240 THE THEORY OF THE GENE 



succeeding generation, the sporophyte (zygote) arising 

 from the fertilization of the egg by the sperm, is some- 

 times said to be sexless or asexual. It has however one 

 X- and one Y-chromosome. 



Some unnecessary confusion has arisen between the 

 terminology used for the moss and liverwort and that 

 employed for dioecious flow^ering plants in which the terms 

 female and male apply to the sporophyte (diploid) gen- 

 eration and not to the egg-cell (which is part of the haploid 

 generation within the embryo sac) and to the pollen grain 

 (which is also part of the haploid generation). It may 

 seem at first sight, that male and female are used in these 

 two groups in a different sense. There is no real contradic- 

 tion, however, except a verbal one arising from phyloge- 

 netic implications. If the two cases in question are stated 

 in terms of genes, the imagined difficulties disappear. In 

 the liverworts, for instance, the balance of genes in the 

 haploid gametophyte, containing the large X-chromosome, 

 leads to the production of egg-cells and the balance of 

 genes in the haploid gametophyte containing the small 

 Y-chromosome leads to the production of sperm-cells. The 

 egg-bearer is here called female, the sperm-bearer male. 

 In the diploid generation of those flowering plants that 

 are dioecious, where a differential pair of chromosomes 

 is present in the male, the balance of the genes in the 

 diploid generation between the autosomes and two X's 

 gives a female (an individual that produces eggs), and the 

 balance of the genes in the diploid generation between the 

 autosomes and the XY pair gives a male (an individual 

 that produces sperm-cells). Both in the liverwort and the 

 flowering plant the situation turns on a balance between 

 sets of genes. It may be that the same sets of genes are not 

 involved in both cases, or that some of them are the same, 

 others are different. The essential point is that in both 

 cases, differences in balance lead to two kinds of indi- 



