FOUNDATIONS FOR SEX 



entirely in some species or may be redupli- 

 cated in others. Males which form two 

 types of sperm are often called heterozygous 

 for sex, although the term digametic may 

 be better, whereas the females are homoga- 

 metic. 



In other species the females show the 

 chromosomal differences whereas the males 

 are uniformly homogametic. These types 

 are principally known in the Lepidoptera, 

 more primitive Trichoptera, Amphibia, 

 some species of Pisces, and Aves. The single 

 accessory chromosome is present in the fe- 

 males whereas the males have two. It may 

 l)e unpaired or be with another chromosome 

 of different morphology and size. The sex 

 types may be symbolized by designating 

 the accessory chromosome by Z and its 

 mate W as a means of separating possible 

 differences between them and the X and Y 

 chromosomes. The significance of these dif- 

 ferences is not clearly established with the 

 consequence that some investigators prefer 

 to substitute the XY designation for the fe- 

 males and the XX condition for the males 

 of these species. The zygotic formulae are: 



Sperm ZA + egg ZA = 2Z + 2A male 

 Sperm ZA + egg WA = ZW + 2A female 



A third major chromosomal arrangement 

 accompanying sex differentiation came to 

 light as a result of Dzierzon's 1845 discov- 

 ery of parthenogenesis in bees. Chromoso- 

 mal and genetic studies have shown both 

 Apis and Habrobracon of the Hymenoptera 

 to be haploid, N, for each germinal cell in 

 the male complex and diploid, 2N, in the fe- 

 male. N may stand for any number of chro- 

 mosomes, as 16 for Apis or 10 for Habro- 

 bracon. The same chromosomal pattern 

 characterizes, so far as known, the other 

 genera of Hymenoptera. Haploidy vs. dip- 

 loidy is viewed as the most obvious feature 

 predicting differentiation toward the given 

 sex type even though, as in Habrobracon, 

 there is evidence for a particular chromo- 

 some of the N set carrying a locus for sex 

 differentiating genes. The zygotic sex for- 

 mulations are: 



No sperm + egg N = N male 

 Sperm N + egg N = 2X female 



In development, as in some other species 



oi widely diverse origins, chromosome pol- 

 yploidy may take place causing the soma 

 cells to differ from the germ cells in their 

 chromosome coniponents. 



The common phenotype for plants and 

 lower animals has differentiated sex organs 

 which are combined in the same individual. 

 Plant species seldom depend on any but 

 hermaphroditic types for their reproduc- 

 tion. Lewis' (1942) tabulation for British 

 flora had but 8 per cent of all species de- 

 pend on other than this form of reproduc- 

 tion. Those that had perfected dioecious 

 systems were not all alike in the system 

 adopted, although species with XY + 2A 

 males and XX + 2A females were in high 

 frequency. Dioecious reproduction was 

 rarely the system common to a whole 

 genus. Recent and multiple origins of dioe- 

 cious types are indicated by the irregular 

 distribution of species with bisexual repro- 

 duction within the different genera and 

 families. Methods for preventing inbreed- 

 ing have taken other channels as self- 

 sterility genes reminiscent of fertility or 

 sex alleles found in the older bacteria and 

 protozoa. Animals of the lower phyla are 

 those which are most frequently hermaph- 

 roditic. Within hermaphroditic species 

 chromosomal distinctions are ordinarily ab- 

 sent. The higher forms with sex and chro- 

 mosome differences, on the other hand, may 

 show reversion to the hermaphroditic con- 

 dition from the dioecious or bisexual states. 



Other less common cytogenetic controls 

 of sex development have become recognized 

 and better understood. Discussion of their 

 gene and chromosomal arrangements will 

 be considered when these cases arise. The 

 al)ove types will be sufficient to furnish a 

 basis for interpreting the newer data. 



C. CHANGING METHODS OF CYTOGENETICS 



The earlier studies of sex determination 

 depended on the natural arrangements of 

 chromosomes found in different species and 

 on occasional chromosomal rearrangements 

 occurring as relatively rare aberrant types. 

 Further development of genetics has in- 

 creased the tools for these studies. Mutant 

 genes have been shown to control chromo- 

 some pairing in segregation (Gowen and 

 Gowen, 1922; Gowen, 1928; Beadle, 1930). 



