CONTINUITY OF THE RACE 



607 



should occur more frequently between 

 genes that lie farther apart in the chromo- 

 somes than between those lying close to one 

 another. For example, in Fig. 24-16 there 

 would be a greater chance for crossing-over 

 to occur between genes A and F than be- 

 tween A and B, because of the distance 

 between them. The frequency of crossing- 

 over can be taken, then, as a criterion of 

 the distance between genes. Thousands of 

 experiments with Drosophila, maize, and 

 others have resulted in the chromosome 

 maps (Fig. 24-4) which indicate rather 

 precisely the loci for several hundred genes. 

 Crossing-over, together with the other 

 methods of mixing o-enes during meiosis, 

 accounts for the vast array of combinations 

 that are possible in offspring, particularly 

 in organisms where the chromosomes are 

 numerous. Consider the case of man, with 

 24 pairs of chromosomes. If each of the ma- 

 ternal set could be designated A, B, C, and 

 so on, up to 24 and likewise each of the 

 paternal set labeled A', B', C up to 24, 

 following meiosis some sperms may have a 

 majority of their genes from one parent or 

 the other, but most of them will contain 

 a well-mixed set. The number of possible 

 combinations that every couple is capable 

 of producing is tremendous (see p. 558). 



MUTATIONS: ALTERATION OF 

 GENE STRUCTURE 



We already have learned that genes are 

 considered to be nucleoproteins and there- 

 fore subject to change, as are all molecules. 

 This may occur spontaneously or be pro- 

 duced artificially as a result of physical 

 forces from the outside. When the molecule 

 changes, its properties change correspond- 

 ingly. If genes are synonymous with these 

 molecules, some change must take place in 

 the genes from time to time; such gene 

 changes are called mutations, a term we 

 have used from time to time without expla- 

 nation. 



Gene mutations can be detected only 



when they produce visible changes in the 

 organism, and since most mutations are re- 

 cessive, they only become effective when 

 homozygous. A great many mutations may 

 occur but because they are recessive they 

 remain hidden in heterozygotes, perhaps 

 for many hundreds of generations. Eventu- 

 ally, by pure chance, they will show up, 

 which probably accounts for the apparent 

 spontaneous occurrence of anomalies in 

 many plants and animals, including man. 

 Strange as it may seem, most mutations are 

 harmful to the species and soon after they 

 appear are eliminated, because the animals 

 possessing them are not as well fitted to 

 cope with their environment as the un- 

 changed wild type. Through centuries of 

 selection the wild type probably already 

 possesses the best possible combinations of 

 genes to fit it for its particular environment, 

 so that any change that might be made is 

 more likely to result in an organism less 

 suited for survival. 



Undoubtedly gene mutations occur in 

 the soma cells, but when they do, there is 

 no effect on subsequent generations be- 

 cause the mutation is not passed on to the 

 offspring unless the change occurs in the 

 sex cells. Aside from the results in the indi- 

 vidual possessing them, somatic mutations 

 are of no importance in evolution. 



Occasionally a mutation may reverse it- 

 self, that is, mutate back to its original con- 

 dition. Such back mutations, as they are 

 called, rarely occur, but when they do the 

 resulting organism is apparently in no way 

 different from the original stock. Such re- 

 verse actions might well be expected in 

 the light of similar actions in protein mole- 

 cules. Similarly, certain mutations seem to 

 occur again and again in a stock of animals. 

 This might mean that under certain envi- 

 ronmental conditions genes are receptive to 

 change, and such changes are more likely 

 to occur when these conditions are met. 

 Moreover, similar mutations have been 

 known to occur in closely related species of 

 organisms under similar conditions. Such 



