GENETIC FACTORS IN THE ORIGIN OF DIVERSITY 385 



mary oocyte divides is determined by chance. Thus it is as likely that the 

 paternal "long chromosome" will be discarded in the first polar body as it 

 is that the maternal one will be. In this case the secondary oocyte, and hence 

 the ovum, would contain M. Hence in the long run half the ova produced 

 by such a heterozygous female may be expected to contain M, half to con- 

 tain m. 



As a result of meiosis in both sexes haploid germ cells are produced. 

 When a sperm cell (haploid) fertilizes an ovum (haploid) the two cells 

 fuse and the diploid number is restored (Fig. 17.4). The fertilized ovum 

 undergoes mitosis, dividing into two cells each with the diploid number. 

 Such mitoses continue and eventually an embryo takes shape, the cells all 



SPERM 

 (haploid) 



OVUM 

 (haploid) 



FERTILIZED OVUM 

 (diploid) 



FIG. 17.4. Fertilization. Haploid germ cells (gametes) unite to form a diploid 

 ovum (zygote). 



containing the diploid number of chromosomes derived from the fertilized 

 ovum. Some of the cells in the embryo are set aside as primordial germ 

 cells, which in time undergo meiosis, and so the cycle is continued genera- 

 tion after generation. 



Returning to the genetic implications of meiosis, we note that the 

 behavior of the chromosomes in this process provides the mechanism for 

 Mendel's "law of segregation" (p. 376) — the means by which each germ 

 cell receives but one member of each pair of genes. The chromosomes 

 also provide the mechanism for the Mendelian "law of independent 

 assortment." This is the principle that different pairs of genes are inde- 

 pendent of each other in the manner in which they are distributed to the 

 germ cells. 



Independent Assortment 



As an example we shall employ the guinea pig, a form whose genetics 

 is more thoroughly known than is the genetics of the European hamster. 



