GENETIC FACTORS IN THE ORIGIN OF DIVERSITY 383 



male hamster. Let us suppose that he inherited the gene for mehmism 

 (M) from his mother, the gene for grayness (m) from his father, and that 

 these genes are in the "long chromosomes" (Fig. 17.2). When each 

 chromosome duplicates itself to form a pair of chromatids the genes are 

 duplicated, too (as shown in the primary spermatocyte of Fig. 17.2). In 

 the figure the maternal "long chromosomes," with their duplicated M 

 genes, are shown going to the secondary spermatocyte on the right, the 

 paternal "long chromosomes," with their "m" genes, to the secondary 

 spermatocyte on the left. Then when the chromatids separate, each of the 

 pair of spermatids on the right receives a maternal chromosome contain- 

 ing M, and each of the pair of spermatids on the left receives a paternal 

 chromosome containing m. As a result, half the sperm cells in such a 

 heterozygous male receive maternal chromosomes containing gene M, 

 half receive paternal chromosomes containing gene m. It will be readily 

 appreciated that the hamster in which this occurred might have inherited 

 m from his mother and M from his father, in which case the m gene would 

 have been contained in the maternal chromosome, the M gene in the 

 paternal one. But the genetic results would have been the same: half 

 the sperm cells would contain M, half would contain ni. 



Meiosis in females differs from meiosis in males only in details. The 

 primordial germ cells are called oogonia. These multiply by mitosis. Even- 

 tually each daughter oogonium increases in size, and the chromosomes 

 duplicate themselves and pair in synapsis, forming a primary oocyte 

 stage (Fig. 17.3). Whereas the primary spermatocyte divides into two 

 secondary spermatocytes of equal size, the primary oocyte divides into two 

 cells of very unequal size: the secondary oocyte and the first polar body. 

 The secondary oocyte contains practically all the cytoplasm of the primary 

 oocyte, the polar body containing only enough cytoplasm to enclose the 

 chromosomes. Despite the unequal partitioning of cytoplasm, the secondary 

 oocyte and the polar body contain equivalent chromosomes: in our exam- 

 ple each contains the chromatids representing one "long chromosome" and 

 one "short chromosome" (Fig. 17.3). 



When the secondary oocyte divides, the cytoplasmic division is again 

 unequal, the products being the large ovum and the tiny second polar body. 

 But, as shown, each receives one chromatid of each pair contained in the 

 secondary oocyte. The polar bodies disintegrate; hence each primary oocyte 

 gives rise to but one ovum. This ovum contains one chromosome for each 

 pair of chromosomes contained in the oogonium from which it arose. Thus 

 meiosis in the female resembles meiosis in the male in that haploid germ 

 cells are produced from diploid primordial cells. 



