Sex Determination 



109 



mixed constitutions: XXX/XO; XX/XO; 

 XY/XO, XXY/XX; XXXY/XY. Such 

 cases are usually due to one or more errors 

 in chromosome distribution among the 

 daughter nuclei produced after fertilization. 

 Although such individuals are sex-chromo- 

 some mosaics, and some may even have 

 one ovary-like and one testis-like gonad, 

 they are not gynanders in superficial char- 

 acteristics because of their whole-body dis- 

 tribution of sex hormones. Although the 

 XXY male is often clearly an intersex, the 

 X0, XXX, and so on females that show in- 

 complete maturity are best considered infra- 

 females, being underdeveloped sexually. It 

 should now be clear that some specific 

 phenotypic sexual abnormalities may be 

 based primarily either on an abnormal en- 

 vironment or on an abnormal chromosomal 

 composition (recognizing also the possibility 

 that mutants other than those involving an 

 abnormal number of sex chromosomes can 

 affect sex). Accordingly, chromosomal 

 counts are often desirable in order to deter- 

 mine the cause — and, hence, the treatment 

 — of sexual abnormality. 



Human Sex Ratio 



Consider how the genotype is related to the 

 sex ratio, that is, to the relative numbers of 

 males and females born. On the average, 

 106 boys are born for each 100 girls. This 

 statistic might be surprising at first, since 

 half the sperm are expected to carry X, half 

 Y, and all eggs, an X, the ratio of boy to 

 girl expected at conception is one to one. 

 Even if the four meiotic products of a given 

 cell in spermatogenesis usually carry X, X, 

 Y, Y, there is the possibility that during or 

 after spermiogenesis (conversion of the telo- 

 phase II cell into a sperm) some X-bearing 

 sperm are lost. This possibility is supported 

 by a report 4 that human ejaculates contain 

 sperm heads of two sizes and shapes (Fig- 

 ure 8-6); the smaller type sufficiently in 



4 By L. B. Shettles (1960). 



figure 8-6. Head shapes in human sperm. 

 Round-headed sperm are reported to be smaller 

 and more numerous than oval-headed sperm, 

 suggesting these carry the Y and X chromo- 

 somes, respectively. {Courtesy of L. B. 

 Shettles.) 



excess to explain an excess of males at fer- 

 tilization provided the smaller sperm con- 

 tains the small Y chromosome, and the larger 

 sperm carries the larger X chromosome. 

 Other evidence suggests that at conception 

 males are much more numerous than fe- 

 males; since more male fetuses normally 

 abort than female, the numbers of boys and 

 girls are more nearly equal at the time of 

 birth than they were at conception. 



A study of the sex ratio at birth shows 

 that the ratio 1.067:1.000 is found only 

 among young parents, and that it decreases 

 steadily until it is about 1.036: 1.000 among 

 the children of older parents. How may 

 this significant decrease be explained? Per- 

 haps in older mothers there is a greater 

 chance for chromosomally normal male 



