Sunday, August 19, 2012

The Family DNA - the female side, part 1

When I began writing about the use of DNA in confirming some of the stories of history, and challenging others, I was really still only aware of the two major initial areas of study through which DNA was being applied. The first of these, the topic of the last post, was the male Y-chromosome, which is passed down from father to son through the generations, while the second was the matriarchal line, which is established through changes in the mitochondrial DNA passed down from a mother to her children. The study of DNA has, however, advanced considerably in the last few years, and it is now possible to get much more information from a sample and it is worth pausing to describe some of this background, since it makes later discussion a bit easier to follow.

When the body of an individual is being developed, the engine of growth needs a blueprint to follow as the various parts of the body evolve from the original egg. This direction comes from the set of instructions which are held for that individual in the arrangements of the series of sugar molecules, separated by phosphate molecules that make up that person’s Genome.

The solving of the structural shape of the Human Genome rightly brought Watson and Crick a Nobel Prize, as it explained the arrangement and nature of these instructions which lead us to become our individual selves. Our deoxyribonucleic acid (DNA) is made up of alternating sugar and phosphate molecules, with the sugar molecules being one of four basic types or nucleotides, adenine (A), cytosine (C), guanine (G) and thymine (T). The Watson and Crick breakthrough was to see that these nucleotides, or bases, would only fit together with A connecting to T, and C connecting to G, and that these linkages formed the rungs of a ladder which formed the two strings into a single double helix.

Figure 1. The interaction between the nucleotide bases to form the double helix of the DNA molecule (Technyou)

The total set of information comes divided into twenty-two pairs of chromosomes, (or autosomes) together with either a pair of X chromosomes, if a girl, or an X and a Y chromosome if a boy. Normally these cannot be seen, except when they concentrate at the time of cell division, and they normally appear as small thread-like strings within the nucleus of the cell.

Figure 2. The pairs of chromosomes in a human cell, note that they are sorted by size, and that the X and the Y are often referred to as the sex chromosomes because they dictate the sex of the individual based on the presence or absence of the Y. (Genetics Home Reference )

Because the autosomal chromosomes are sorted by length, pictures of them are also (and most typically in genealogical material) laid out in more of a tabular display. I will come back to this layout in a future post.

Figure 3. Layout of the chromosomes, (23and me), the shorter length is referred to as the p-arm, and the longer as the q-arm with the constriction being the centromere.

When a new person is born their DNA forms in their pairs of chromosomes, of which one of each pair comes from the father and the other from the mother. However the two separate parts of the new DNA each are combined out of the information in the pair of chromosomes that come from the parents of each of the new parents. This recombination of two separate strings is not as yet fully understood, since parts of the string come from one parent and part from another in segments, and I will return to this in a later post. But out of the separation and recombination of the parental strings comes the DNA that lies in the new chromosomes of the cell nucleus.

The order of the bases, i.e. the sequence in which they run – for example ATTCAG – carries an instruction, but when the strings separate because of the matching requirement, the mating string on the other side will always form in the same way – in this case TAAGTC, tailored to match the form of the separated string. This occurs all along the string and forms the new DNA within the cell. And after the formation and division of the cells, the DNA spreads out again into almost invisible threads. The DNA itself does not carry out the instruction, but rather provides the information, through a grouping of the bases in sets of three, or triplets. Thus, as Bryan Sykes illustrates, the sequence ATGACCTCCTCC becomes the instruction ATG-ACC-TCC-TCC, with each triplet specifying production of an amino acid (in the sequence, methionine, threonine, serine, pheylalanine, and phenylalanine which are part of the sequence required to generate the amino acids that combine to form keratin which is produced as hair from the hair cells on your head).

However not all the DNA material and the information is held inside the nucleus of the cell. Around the outside of the nucleus, but inside the cell wall, lies the cytoplasm which contains some very small structures called the mitochondria, which are also DNA strings. As with the Y-chromosomes these small units have segments that remain the same from generation to generation, as they are passed down. But, in contrast with the information in the Y-chromosome, that in the mitochondria comes from the original egg, and that originates with the mother. Thus the mitochondrial DNA (mtDNA) comes down the maternal line.

Figure 4. The descent of mtDNA from generation to generation, note that it goes to children from the mother, regardless of their sex, but nothing comes from the father.

