ANATOMY OF OUR GENES
The Human Body
The human body is made of some 50 trillion to 100 trillion cells, which form the basic units of life and combine to form more complex tissues and organs. Inside each cell, genes make up a blueprint for protein production that determines how the cell will function. Genes also determine physical characteristics or traits. The complete set of some 20,000 to 25,000 genes is called the genome. Only a tiny fraction of the total genome sets the human body apart from those of other animals.
Most cells have a similar basic structure. An outer layer, called the cell membrane, contains fluid called cytoplasm. Within the cytoplasm are many different specialized “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 mitochondrion. 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 cell’s nucleus would stretch to over six feet 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-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.
Shuffling the Deck
For most of our genome we receive half of our genes from our father and half from our mother. Each half represents a shuffled combination of DNA passed down to us from our ancestors. This recombination process makes it difficult to study lines of descent because 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.
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 an identical 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 our genome. This allows the Y to be passed down through a purely male line, changed only by random mutational events.
Mitochondrial DNA (mtDNA)
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 (HV 1 and 2), where the rate of mutation has been shown to be up to a hundred times greater than that of the nuclear genome. Because of its much shorter length (several hundred nucleotides versus millions of nucleotides for the Y), the HV region 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 into two unique strands. The individual strands duplicate themselves for the next generation, but the process is not always perfect. Random “copying errors” along a 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 favors 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.
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 with population size. Smaller populations are subject to much greater genetic drift for the same reason that a result of seven “heads” in ten coin flips is much more likely than 700,000 “heads” in one million coin flips. The effects of genetic drift in a small population can be quite dramatic. Random genetic changes in those individuals whose descendents survive can then be repeated in 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 replication, 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 “nodes” on the tree where a mutation split a branch into two directions. Eventually these branches can be followed backward all the way to a common African root—a common ancestor.