So far, we have pretended that there is only one pair of chromosomes, each of the pair carrying merely one gene. In fact, every chromosome carries a great many genes—it is a large railway-carriage packed full of passengers—and most organisms have many more than one pair of chromosomes. For the moment, we need not concern ourselves with what happens when a pair of chromosomes carries more than one pair of genes; nor with the possibilities arising from 24 pairs of genes each in a different one of man’s 24 pairs of chromosomes. But it is impossible to understand one of the most important things about heredity if we look only at a single pair of chromosomes, so now we will take three of the human pairs for an example. It is easiest if we name them after playing-cards, so, the first chromosome is the Ace, and then come the King and the Queen, and we can suppose that one parent comes from a pack with Black backs, the other from a pack with White.
The child of these parents—call it a girl—must inevitably have a Black Ace, King, and Queen, and corresponding White cards. And we must suppose that she grows up and mates with a boy of similar parentage.
Now, when we were looking at only two chromosomes, we saw that in the ‘reduction division ‘that precedes the formation of gametes, the two chromosomes always separated from one another. And the same thing happens this time—if the Black Ace, that is, goes to the top of the cell, the White will go to the bottom. The Kings likewise will separate from each other, and so will the Queens.
But all the Black cards do not go to one end of the cell, all the White to the other. Each pair sorts itself out independently of the other pairs, so that in one gamete there may be a Black Ace, and the two other cards White; in the next a White Ace and King, but a Black Queen. There are obviously many other possible combinations.
And now, to round off things, let us find genes and characters to fit these chromosomes: Ace contains a gene governing eye-colour (Black = dark; White = light). King contains a gene governing hair-form (Black = curly; White = straight). Queen contains a gene governing jaw-bone (Black = Hapsburg; White = normal).
This grandchild, therefore, has hazel eyes, taking light from his maternal grandfather, dark from his paternal grandmother; pure curly hair, taking the genes from each of his grandmothers; and a normal chin, taking the genes from each of his grandfathers. He is a chance assortment of all the factors which his four grandparents handed on to his two parents. In the dance of the chromosomes, not only are opposite numbers bound to separate completely, but the different companies of the same ‘army ‘are liable to do so.
THE CHANCE THAT DECIDES THE NEXT GENERATION THIS is very different from the vague idea that most people have of the relation between parents and children. They wrongly tend to look upon father and mother as rather like
jugs of black coffee and white milk, respectively. The pale-brown cup of cafe an lait is the child they produce, and that child, in its turn, hands on cafe au lait to the third generation.
This is the wrong analogy. You will get the right one if you will take two packs of patience cards—let us say red and blue—and call them the gametes produced by the first generation. Shuffle them together to make a child (second generation). Now pick out a complete pack from that child, but without looking at the backs of the cards, which will therefore be a mingling of red and blue. This is a gamete for making the third generation.
Do it all over again, but this time start with a yellow pack and a green pack. Now shuffle together your red-blue and your green-yellow pack. This is the grandchild, the third generation, which will show all four colours in its gametes.
Start all over again with purple-and-gold and orange-and-silver, finally combining the ultimate gamete with that from the red-blue-green-ycllow series, and so produce the fourth generation.
It will not be very long before the shop fails to supply you with any new colours, so you will be driven to combine the final gamete of your series with one of the old colours, so that both your Aces of Spades, for instance, will have green backs— the child, that is, will be pure for the set of genes carried in that pair of chromosomes, though most of the others have come from a variety of ancestors. This is very much what happens in life itself. (The human double ‘pack,’ by the way, consists of only 24, not 52, pairs of cards; but the genes in a chromosome are very much more numerous than the pips on any card.)
Since each chromosome carries so many genes, we should expect to find that two or more characters are inherited together; and in fact a great many instances of this ‘linkage ‘are known. In Drosophila, the fruit fly, for example, grey body and straight wings go together, black body marches with curved wings. In sweet peas, the kind with long pollen grains are purple, while the red flowers have round grains. You can say that the top pip of the three of hearts controls the pollen shape, while the bottom pip controls the flower-colour. Alternatively, ‘linkage ‘may be described as two genes being passengers in the same railway-carriage. It is difficult to know the linkages in man, owing to the large number of chromosomes and the impossibility of experimental breeding.