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10 The Discovery of Sex Chromosomes

Elof Axel Carlson Indiana University Press ePub

The idea of the continuity of chromosomes arose among German cytologists in the 1880s. At that time, many American biologists would get their PhDs (especially at Johns Hopkins University) and take a trip to Europe to visit the laboratories of German, Belgian, Dutch, French or Italian biologists where much of the work on meiosis, mitosis, and reproductive biology was taking place. They would then return to the United States to begin their own cytological studies. In 1891, the German biologist Herman Henking (1858–1942) studied the fire wasp, Pyrrhochoris,1 which is not actually a wasp, or Hymenopteran, but a true bug, or Hemipteran. He noted that, during the spermatogenesis of the fire wasp, there is an unusual chromosome: a nucleolar object that takes on a very dark stain in the first meiotic division. In the second division, this unit did not divide, and it appeared to remain in only one of the two cells produced. Because it was unusual in its staining, its morphology, and its behavior, Henking called it an X element, using X as a mathematical symbol for an unknown to be solved. The next year, when Henking was given an opportunity to take on an important and more financially rewarding position in German fisheries, he dropped cytology, focusing on fisheries for the rest of his career. Henking made no association between his X element and sex determination.

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7 The Discovery of Sperm in Higher Eukaryotes

Elof Axel Carlson Indiana University Press ePub

Semen has long been recognized as necessary for producing offspring. It is liquid, somewhat viscous, and usually clear or slightly cloudy in appearance; certainly the unaided eye can see no visible body within it. The Greeks, especially through Hippocrates and later Galen, embraced a theory of vital fluids, which they called humors. Blood was considered the major constituent of life, at least among vertebrates. It was considered the progenitor of semen in the male body, and believed to be the hereditary material that allowed a species to generate offspring in its likeness.

Semen was endowed with a capacity to impose form on the pliable material supplied by females. That material was also thought to be blood: sometimes it was associated with menstrual blood, and sometimes it was thought to be another type of semen. Female semen was not clarified, like male semen, but still bloodlike and clotted—a type of miniscule clay ready to be molded into shape by the empowering effect of male semen. For more than two thousand years, arguments were made about the relative roles that males and females play in forming a new individual through their fluids, which were commingled after copulation. There were inside–outside theories in which the male supplied the outer components of the new baby. There were theories in which the female role was passive, being shaped exclusively by the male, forcing some observable phenomena, such as the equal contributions made to the skin color of the offspring of a black person and a white person, to be swept under a mental rug.

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8 The Discovery of Sex Hormones

Elof Axel Carlson Indiana University Press ePub

In 1902, William Bayliss (1860–1924) and Ernest Starling (1866–1927) introduced the term “hormone.”1 Hormones are substances produced by one organ, an endocrine gland, that acts at a distance on another organ. The field of science that studies this is called endocrinology. The names of hormones were all coined in the twentieth century, but the idea that there was something like hormones has existed since antiquity. For example, Chinese medicine frequently made use of extracts from human urine that were used to treat disease.

Since human history began and medical treatments were attempted, physical changes associated with endocrine glands have been known. Castrated males, since antiquity known as eunuchs, lose their capacity to grow a beard, may develop enlarged breasts, and become effeminate. Eunuchs have had a long history serving as guards of harems in the Middle East, where plural marriages were common and reflective of wealth and power, and they served as political advisors in the Forbidden City in Beijing during the rule of Chinese emperors. Eunuchs were usually castrated as young men, but a special category of eunuchs were castrated as preadolescent boys. These were called castrati. During the Renaissance and until the eighteenth century, boys in choirs who were aged six to ten and who had a talent for singing and reading music were castrated and groomed to become prized singers because of their “celestial” soprano-like upper voice range. They differed from typical eunuchs, who lost their testes as adults, tending to be taller than average and appearing “etiolated,” with unusually wide hips in an otherwise slender frame.2 Castration was also applied to slaves in Greece about 400 BCE because they were considered to be more docile. In Jewish tradition, eunuchs were excluded from religious ceremonies. Early Christian monks sometimes practiced castration to remove the temptation of sexual attraction.3

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20 The Evolution of Sex Determination

Elof Axel Carlson Indiana University Press ePub

In 1871, Charles Darwin published his long awaited Descent of Man and Selection in Relation to Sex.1 Darwin considered sexual evolution as a lesser form of natural selection and he gave it the term sexual selection. This distinction made sense because natural selection operated on all aspects of survival, while sexual selection acted mostly on the differences between the two sexes. Adult male humans are taller, heavier, more muscular, and their body fat and muscles are distributed differently from females. Females have wider hips than shoulders; males have the reverse. The secondary sexual differences in breast enlargement and facial hair may play a role in sexual attractiveness. They may also alter the relation of infants to their parents, making the mother-child bond more important for survival. In many animals, this sexual dimorphism between males and females is even more pronounced. The peacock is elaborate in color, size, and distribution compared to the peahen. Male deer have large antlers that they use for display as well as for combat with other males in their rivalry for access to females. Female deer lack those antlers. Darwin felt that sexual reproduction gives an advantage to a species by fostering what was later called “hybrid vigor”—relatively specialized breeds of animals are less likely to survive in the wild than mongrels or hybrids of those strains that are closer to their ancestral wild type.

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11 The Balance Theory of Sex Determination

Elof Axel Carlson Indiana University Press ePub

From 1907, when Thomas Hunt Morgan began working on fruit flies, until 1915, he and his students believed that their sex chromosome composition was 2N = 8,XX for females and 2N = 7,XO for males because they had misinterpreted a paper that Nettie Stevens wrote in 1907. Once Morgan and his students realized that Drosophila melanogaster used the XX female and XY male system for sex determination, they had to reconcile the role of the Y chromosome in sex determination.1 Since it was well known from Edmund B. Wilson’s and Stevens’s work that some Diptera had XO males and others had XY males, they concluded that the Y could not be playing a role in the sex determination of males.

That inference was reinforced when Calvin Blackman Bridges (1889–1938) discovered a phenomenon he called nondisjunction, the topic on which he wrote his PhD dissertation, which was published in 1916.2 Bridges found an unexpected appearance of a white-eyed male in a cross that should have given red-eyed males. If one parent is a white-eyed male and the female parent is red eyed, all the progeny should be red eyed. When Bridges tried to mate the white-eyed male, he found it was sterile. He also found that if he did a cross with a white-eyed female and red-eyed male, the offspring should be white-eyed sons and red-eyed daughters: a distribution that the laboratory referred to as crisscross inheritance. But Bridges found a female that was white eyed on some occasions. That exceptional female was fertile, and when mated to a red-eyed male she gave an unusual distribution of progeny: about eight percent of the offspring being of an unexpected kind with respect to their eye color and sex.

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