CYTOLOGY OF SEX DETERMINATION 



In bisexual species, either plant or animal, it can generally be pre- 

 sumed that one or the other of the two sexes is heterogcunetic, i.e.. that 

 it produces two kinds of gametes, one female-determining and the other 

 male-determining. In the majority of bisexual animals as well as in 



Figure 8-9. Photomicrograph of 12 Bivalent Chromosomes at First 

 Meiotic Metaphase in Diploid Male of Melaudrium. Note heteromorphic XY 

 pair. (From Warmke. H. E., 1946. "Sex Determination and Sex Balance 

 in Melandrinmr Am. J. Botany, 33, Fig. 1, p. 651.) 



many plants, the heterogametic sex may be distinguished cytologically 

 by the fact that it contains a heteromorphic pair of chromosomes, the 

 segregation of which at meiosis leads to production of the two kinds of 

 gametes. The heteromorphic pair is generally known as the X-Y pair 

 (Figure 8-9). The Y element is missing from the hotriogametic sex and 

 the X element is represented twice. In some cases, as in several species 

 of grasshopper, the Y element is missing altogether so that the male is 

 XO and the female XX. In other cases, the praying mantid, for example, 

 the X element is compound and consists of two elements which pair 



CYTOGENETICS / 189 



