The writer, Ray Buschmann, and his wife, Angie, have donated their DNA to participate in the U.S National Geographic “Genographic Project”. Physically this means taking a swab inside of your cheek with a supplied sanitised scrapper. The scrapper has a snap-off head which allows the DNA to be passed into a collection tube and sealed without any contamination from hands. The device is then placed in a special mailing parcel and despatched to the National Geographic Society in Memphis, Tennessee USA. Waiting time is about 2 months when your results are mailed to you. Your name is never associated with the DNA results, rather a code number than only you can decipher. No health or any other personal analysis is done – this is an anthropological study only.
By using sophisticated laboratory and computer analysis of DNA contributed by hundreds of thousands of people world-wide, migratory patterns emerge – the secret to unlock the patterns are newly found markers in some people’s DNA: it is our good luck that the writer and his spouse carry this marker and so geneticists can discover the human journey that our very own ancestors travelled, starting in Africa which is now known as the common root for all families.
In order to understand the process, the following is a summary of how the human body is able to carry, in today’s world, markers of some 60,000 years ago.
The human body is made up of some 50 – 100 trillion cells which form the basic units of life and combine to form the basic units of life and combine to form more complex tissues and organs. Inside each cell, genes comprise a “blueprint” for protein production that determines how the cell will function. Genes also determine physical characteristics or traits. The complete set of some 30,000 to 40,000 genes is called a genome. Only a tiny fraction of the total genome sets the human body apart from those of animals.
Most cells have a similar basic structure. An outer layer called the cell membrane, contain fluid called cytoplasm. Within the cytoplasm are many different specialised “little organs” called organelles. The most important of these is the nucleus which controls the cell and houses the genetic material in structures called chromosomes. Another type of organelle is the mitochondria. These “cellular power plants” have their own genome and do not recombine during reproduction.
Chromosomes carry hereditary, genetic information in long strings of DNA called genes. Humans have 22 numbered pairs of chromosomes and a single pair of sex chromosomes; XX in females and XY in males. Each chromosomal pair includes one inherited from the father and one from the mother. If unwound, the microscopic DNA strands in one’s cell nucleus would stretch to over 2 metres in length.
DNA (deoxyribonucleic acid) is the set of genetic instructions for creating an organism. DNA molecules are shaped like a spiral staircase called a double helix. Each stair is composed of the DNA bases A, C, T and G. Some segments of these bases contain sequences like A-T-C-G-A-A-C-T-A-G, which constitute individual genes. Genes determine which proteins individual cells will manufacture and thus what function particular cells will perform.
For most of our genome we receive half of our genes from our fathers and half from our mothers. Each half represents a shuffled combination of DNA passed down to us from our ancestors. This shuffling process makes it difficult to study lines of descent – it creates a genetic mix of everyone who has come before. Fortunately for anthropological geneticists there are parts of the genome that are passed down unshuffled from parent to child. In these segments the genetic code is varied only through occasional mutations – random spelling mistakes in the long sequence of letters that make up our DNA. When these mutations are passed down through the generations they become markers of descent.
Male Lineage – the Y Chromosome: The Y chromosome is the sex determining chromosome in humans. While all other chromosomes are found in matching pairs, it is the mismatch of the Y with its partner, the X chromosome, that determines gender – men have a mismatched pair (Y and X), while women have two X chromosomes. Because the Y does not have a matching chromosome, most of it (the non-recombining region or NRY) escapes the shuffling process known as recombination that occurs every generation in the rest of the genome. This allows the Y to be passed down through a purely male line, changed only by random mutational events. These mutations have given rise to many useful genetic markers, such as varying geographic distributions in men around the world. Female Lineage – Mitochondrial DNA: If the Y chromosome traces the male lineage back through history, then the mitochondrial genome (mtDNA) can be considered its female counterpart.
Mitochondria are self-reproducing structures found inside the cells of all higher organisms, typically present in hundreds of copies per cell. They are responsible for generating most of the energy used by the cell. Because there are no mitochondria in the head of a mature sperm, they are passed down solely from mother to offspring. One region of particular importance in mtDNA is the hypervariable region (HVR 1&2), where the rate of mutation has been shown to be up to 100 times greater than that of the nuclear genome. Because of its much shorter length (several hundred nucleotides, versus millions of nucleotides or Mb, for the Y), the HVR can be quickly scanned to reveal many informative mutational events that have been passed down through the maternal line. Mutations are random changes in an individual’s DNA sequence which occur very rarely in each new generation. During reproduction each cell’s DNA double helix separates. The individual strands duplicate themselves for the next generation but the process is not always perfect. Random “copying errors” along genome’s long spelling sequence of base pairs are called mutations.
As genetic markers are inherited they are passed down through generations forming a complex story that can be traced backward in time. The exact shape of this tree is affected by other evolutionary forces: natural selection, genetic drift and migration. Natural Selection: is an evolutionary process that favours beneficial genetic mutations and limits harmful ones. Organisms that possess an advantageous trait either attract mates more easily or survive in greater numbers. Such traits are passed on to increasingly larger numbers of individuals with each successive generation. The cumulative effect of natural selection produces populations that have evolved to succeed in their unique environments. Genetic Drift: some genetic changes such as allele frequency occur randomly within a population and are passed from parent to offspring. The effect of this “genetic drift” varies from population. Smaller populations are subject to much greater genetic drift. Initially random or infrequent changes in a few individuals can subsequently be repeated throughout the ever growing numbers of successive generations. The Human Family Tree: Y chromosome DNA, passed from father to son, and mitochondrial DNA passed from mother to daughter are varied through the generations only by occasional natural mutations called markers. These mutations, occurring in an otherwise continuous string of genetic replications serve as genetic signposts for tracing human evolution.
By following a marker back through time to its origin, geneticists can identify the most recent common ancestor of everyone alive who carries a given marker. The divergent branches of the human family tree, represented by groups carrying a given marker, can be followed back to the “nodes” on the tree where a mutation split a branch in two directions. Eventually these branches can be followed backward all the way to a common African root – a common ancestor.
Want To Know More? Contact Ray to answer any questions you may have, or go directly to the Genographic website at www.genographic.nationalgeographic.com for more information and how you can become a participant.