DNA has become an important part of genealogical research, but it’s a complex subject with more than one kind of DNA to test. This is a somewhat simplified explanation of how DNA works, written as a supplement to our DNA study, for those who are curious or who want to understand better the principles upon which DNA studies depend.
Cells are complex little things that are typically specialised to a particular purpose. They have a great deal of internal structure as you can see from the Wikipedia description of the cell nucleus. Mitochondria and the cell nucleus are just two of many organelles found in most cells and, with some notable exceptions (eg human red blood cells which have no nucleus) both carry DNA, although only nuclear DNA affects our physiology such as hair and eye colour etc.
A chromosome is a division of the whole of the genome. The human nuclear genome is made up of 23 pairs of chromosomes (22 autosome pairs and one allosome or sex-specific pair). Each chromosome carries a series of genes (each of which is made up of a series of base pairs) and quite a lot of structure required for the cell division and replication process including telomeres which protect the ends of each chromosome and centromeres which have to do with how chromosomes are correctly paired up. There’s a great deal more to it, but none of it particularly important to a basic understanding of how genetics works.
Side note for curiosity’s sake: humans and the great apes, our nearest living non-human relatives, share about 97% of nuclear DNA in common (up to 99% in the case of chimpanzees), but there’s an extra twist (pun not intended): the great apes have 24 pairs of chromosomes rather than 23. If species with differing chromosome counts can’t interbreed, how is it possible that H. sapiens can be related to any of the great apes?
The answer is that, at some point in our past, two chromosomes in some long-distant common ancestor fused to become what, by convention, is called chromosome 2. The evidence for this is that, unlike every other chromosome, chromosome 2 contains two centromeres plus the vestigial remains of telomeres within the chromosome, and the genes found on chromosome 2 correspond with the same genes found on two separate chromosomes in great apes.
Cells are constantly dying off, typically by apoptosis or programmed cell death that replaces damaged cells, by the process called mitosis whereby a cell divides into two theoretically identical cells. I say “theoretically” because mitosis isn’t perfect and sometimes errors are introduced into the new copy. It is known that, as a person ages, the telomeres get shorter and it is thought that shortening telomeres contribute to the physical effects of aging of the body.
There is a related but specialised form of cell division called meiosis, which is the means by which gametes (sperm and ova) are produced. There are two important differences between mitosis and meiosis:
- each division results in three cells instead of two, each of the two new cells containing only half of each pair of chromosomes (ie 23 chromosomes instead of 46), and
- there’s an extra step wherein autosomes mix up the genes found on each of the pair of chromosomes, a process called chromosomal crossover and genetic recombination.
Chromosomal crossover is why children share some physical attributes with their parents and why, excepting identical twins, there can be such variation between children of the same two parents. Chromosomal crossover is also why autosomal DNA “dilutes” so quickly: if about 50% of the genes are swapped with each generation, the third generation shares only 1/8 (12.5%) of DNA with the ancestor and the fifth generation shares only 1/32 (3.1%). That weakens confidence of a match exponentially with each generation.
Chromosomal crossover can’t happen with the allosomes because in boys, the two chromosomes of the pair are of different types, X and Y, and though girls have two X chromosomes, one of them is always deactivated and becomes what’s called a Barr body and therefore is not available for gene exchange.
Side note for curiosity’s sake: the X-inactivation process doesn’t always deactivate the same X chromosome in every cell, which gives rise to some interesting results by way of a phenomenon known as mosaicism.
In cats, the genes for fur colour are carried on the X chromosome and code for either red or black fur. White fur results when the colour gene is turned off altogether. Some female cats carry the gene for red fur on one X chromosome and black fur on the other X chromosome, and the resultant tortoiseshell coat depends on which of the two X chromosomes is active in any given spot.
Strangely, male tortoiseshell cats do exist but by a rare and completely different phenomenon known as chimeraism in which two genetically distinct sets of cells get stuck together at a very early stage of embryological development. Instead of developing into distinctly individual cats, you get one cat with two sets of distinct nuclear DNA which can result in a tortoiseshell tom.
In humans, the genes for colour vision are carried on the X chromosome. It is thought that there are variations of these genes that code for cone cells (those responsible for colour vision) that have different peak wavelength sensitivities and that, in the same way that female cats can have a bi-coloured coat, both sets of colour genes are expressed in the retina. That means that instead of having three types of cone (trichromacy), some women may have four, ie be true tetrachromats. Such women may be able to discern differences in colour that all men and most other women cannot.
The Y chromosome and mitochondrial DNA are so useful for genealogical purposes because they have no complement with which to swap genes, meaning that lineage can be traced over hundreds of thousands of years. If there were no mutations over the ages, then both would be an exact copy of the oldest ancestor whose Y-chromosome or mitochondrial DNA survives today but, in practice and as mentioned before, mutations do happen. If the rate of mutations remains roughly constant, then it should be possible to tell how closely any two individuals (or groups of individuals, eg haplogroups) are by the number of differences between Y chromosomes of two men or mitochondrial DNA of two women — but only when there is a direct common male or female ancestor to pass down the same DNA.
This idea was put to practical use at a group level in the Mitochondrial Eve hypothesis and its male complement, Y-chromosomal Adam. These hypotheses are not without criticism but regardless of their merits, it’s clear that there is far greater genetic diversity amongst Africans than there is amongst the rest of the world, which is why it is thought that H. sapiens originated in Africa.
It should be noted that neither hypothetical “Adam” nor “Eve” were any one specific individual, they certainly are not literal references to the Biblical Adam and Eve, they were probably not contemporaneous and there were probably multiple such Adams and Eves because humans didn’t suddenly spring into existence as we are today; rather, we slowly morphed from H. erectus and H. neanderthalensis into the H. sapiens of today. Speciation, the process of a population dividing into distinct and separate species, is as much a matter of opinion as it is fact, and it can be difficult to pin down exactly where and when two populations became distinct species.
Side note for curiosity’s sake: a direct, modern-day example of this difficulty is the phenomenon of ring species. A ring species is one in which a series of neighbouring populations can successfully procreate together, but for which there are “end” populations which are too distantly related to breed despite the unbroken chain of populations that can interbreed.
Putting it all together, then:
- Everyone has autosomal nuclear DNA, but genetic recombination limits its usefulness except where you have reason to suspect that two individuals are reasonably close cousins.
- Everyone has at least one X chromosome, but its usefulness is limited by the fact that only one of a female’s X chromosomes is active and it’s luck which one you get. At best, it can only supplement autosomal DNA tests.
- Everyone has mitochondria, but its usefulness is limited by the fact that it follows the female line and the fact that the female line is so difficult to trace because of the Western habit of women taking their husbands’ surnames.
- It’s no coincidence that tracing the male line is the easiest by conventional genealogical techniques, and so it is also with DNA. The Y chromosome (usually) follows the surname, which is why Y chromosome testing is the most common DNA testing done.