Sex Determination {11) 



so because, at one place along the length of 

 the chromosome, pairing is between two 

 homologs, and, at another place, it is between 

 one of these two and the third homolog. 

 In this way, then, although pairing is two-by- 

 two at any given level, all three homologs are 

 held together as a trivalent. At the first 

 meiotic division, the two homologs synapsed 

 at their centromeric regions separate and go 

 to opposite poles, while the third homolog 

 goes to either one of the poles. At the end 

 of the second meiotic division two nuclei 

 each have one homolog of the trivalent and 

 two nuclei each have two homologs. The 

 same result obtains when synapsis is entirely 

 between two homologs and excludes the third 

 homolog. Since each of the four trivalents 

 present at metaphase I segregate in this 

 manner independently of the others, eggs 

 may be produced which have each chromo- 

 some type singly, and so contain one complete 

 genome (type 1), or contain two of each type, 

 and so carry two genomes (type 2), or have 

 any combination in which some chromosomes 

 are represented once and others twice (type 3). 

 Type 1, being a haploid egg, will produce 

 normal males and females when fertilized by 

 sperm from a normal male. Eggs of type 2, 

 being diploid, will produce triploid females 

 when fertilized by X-bearing sperm. When 

 the chromosomal content of intersexual sur- 

 vivors is determined cytologically, they prove 

 to have three sets of autosomes plus either 

 XXY or XX. The former type was derived 

 from a type 2 diploid egg containing two sets 

 of autosomes plus XX which was fertilized 



m.'- 



FIGURE 14-4. Some abnormal sex types in Dro- 

 sophila: A = superfemale ; B = super male ; 

 C = intersex. {Drawn by E. M. Wallace.) 



by a Y-bearing sperm; the latter type came 

 from a type 3 egg containing two sets of auto- 

 somes plus X which was fertilized by an 

 X-bearing sperm. 



Close observation reveals two additional 

 sex types among the progeny of triploid Dro- 

 sophila (Figures 14-4, 14-3). These appear, 

 not as intersexes, but as supersexes — some of 

 these individuals show characteristic female 

 traits even more strongly than do normal 

 females (these are called superfemales, meta- 

 females, or ultrafemales), the others show 

 characteristic male traits even more strongly 

 than do normal males (these are called super- 

 males, metamales, or inframales). Chromo- 

 somally, superfemales contain two sets of 

 autosomes and three X's, derived from a type 



