Sex Chromosomes and Sex Determination

In humans and many other animal species, sex is determined by specific chromosomes. But how did researchers discover these so-called sex chromosomes? The chromosomes that carry those genes related to sexual characteristics have a long and intriguing story of exploration. As often happens during such a long course of discovery, scientists noted the appearance of these chromosomes before knowing exactly what these structures were.

The first inkling that sex chromosomes were unique from all other chromosomes came from experiments conducted by German biologist Hermann Henking in 1891. By this point in time, scientists could easily view mitotic cell division using light microscopy. Indeed, while using this method to study sperm formation in wasps, Henking saw that some wasp sperm cells had 12 chromosomes, while others had only 11. Moreover, while observing the stages of meiosis that formed these sperm cells, Henking noticed that the mysterious twelfth chromosome looked different from all the others. He thus named this chromosome the "X element," to represent its unknown nature. Interestingly, in subsequent studies of female grasshopper egg formation, Henking was unable to spot the X element (Brown, 2003; Pierce, 2005). Based on these observations, Henking hypothesized that this extra chromosome must play some role in sex determination in insects. However, he was unable to gather any direct evidence that supported this hypothesis.

About 10 years after Henking's studies, American zoologist C. E. McClung began extensive research into the idea that the X element must have something to do with sex determination. First, McClung asserted that this strange chromosome be called the "accessory chromosome," because it appeared to have a separate purpose compared to the other chromosomes. Next, he decided to undertake a wide comparative study of spermatogenesis not just in insects but in many different organisms, including spiders and mice. While observing sperm generation in grasshoppers, McClung noted that the accessory chromosome did not behave as the other chromosomes did during the first meiotic division. Specifically, this "twelfth chromosome" was not distributed equally to the four sperm cells that arose from one grasshopper spermatogonium; instead, it was present in only half of them. This consistent pattern of segregation in the spermatogenesis of many different organisms caused McClung to speculate that there must be some fundamental reason for the generation of two types of sperm cells. McClung thought about the consequences of this 50/50 distribution of the accessory chromosome, and he pondered how this distribution probably resulted in two different kinds of zygotes. He also noted that there was really only one characteristic that varied among zygotes of many different species in 50/50 proportions, and that characteristic was sex. Thus, when he assembled all the evidence together, McClung came up with the proposal that the accessory chromosome must influence the determination of sex in a zygote (Brown, 2003). McClung was so eager to formally communicate his hypothesis that he wrote a preview of this idea (McClung, 1901) that was published as a short letter approximately a year before his full manuscript appeared in a journal (McClung, 1902).

At around the same time, Walter Sutton, who was one of McClung's former students, published a separate paper that fully examined the chromosomes in the grasshopper and provided data that strongly supported McClung's hypothesis regarding the accessory chromosome. In his study, Sutton closely followed the path of chromosomes throughout meiosis in sperm cells, and he demonstrated with drawings of successive meiotic stages that sperm cells contained half the total number of chromosomes present in grasshopper zygotes. Sutton's careful consideration of how chromatic material segregated into sperm cells became the launching point for the chromosome theory of inheritance. To develop this theory, Sutton counted every chromosome present in grasshopper sperm cells and measured their size. He then ranked each chromosome according to these size measurements. Sutton's careful observation of chromosome number and segregation led him to conclude that gametes carry one-half of the total chromosomal collection in a zygote; he also determined that the merging of two gametes during fertilization must be the mechanism by which chromosome pairs are reformed.

Throughout the many observations that led to this conclusion, Sutton made careful note of the path of the accessory chromosome. Based on these observations, he asserted that, though it was different in its segregation into sperm cells, the accessory chromosome was indeed a true chromosome (Sutton, 1902).

The notion that sex could be determined by chromosomes was a new idea. Prior to Henking, McClung, and Sutton's reports, sex determination was attributed to factors other than gametes, such as the environment in which egg cells existed. Thus, the issue of the accessory chromosome was a lively topic of debate in the early twentieth century. Controversy soon began to fade, however, as several other scientists reported similar cases of "mismatched" chromosomes in other organisms. For example, when examining differences between male and female beetles, American researcher Nettie Stevens recognized patterns of chromosomes that differed between the two sexes. In particular, she found an unbalanced pair of chromosomes in male cells, and she named these structures "heterochromosomes." Similarly, American geneticist E. B. Wilson worked with several different insect species and observed that, although there were equal numbers of chromosomes in male and female cells, one pair in the males was different in size. He called these mismatched pairs "idiochromosomes," perhaps because they were idiosyncratic (Wilson, 1905). Wilson became convinced that the larger of the pair was actually the "accessory chromosome," and that its smaller, more crumpled partner chromosome had gone undetected by Henking, McClung, and Sutton. Thus, according to Wilson, while the accessory chromosome did indeed have something to do with determination of sex, it was not a lone, unpaired chromosome—in most cases, it had a partner. Figure 1 provides an idea of why this partner chromosome may have been overlooked by researchers prior to Wilson. (In this image, the "X" chromosome is on the left, and its small partner is to the right.)

Following Wilson's discovery, there emerged some confusion about what to call these apparent differences in chromosome sorting seen in each sex. Part of this confusion related to the fact that numerous species were being examined, and these species varied in terms of chromosomal number and arrangement. Thus, challenges remained to the development of a unifying theory about how chromosomes actually determine an organism's sex.

With this combination of evidence from different species, a connection was observed between specific chromosomes and the sex of an individual. Interestingly, it was the pursuit of sex determination that enabled scientists to see a direct link between chromosome distribution and specific traits. Sutton's suggestion that the gamete chromosome separation pattern was similar to the proportions of male and female offspring opened up the possibility that chromosomes had something to do with inherited traits. But it wasn't until a wide variety of species were investigated that some of the mysteries of the accessory chromosome could be solved. For his broad analysis of different species' chromosome counts, E .B. Wilson is credited with the idea that there can be different types of sex chromosome systems among different animals (Wilson, 1910). However, the initial idea that sexual characteristics were physically represented by chromosomes and inherited through the transfer of chromatin originated with Sutton and his careful chromosome counts in the grasshopper.