242 PRINCIPLES OF ANIMAL BIOLOGY 



the two maturation divisions^ (the first in some animals, the second in 

 others) , the chromosomes do not spHt as in ordinary mitosis, but the chro- 

 mosomes that have come together merely separate again. Each daughter 

 cell receives one of the chromosomes of each pair, not halves of both of 

 them. The number of chromosomes in each daughter cell is thereby 

 reduced to one-half. In the fruitfly Drosophila, whereas the somatic 

 cells and the oogonia and spermatogonia have eight chromosomes, the 

 mature germ cells have only four. 



Moreover, as a result of this reduction division in maturation the 

 germ cells may be different from one another. If the two chromosomes 

 of a pair are precisely alike in all their genes, then all the mature germ cells 

 will be precisely alike with respect to those genes. If, however, the chro- 

 mosomes are unlike in some of their genes, the mature germ cells will be 

 unlike. Thus, if an individual contained the two chromosomes repre- 

 sented in Fig. 189, the mature eggs produced by it would all be alike with 

 respect to their genes for size and shape of wings and genes for shape of 

 bristles, but would be of two different kinds with respect to their genes 

 for body color, eye color, and extent of eye. The fact that the different 

 mature germ cells of one and the same animal may carry different genes is 

 of capital importance in heredity. 



Terminology „ — The distribution of different genes to different germ 

 cells is called segregation, and it results from the phenomenon of reduction 

 (the separation of homologous chromosomes withput division) in one of 

 the maturation divisions. The hereditary constitution of an animal 

 (with certain exceptions) involves two genes for each Idnd of character; 

 the hereditary constitution of a germ cell includes but one gene for each 

 kind of character. The formula of an individual is thus a double one, 

 that of the germ cells a vsingle one. Thus, since genes are usually symbol- 

 ized by letters, if the formula of an animal with respect to any character 

 were AA, the formula of its germ cells would he A. If the formula of an 

 animal were Aa, the genes for one kind of character being unlike, the 

 germ cells would be of two kinds, some with the formula A, others with 

 the formula a. An animal in which the two genes for a given kind of 

 character are alike {AA or aa) is called a Jwmozygote, and is said to be 

 homozygous. If the two genes of the pair are different (Aa), the animal 

 is a heterozygote, or is said to be heterozygous. A given animal mg,y be 

 homozygous for some characters, heterozygous for others. A fly con- 

 taining the chromosomes of Fig. 189 would be homozygous for four 

 characters, heterozygous for four others. 



With the foregoing facts concerning the germ cells and their chromo- 

 somes in mind, it is possible to analyze the results of breeding experiments. 

 The better known cases rest, as is shown below, upon a very simple foun- 

 dation. 



^ This statement would have to be altered to be applicable to plants, since the 

 time of reduction of the chromosomes varies. ^ 



