224 PRINCIPLES OF ANIMAL BIOLOGY 



Heredity, Mendel concluded, juggles characters, not individuals; it 

 deals with traits, not ancestors and descendants. The complete inde- 

 pendence which he supposed characters to have is illustrated by the peas 

 which he studied. He found that shape of pod, color of seed, height of 

 stem, etc., were entirely free to go to the various offspring without refer- 

 ence to the other characters. Thus there arose different combinations 

 of the characters in different plants. One would have constricted pods, 

 green seeds, and tall stems; another inflated pods, green seeds, and dwarf 

 stems; a third constricted pods, yellow seeds, and dwarf stems; and so on. 

 This freedom of assortment proved later, in heredity in general, to be 

 less than Mendel supposed, but it is very ^\idespread. 



Mechanism of Heredity. — How heredity operates will be more 

 easily understood if its mechanism is known. Inherited characters are 

 represented in the cells of an organism by minute bodies called genes. 

 These genes are located in the chromosomes and are demonstrated in 

 some animals and plants to be in a row, from one end of the chromosome 

 to the other. There are two genes representing each character in each 

 cell, one of them derived from the mother, the other from the father. 

 These two genes must, from their source, be in two different chromo- 

 somes, one of which has come from the individual's mother, the other 

 from its father. The genes in one of these chromosomes all relate to 

 the same characters as do the genes in the other chromosome. Two of 

 the chromosomes in the cells of the vinegar fly Drosophila are diagram- 

 matically sho^vn in Fig. 192. Two chromosomes having corresponding 

 genes, as these do, are said to be homologous with one another (see 

 page 252 for homology). The genes in them are likemse homologous; 

 the gene for yellow body is homologous with the gene for gray body, 

 white eye with red eye, complete eye with bar eye, and so on. 



All the chromosomes in a cell are members of such homologous pairs. 

 One chromosome of each pair has come from the mother, the other from 

 the father. The two homologous chromosomes come together in a 

 pair in the oocytes and spermatocytes early in the maturation of the 

 germ cells, as in Fig. 164. In the reduction division they are separated 

 again, one going to each of the cells produced by that division. Since 

 the genes are in the chromosomes, the two homologous genes of every 

 pair part company at the reduction division, one gene going to each of 

 the cells produced. At the end of maturation in the male, each sper- 

 matozoon contains one gene of every pair, never both of any of them. 

 In the female, each mature egg contains one gene of every kind, ncncr 

 both. Polar bodies receive their share of the genes, but these genes are 

 lost as the polar bodies degenerate. 



As a result of the reduction division, therefore, the mature germ cells 

 have a single set of genes, one of every kind. Body cells, on the con- 



