FOR the sake of lucidity, we have so far considered only well-known or easily visualised characters, such as tall and short, black and grey, and have stuck to those that depend on a single pair of genes. But obviously such big and important characters as milk-yield in cattle, health or intelligence in man, or disease resistance in plants must usually have a much more complex hereditary basis. Most of them, indeed, depend on many pairs of genes, each perhaps in a different pair of chromosomes. One simple example (that has been several times known) will explain how this works.
A White and Negro married, producing mulatto (cafe au lait) children. One of these married another mulatto and had a large family—one child much whiter than either parent, one much blacker, and the rest an assortment of varying coffee shades. Evidently there has been no blending (or all the children would be the same colour), and you can see what has happened if you suppose that there are six pairs of genes involved (the exact number is not known) in as many chromosome-pairs.
Both the mulattoes, therefore, have six black genes and six white; but each of their children is likely to get a different assortment—one with eight black genes and four white; the second, the opposite; a third, half and half (like the parents); a fourth, eleven white genes and only one black—and so on. Straightforward cases of this sort are called cumulative genes.
The ‘rose-pea-walnut ‘series of comb-shapes in chickens is a simple example of another very common kind of complication called complementary genes, which you will find explained in the books mentioned at the end of this section.
A whole large complex of such complementary and cumulative genes, spread out over many chromosomes, must be responsible for such complicated things as the different varieties of the normal human brain, the fitness of certain plants for various soils and climates, the excellence (or otherwise) of some dogs for driving sheep, and similar characters.
Let me hastily beg you not to be discouraged by this complexity of the hereditary machine. It was necessary to mention the wheels within wheels. Now you can put those details to the back of your mind, and only keep clear about the simple main principles. After all, you do not need to understand the theory of electro-magnetics in order to turn on the radio, nor
even to be a practical wireless engineer. And complicated characters can be understood (and developed or eliminated) with no more than the outline of genetical principles. The great improvement in English livestock and in most cultivated plants was achieved by men who had never heard of genes and chromosomes.