When a new person is formed the mtDNA in the child replicates that of the mother. And while the nuclear chromosome is some 3 billion bases long, the mtDNA is only 16,569 bases long and it is evaluated over a range of 340 to 1,000 bases. (Smolenyak). Initially as Sykes noted the analysis was kept to a control block of some 500 bases, since this region known as the control region, where variations in the mtDNA crop up more frequently but, because the section has not defined purpose they do not select out in the next generation. (Mutations in other parts of the DNA chain induce consequences in the person when the DNA is implemented and if the variation causes a significant change, then the person may not either survive or reproduce – thus the code is, to a degree self-regulating).

In passing from one generation to the next there is not normally any variation in the sequences in the mtDNA control, but there are the occasional changes that can slip in. These are very rare, but when one does occur and can survive, then it will be passed on to the generations that follow as children, and then daughter to daughter down through the generations. That change however is unique to a branch of the main family tree, and those not on it do not carry that particular variation as the other branches extend on through time, though they make pick up others. As a result it becomes possible, by looking at the different variations at different points along a person’s mtDNA to establish were along the path of human development, an individual’s family moved. In the same way as with the Y-chromosome the variations are cumulative, so that both the points where branches diverged, and the trunk from which they diverged, can be identified.

In this way it has proved possible to trace back all of those currently alive to an individual, who lived in Africa roughly some 160,000 years ago, and who has been described as “Mitochondrial Eve.” (mtEve)

By looking at the major variations in the mtDNA control, it is possible to identify different changes that subdivide the whole into different subsets, or haplotypes. These (branches) split away at different points in history, and have been given different identifying letters, to help clarify the division.

Figure 5. The sequential change in different segments of the control region mtDNA that has created different Haplogroups. (io9)

Because some of the populations at different points along the paths that humanity has followed from the time of mtEve have stayed in the same place since that time (something which Spencer Wells has documented in Deep Ancestry- Inside the Genographic Project) it then begins to be possible to show the track of an individual’s ancestors.

As it happens we had sent away the Nurse’s sample at the same time as mine, and her result came back showing that the female ancestral line had followed the following path:

Figure 6. Path of our female ancestors through time, showing the different spin-offs of different haplogroups along the way. (The Genographic Project).

The movement out of Africa occurred with the group that was founded by the woman with the variation designated L3, who lived about 80,000 years ago and whose family moved north as the climate conditions changed, first into North Africa, and then over into what is now Saudi Arabia. Along the way the group divided, and while one group (M) moved East and eventually occupied land all the way down to Australia, our group went North and was given the designation N. This occurred around 60,000 years ago, and since there were still Neanderthals around at the time, it is perhaps no surprise that some Neanerthal DNA is now also part of the mix.

Out of the N group a smaller subset at the time, designated R, began to spread away from the central focus of the group. Given the climate conditions of the time, conditions they spread over much of Central Asia and provide one of the common groupings found almost everywhere now. (The changes that allow the discrimination between groups can be approximated – as suggested by Bryan Sykes) to occur roughly every 20,000 years, so this is around 40,000 years ago).

Out of group R the group that would form into the two haplogroups H and V now began to move West. And with the coming of the Last Glacial Maximum moved down into Spain to where it was warmer. This group (and it is interesting to note that my male line turned up in the same place at the same time) became Cro-Magnon man and now dominates the female part of the European landscape.

Bryan Sykes, in his book “The Seven Daughters of Eve”has given the different haplogroups names rather than letters. Since the variations that create the divisions occur at roughly regular intervals he can also give a rough time frame for when the original clan mother lived. (Which are in parentheses). Thus he refers to the branches as Ursula (45,000 BP), Xenia (25,000 BP), Helena (20,000 BP), Velda (17,000 BP) , Tara (17,000 BP) , Katrine (15,000 BP) and Jasmine (10,000 BP), rather than just using the letters, and 95% of European women fall into one of these groups. Our mtDNA designates us as falling into the Helena haplogroup.

While I would otherwise leave the story there for today, there is a small corollary. Bryan Sykes went on to write another book “Saxons, Vikings and Celts:The Genetic Roots of Britain." For what it is worth I think his analysis short changes the region of the borders between England and Scotland, which is where we come from, but the book does show an initial approximation of the Helenic distribution around the isles: (I say initial because the sample size is relatively small.)

Figure 7. Matriarchal clan percentages in Scotland (Bryan Sykes)

Figure 8. Matriarchal clans in England (Bryan Sykes)

I will have a bit more trying to show how to get from Cro-Magnon man to where the family ended up, in another post, and then I will try and tie some of this information back into the Europeanization of America, and back to the original topic, in posts that will follow after the family summary.

1 comment:

  1. Can you please tell me who owns the double helix icon you used here. I want permission to use it in a book. Many thanks for your great article, also.