274 EVOLUTION, GENETICS, AND EUGENICS 



ent allelomorphs. — To use Mendel's own expression, "the relation of 

 each pair of different characters in hybrid union is independent of the 

 other differences in the two original parental stocks." This third 

 law is only discoverable when we try to follow the assortment and 

 recombination of at least two pairs of allelomorphs up to the second 

 hybrid generation (F 2 ). If each allelomorph be studied by itself, it 

 will show nothing more than the facts indicated in the first two laws, 

 but as soon as we try to follow the modes of inheritance of more than 

 one character simultaneously, we find that we are merely dealing with 

 the independent shuffling and assorting of two or more genes. The way 

 in which we explain the third law is that all genes that exhibit inde- 

 pendent assortment are located in different chromosomes. If two 

 allelomorphs were in the same chromosome, it is obvious that their 

 association with each other in heredity would be much closer than 

 if they were in different chromosomes. Remember this when we come 

 to consider a later proposition called linkage. 



Mendel's fourth law: the law of recombination. — According to 

 Mendel, this law means "that the constant characters which appear in 

 the several varieties of a group of plants (or animals) may be obtained 

 in all the associations which are possible according to the mathematical 

 laws of combination." The genes carried by the chromosomes are 

 shuffled about like a pack of cards and dealt out in all possible combina- 

 tions according to the laws of chance. The result of this is that the 

 particular deal, or "hand," that happened to be possessed by the parent 

 is likely not to be repeated in any of the offspring if the number of 

 differences involved is at all large. Of course, if there is only one point 

 of difference between the two parents, the character of the parent will 

 be repeated in one out of each four individuals of the F 2 generation. If 

 there are two pairs of allelomorphs concerned, there will be one in 

 sixteen in the F 2 with the same combination as each original parent; 

 if three pairs of allelomorphs, one in sixty-four ; if four pairs, one in two 

 hundred fifty-six; if ten or more pairs, one in hundreds of thousands 

 or millions. Nearly all human beings differ from one another with 

 regard to hundreds of allelomorphs. Is it not remarkable, then, that 

 there is as much resemblance between two brothers as there sometimes 

 is? This condition will be better understood when we come to discuss 

 the limitations of the law of independent assortment, which Mendel 

 failed to discover, and which is explained by the law of linkage. 



The concepts expressed in the above laws may be considered to 

 have originated with Mendel. It must be remembered, however, that 



