuming the mutations do not affect the ability of the new individual to survive and procreate, these little changes accumulate over generations, increasing variability within the population. The different types of DNA change at different rates, however. The mutation rate of the Y-chromosome is faster than that of mtDNA. The most complex example involves nuclear DNA. On top of the recombination that occurs when we get half our chromosomes from each parent, there is the problem of an additional mechanism: sometimes during replication, the chromosomes in a pair will get tangled up with each other and exchange segments. This complicating factor increases the speed and amount of change that builds up in the nuclear DNA.

It is now possible to have your DNA tested. Usually this involves sampling only a portion of your DNA, since to have one’s entire genome read and transcribed would be expensive. (According to the journal Nature, the first complete human genome sequence cost nearly $3 billion, however, several companies are close to developing technology that would allow them to market a complete human genome sequencing for $1000.) Different tests are used for different types of DNA and provide results on different time scales. When Y-chromosomes are examined, portions of DNA at certain locations on the chromosome are analyzed and compared to those of other individuals who have been tested. When mtDNA is tested, usually only certain areas are examined and compared to a reference sample that is used as a standard. Results are then compared to those of other tested individuals. A low resolution mtDNA test can suggest a common ancestor was shared with another tested individual within the last 52 generations (about 1,300 years), whereas a high resolution test might decrease that to the last 28 generations (about 700 years). In contrast, because of the complexity of the nuclear DNA changes that occur from one generation to the next, tests comparing this type of DNA from different individuals may not even be able to recognize cousins (those who shared a common ancestor only two generations ago).

So, before rushing out to get your DNA tested, you should understand which type of DNA you wish to examine, based on your research questions. Then you need to understand the different tests that can be done on each type of DNA and what information that could potentially give you. Then you need to choose an appropriate service provider. Lastly, and most importantly, you need to understand the sometimes serious and far-reaching risks involved with DNA testing. These hazards will be the subject of next month’s column.

This article was originally printed in the Bergen News and is being reprinted with permission.

 

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