GENETICS 241 



they are sufficiently accurate. The two genes come from the two parents; 

 one is in the egg before fertihzation, the other is introduced by the sperm. 

 They may be hke one another, or of different kinds, as is pointed out 

 later. When the fertilized egg develops into an adult organism, the two 

 genes for each character are handed on through successive cell divisions 

 to the body cells of the adult. Thus every animal produced bi-sexually 

 has a double hereditary constitution that may be represented by a 

 formula naming the genes which it contains. Theoretically, at least, 

 this formula is the formula of every cell of the body; for the ordinary 

 processes of cell division seem to insure that all the genes contained in the 

 fertilized egg are passed on to every cell produced by that egg. 



The mechanism which appears to guarantee that the same group of 

 genes shall be assembled in the daughter cells as was present in the mother 

 cell is found in the equal division of the chromosomes in mitosis. For 

 reasons which will appear as this discussion proceeds, the genes must be 

 regarded as being in the chromosomes. There is strong evidence to 

 show that they are arranged in a row through the length of the chromo- 

 some, as shown in Fig. 189. When the chromosome divides in the meta- 

 phase of cell division, each gene presumably divides, so that each daughter 

 cell has a chromosome containing the same set of genes as did the parent 

 cell. In animals capable of regenerating missing parts, the general dis- 

 tribution of genes throughout the body may be demonstrated. If the 

 worm Lumbriculus is cut in two, in certain regions, a tail develops 

 from one of the cut surfaces, a head from the other cut surface. Genes 

 for the structures of both head and tail must have resided in the cells at 

 the level of the cut. It is possible, of course, that some unknown feature 

 of cell division may cause the splitting of the chromosomes to be unequal, 

 but the higher animals afford no means of demonstrating such inequali- 

 ties. By analogy with the lower animals, one would expect genes for 

 eye color to be lodged also in the cells of the liver, and genes for intelligence 

 in the cells of the leg bones. 



The t wc) genes for a given kind of trait occur in two separate chromo- v/ 

 somes. One of these chromosomes is maternal, having been in th*e unfer- 

 tilized egg; the other is paternal, having been introduced by the sperm. 

 Other genes may reside in the same two chromosomes, but the genes of the 

 one chromosome have to do with the sarae kinds of characters as do the 

 genes of the other chromosome. Such chromosomes may Ije said to be 

 homologous with one another. Figure 189 shows a pair of homologous 

 chromosomes, one maternal and one paternal, of the fruitfly Drospphila. 



The germ cells, prior to maturation, have the same outfit of chromo- 

 somes, and hence of genes, as do the body cells. During maturation, 

 however, their genetic composition is considerably altered. As was 

 pointed out in Chapter X, homologous maternal and paternal chromo- 

 somes meet in pairs during or before the growth period. Then in one of 



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