200 



DISCOVERY 



This; halving of the chromosome-number before the 

 formation of tlie reproductive cells is a universal 

 occurrence in animals or plants which possess sexual 

 reproduction, and is known as reduction. Thus at 

 fertilisation, one complete set of chromosomes derived 

 from the father is united with a second complete set 

 derived from the mother. 



Wlien these facts were first discovered, the theorj' 

 was promptly advanced that it was by means of the 

 chromosomes that inheritance was to be explained. 

 The ovum and sperm differ from each other in 

 almost every particular of size and shape and con- 

 stitution. There is only one point in which they 

 agree, and in that they are identical : both possess 

 one complete set of chromosomes. It was therefore 

 natural to suppose that it is by means of these chromo- 

 somes that organisms — whether plants, animals, or 

 human beuigs — inherit characters equally easily, as 

 we know they do, from their mother's or their father's 

 side. We shall see later that this theory has been 

 abundantly justified by the facts that have since been 

 discovered. 



Meanwhile, let us not think too much of theories, 

 but merely of the established fact that all higher 

 living things possess in every cell of their body two 

 complete sets of chromosomes, one coming from the 

 father, one from the mother, and that each of these 

 is accurately divided every time that a cell divides. 



WTiile one set of workers was establishing these 

 facts, others were engaged on extensive plant- and 

 animal-breeding experiments, with results which will 

 be the subject of the next section. 



(2) Breeding Experi.ments and the Idea of 

 Unit-Factors 



The other line of attack on the problem of heredity 

 has, of course, been by breeding experiments. Enor- 

 mous numbers of crosses between different varieties 

 both of animals and plants were carried out during 

 last century, and a number of general conclusions 

 arrived at. But there remained a feeling that we had 

 only scratched the skin of the problem, without any 

 real penetration of its hidden secrets. Then, in 1900, 

 a dramatic event in scientific history took place. 

 Two independent workers, reading through the 

 literature on the subject in the hope of hitting on 

 something to illuminate their results, happened upon 

 an account of some experiments carried out and 

 recorded as far back as 1866 by an Austrian monk, 

 Gregor Mendel, Abbot of Briinn. These had lain 

 unproductive for half a century, like seed in a box. 

 Now at last they were to germinate. Both men 

 realised their importance ; they have served as the 

 basis of all the enormous advance that has since been 

 made ; and there has sprung into being a new science, 



of animal- and plant-breeding carried out on the 

 Abbot's principles, and called Mendelism in his honour. 



What were the new principles that proved so illu- 

 minating ? Put as briefly as possible, they are as 

 follows : W'herever one variety of an animal differs 

 from another variety in a constant and well-marked 

 way, these differences can be shown, by cross-breeding 

 experiments, to depend on a definite number of separate 

 characters which can be passed on from parent to 

 offspring — in other words inherited — and each of 

 which can, by proper crosses, be separated from the 

 others. An example will make this clear. There are 

 two races of fowls, the one black, the other white with 

 black splashes. When crossed together, all their 

 offspring in the first, or, as it is often called, the Fi 

 generation, are ahke ; but they do not resemble either 

 parent. This form of fowl is bluish-black with black 

 lacing, and is called the Blue Andalusian. If these 

 Andalusians are bred together, the next, or F2, genera- 

 tion will consist of three different sorts of fowls- 

 white exactly like their white grandparents, black 

 exactly like their black grandparents, and Andalusian 

 exactly like their parents. These will always appear, 

 if a sufiicient number is bred, in the same proportion, 

 the whites averaging 23 per cent., the Andalusians 

 50 per cent., and the blacks 25 per cent. Mendel, as 

 a matter of fact, worked not wth fowls, but with 

 different strains of peas, but got preciselj' similar 

 results. The explanation he gave was that something 

 which made the black fowls black was present in 

 their reproductive cells, and so handed to their off- 

 spring ; while in white fowls, a slightly different 

 something w^as handed on in the same way. These 

 " somethings " may be called /(Jc/ors, and the appear- 

 ances they produce in tlie adult fowl characters. When 

 whites are crossed with blacks, the fertilised egg 

 receives one white-producing and one black-producing 

 factor. The resulting character is neither black nor 

 white, but a compromise, the bluish colour of the 

 Andalusian. '\Mien the Andalusian fowls come to 

 reproduce, the Mendelian theory supposes that all 

 the reproductive cells \vill receive either a black- 

 producing or a white-producing factor, but not both. 



Let us for convenience call the former B and the 

 latter b. There are, then, four possibihties for the 

 next generation. A sperm with B can equally well 

 fertilise an ovum with B or one with b ; and the 

 same two possibilities are open to sperms containing 

 b, and eacli of these unions is equally likely to occur. 

 Thus on the average, out of every large lot of resulting 

 fowls, one-quarter will contain B -f B, one-half B + b, 

 and the last quarter b + b. But B -|- B will give a 

 black fowl, B -t- 6 an Andalusian, and b + b a. white. 

 We thus have an explanation both for the sort of 

 fowls produced in the Fi generation, and of the 



