Out of Eden: The Peopling of the World by Stephen Oppenheimer

would be donated by each parent to each offspring plant. Thus, each offspring inherited a mixture of characters from each parent, withthe combined effect of two genes determining each physical character at each generation. Since either parent could have different functioning gene types, for instance different petal colours, for each (and any) pair of genes, and only one of the two was chosen at random, the proportions of different varieties of offspring formed a pattern that could be predicted from a knowledge of the characters possessed by each parent. In this way, Mendel showed that inheritance of characters took place by the transfer of discrete packets of information. The variation in the offspring was down to the precise but random mix of these packets, or genes, in each individual, whether it be a pea seed or a human.
    Mendel was careful to choose simple, common, easily distinguished characters and to study them individually. In reality, the expression of some physical characters is determined by more than one gene, and we all have around 30,000 pairs of functioning genes because we are rather complex organisms. Thus the visible random variation between siblings in the same family is not the result of any vagueness in the process of heredity, but arises because there are large numbers of gene pairs being randomly chosen during sexual procreation. With so many gene pairs in different combinations, there is huge potential for variety. By way of contrast, the extreme similarity of identical twins gives us a glimpse of how precise the conversion of heredity into physical form really is. The extraordinary achievement of Watson, Crick, and Rosalind Franklin was to translate Mendel’s discoveries into biochemistry – or molecular biology, as this aspect of biochemistry came to be known.
    Cardboard keys to life
    ‘We are the products of our genes.’ The secret keys to this Edwardian truism were traced and cut out on bits of cardboard by two adventurer-scientists, Jim Watson and Francis Crick, in 1953. 34 Their ‘keys’ were scale diagrams of four chemicals (nucleotidebases), whose unique interlocking relationship, set in the double-stranded zip-locking deoxyribonucleic acid (DNA), holds the code for life on Earth. Those bits of cardboard unlocked the mechanism linking Mendel’s work to the theory of evolution by natural selection as set out by Darwin in the
Origin of Species
. Watson and Crick explained exactly how thousands of unique characteristics, varying from one individual to another, are passed on intact from generation to generation. In short, it was the greatest advance in biological understanding in the twentieth century.
    Within each of the cells of our bodies we all have incredibly long strings of DNA. It is the stuff of the genes. It stores, replicates, and passes on all our unique characteristics – our genetic inheritance. These DNA strings hold the template codes for proteins, the building blocks of our bodies. The codes are ‘written’ in combinations of just four different chemicals known as nucleotide bases (represented by the letters A, G, C, and T), which provide all the instructions for making our bodies. We inherit DNA from each of our parents, and because we receive a unique mixture from both, each of us has slightly different DNA strings from everyone else. Our own DNA is like a molecular fingerprint.
    During human reproduction, the parents’ DNA is copied and transmitted in equal proportions. It is important to know that although most of the DNA from each parent is segregated during reproduction, small bits of their respective contributions are shuffled and mixed at each generation. The mixing here is not that of mass random allocation of genes inferred by Mendel, but tiny crossovers, duplications, and swaps between maternal and paternal DNA contributions. This is known technically as
recombination
. Luckily, for the purposes of genetic researchers, there are two small portions of our DNA