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. 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.
In Jewish tradition, eunuchs were excluded from religious ceremonies. Early Christian monks sometimes practiced castration to remove the temptation of sexual attraction. We tend to think of all animals as being diploid, represented as 2N, and their gametes as haploid, represented as N. Exceptions are rare: this is not true for only about 18 of the many thousands of taxonomic groups of animals.
In plants like mosses and liverworts, an alternation of haploid and diploid states is common and among a category of insects called the Hymenoptera, which includes bees, ants, and wasps the presence of both haploid and diploid organisms is virtually the rule. These insects share a common mode of sex determination and also a social structure, called eusociality, in which there is usually one egg-laying queen, a huge number of sterile female workers or helpers, and a small number of drones that inseminate the queen in some species just a single male for just a single encounter serves that role.
Bees have immense commercial value, fertilizing one third of the plants we consume and having been used for honey production since antiquity. They cannot be domesticated, however, and experiments to mate specific males with specific females failed until the s, when people started using artificial insemination to initiate bee genetics. The idea of the continuity of chromosomes arose among German cytologists in the s. 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 , the German biologist Herman Henking — 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 7 Sexes: Biology of Sex Determination by Elof Carlson
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. 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. That inference was reinforced when Calvin Blackman Bridges — discovered a phenomenon he called nondisjunction, the topic on which he wrote his PhD dissertation, which was published in 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.
When Anthony van Leeuenhoek observed the animalcules, as he called them, from different dips of water or from his own body, he did not discuss how they formed. Most of his contemporaries would have said that they formed from spontaneous generation. The idea is as old as written thought. Aristotle believed in spontaneous generation, and so did anyone watching rotting food or meat swarming with maggots.
At least they conceived the process as far back as life goes: Genesis for the pious; after Charles Darwin, some sort of event that led to the formation of the first living cell; or after H. Muller, the formation of the gene, the first replicating molecule that could copy its errors. Microscopy flourished in the last half of the nineteenth century. It spun off the field of histology in medical schools and the field of cytology that led to inquiries about heredity.
It was a necessary tool for the field of microbiology that flowed from germ theory. Louis Pasteur — and Robert Koch — introduced the germ theory of infectious diseases in the s and s. It revealed even smaller organisms than those seen by Robert Hooke and Leeuenhoek.
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By the end of the nineteenth century, scientists inferred the existence of even smaller organisms, which slipped through filters that barred passage of bacteria. In , the first virus, tobacco mosaic virus, was identified. The concepts of hermaphroditism, sexual chimerism, sexual mosaicism, intersexuality, and gynandromorphism are all connected through a common aspect. They involve the presence, in an individual, of the sexual phenotype of both sexes to some degree, either at once or at some time in their life cycle as adults.
In the broadest sense, the term intersex is most inclusive, and it is purged of connotations of the classic mythology of Hermaphroditus and his encounter with Salmacis. Unfortunately, the biological use of the term intersex has usually excluded errors of mitosis leading to mosaicism. Intersexuality implies a genetic or environmental cause for the dual-sexual aspect, in which all the cells of the body have the same genotype and chromosome composition. It has a classical legend behind it: numerous deities were part human and part animal in Egyptian and Greek mythologies.
The Egyptian Sphinx is a part-human, part-lion, chimera that is familiar to most of humanity. In contrast, biologists use the term mosaicism to describe individuals who have a mitotic, nondisjunctional event or gene mutation after zygote formation , leading to two different genetic compositions or cell lines that were derived from one initial fertilization. Time will tell whether this term is specific enough or accurate enough to convey information about the condition, as this new usage is put into public discourse. It is unlikely that the term DSD will be applied to fruit flies or species other than humans.
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Such fine distinctions among these terms are almost entirely of twentieth-century origin, because neither the sex chromosomes nor genes in relation to sex determination were known before then. Calvin Bridges and Thomas Hunt Morgan discovered the existence of dosage differences on the X chromosome of fruit flies when comparing the allele of white eyes called eosin to that of the allele of white eyes called apricot.
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They called the phenomenon bicolorism, but did not make a generalization about it. In a stock of eosin flies, females had a darker eye color than males. In a stock of apricot flies, the eye color of the male and the female was the same. Eosin arose in a bottle of white-eyed flies as a solitary male fly.
It was interpreted as a partial reverse mutation from white to eosin. About a decade later, in , Curt Stern — discovered a mutation called bobbed bristles. It was the first genetic character found on the Y chromosome in fruit flies that was not associated with fertility. This made the gene behave like an autosomal recessive. Normal males and females had two doses, but XO males had a single dose and XXY fertile females had a triple dose. Stern noted that as the number of bobbed alleles increases, there is a normalizing effect.
When Michael Frederic Guyer — looked at human chromosomes in , he estimated the diploid number was 24 in females and 23 in males. Two years later, Hans Jean Chrysostome von Winiwater — doubled that estimate and claimed the human chromosome number was 47 in spermatocytes, but he agreed with Guyer about the XO status of males. Painter used testes from freshly executed prisoners or from castrated patients in mental asylums. He also said there was a definite Y chromosome in the male cells he studied and thus 48,XY was the chromosome number and sexual status of males for another generation.
The status remained stable because techniques did not change in cytology for human cells until the s, when tissue culture techniques improved, hypotonic solutions were used to increase the volume of nuclei, and colchicine was used to arrest cell division at metaphase. In , when two Swedish investigators, Joe Hin Tjio and Albert Levan, combined all these procedures, they got a consistent reading of 46 chromosomes, with very clear XX females and XY males.
By , a standardized way of photographing, enlarging, and clipping out chromosomes was introduced. The chromosomes were measured and aligned in size, place, and by grouping within a particular size range of the location of the centromere that separates the two arms of the chromosomes.
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The X chromosome is in the C group chromosomes of moderate size , and it is slightly submetacentric. The Y chromosome is very small, and the q arm is at least twice as long as the p arm. In , Lionel Sharples Penrose — estimated that the X accounted for about 6 percent of the total DNA of the sperm nucleus in humans, and the Y accounted for about 2 percent. There, I was asked to teach a course in human genetics for medical students, which was a challenge because I had never had a human genetics course.
As an offshoot from my dissertation study on the structure of the dumpy locus in fruit flies, I had published an article on the parallel of that gene complex to the Rh blood groups. That was my only contact with human genetics.