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Genetic Basis of Sex and Sex-linked Traits
The modern science of genetics has provided a scientific explanation about how an offspring becomes either female or male. Based on the discovery that among the chromosomes present in the body cells, a special pair of sex chromosomes exist that bear the genes determining the sex of the offspring. In the human female, these chromosomes are identical and are called X chromosomes (indicated by XX). The male has one X chromosome and one smaller Y chromosome, which is dominant for maleness. During the process of producing reproductive cells (see meiosis), each of these chromosomes is segregated into a different gamete. Thus, when fertilization occurs, according to Mendelian law, 50% of the offspring will be XX (female) and 50% XY (male). Deviations from this rule do occur, but it is generally true.
The rule also helps to explain the inheritance of sex-linked characteristics such as hemophilia (a blood clotting disorder) and red-green color blindness, since the X chromosome also carries some genes for nonsexual traits. The Y chromosome carries very few genes for nonsexual traits; these few (including one for hairy ears) are called holandric genes. Certain inherited characteristics comprise X-linked traits, so called because a single X chromosome occurs in males. A recessive characteristic, e.g., when a gene leads to the expression of a disease such as hemophilia, may locate on the X sex chromosome in males and thus appear in that family.
See study by J. Maynard-Smith (1978).
the sum of the morphological and physiological features of an organism that provide for sexual reproduction, the essence of which amounts in the final analysis to fertilization. When fertilization occurs, the male and female sex cells—gametes—merge into a zygote, from which develops a new organism. In the zygote, two haploid sets of chromosomes of the maternal and paternal gametes are united. In the sex cells of the new organism, haploid sets of recombined paternal and maternal chromosomes are formed. This results from the crossing-over of homologous parental chromosomes and the random distribution of the chromosomes in the newly formed cells during meiosis (seeCROSSING-OVER). Numerous genetically different individuals arise in dioecious populations, establishing favorable conditions for the natural selection of better adapted forms. In this lies the principal advantage of sexual reproduction over asexual reproduction.
Sexual reproduction predominates in animals and higher plants. It also characterizes many microorganisms, for example, conjugation in bacteria is accompanied by partial exchange of genetic material—strands of DNA. The sexual process in unicellular organisms does not require significant sex differentiation. The same cell may be both a somatic cell and a sex cell. Multicellular diploid organisms have special haploid sex cells: females have large nonmotile or slightly motile ones, and males have small, usually motile ones. In most plants but only in a few animals both types of gametes are produced by a single individual (seeHERMAPHRODITISM). In most animals the sperm and eggs are produced by males and females, respectively. Besides producing cells of different sexes, males and females differ in sexual behavior and a number of morphological and physiological features, which ensure the fusion of the sex cells.
Sex determination. All organisms are genetically bisexual, since their zygotes receive genetic information that potentially makes possible the development of male and female features (sexual bipotentiality). In bisexual plants and in some hermaphroditic animals, female and male reproductive organs and sex cells develop from genetically identical cells under the influence of internal conditions (in respect to certain cells the conditions may be regarded as external). The switch mechanism that determines whether the individual will develop female reproductive organs or male reproductive organs has not been discovered. In dioecious species potentially bisexual zygotes only rarely develop into females or males as a result of external conditions. For example, in the marine annelid Bonellia, a larva that settles on the proboscis of the female develops into a male, whereas a larva that settles on the sea bottom develops into a female. The large nutrient-rich tubers of the plant Arisaema japonica give rise to plants with female flowers, but the small tubers yield plants with male flowers. Sex determination by external conditions is called phenotypic, or modifiable.
Genetic sex determination is more common. The zygote also exhibits sexual bipotentiality. However, genetic factors cause one half of the zygotes to develop into males, and the other half into females. A special chromosome mechanism ensures transmission of genes of the female sex to one half the offspring, and genes of the male sex to the other half.
At the beginning of the 20th century it was established that in the males of some insect species the diploid cells have, in addition to pairs of homologous chromosomes, an unpaired chromosome. The females, on the other hand, have two unpaired chromosomes. The males of other insect species all have paired chromosomes, but one pair consists of two morphologically dissimilar chromosomes. These dissimilar chromosomes, which are involved in sex determination, are called sex chromosomes, and all other chromosomes are called autosomes. Sex chromosomes were later discovered in many dioecious organisms.
The sex chromosome of the male that is also found in females is called the X chromosome, and the one unique to males is called the Y. The combination of sex chromosomes of the male is designated by the formula XO or XY, and that of the female by XX. Males with one sex chromosome (XO) produce equal numbers of gametes with and without X chromosomes. Those without X chromosomes have only one haploid set of autosomes (A). All gametes produced by females contain an X chromosome. After the random fusion of male and female gametes, half of the zygotes will have two X chromosomes (XX), and the other half will possess only one X chromosome. The former will become females, and the latter males.
Males bearing X and Y chromosomes produce equal numbers of gametes having an X chromosome and those having a Y chromosome. The female gametes of this type are genetically identical—they all carry one X chromosome. As a result, half the eggs are fertilized by spermatozoa with a Y chromosome, and the other half by those with an X chromosome. XY zygotes will develop into male individuals and XX zygotes will develop into females.
XO males and XY males are heterogametic; XX females are homogametic. In many animals the female is the heterogametic sex. The sex chromosomes of such females are designated by the letters Z and W or XY, and the sex chromosomes of homogametic males are designated by ZZ or XX. In mammals, nematodes, mollusks, echinoderms, and most arthropods the male is the heterogametic sex. In insects and fishes, heterogamety is observed in both males and females. Heterogamety in females characterizes birds, reptiles, and some amphibians.
The sexual bipotentiality of a zygote is conditioned by genes that are localized in the autosomes and that appear only by the action of other genes known as the sex determiners. It is these genes that promote the formation of the female sex by some genes and the formation of the male sex by others. With genetic determination of XO and XX, the determiners of the female sex are localized in the X chromosomes, and those of the male sex are in the autosomes. When one set of female-determining genes in an X chromosome combines with a diploid set of male-determining genes in the autosomes, a male develops. Two X’s are sufficient to override the potential development of a male and thus result in a female. In humans, the Y chromosome plays a sex-determining role. In anomalous cases, it combines with two, three, or even four X chromosomes when there is a normal set of autosomes. Although this leads to pathological deviations, all individuals with such sets of chromosomes are male. The sex-determining role of Y chromosomes has been noted in many animal species and in the plant Melandrium album (white campion).
In Drosophila the Y chromosome has almost no genes, that is, it is genetically inert. The female-determining genes are localized in the X chromosome, and the male-determining genes are in the autosomes. Sex determination is controlled by the ratio of X chromosomes to the set of autosomes (X:A), which is conventionally accepted in the female as 2X:2A = 1; this ratio, known as the sex index, in the male is equal to 0.5 (X:2A = 0.5). An increase in the sex index above 1 leads to excessive development of female sex characters (superfemale), whereas a decrease below 0.5 fosters the appearance of males with more strongly expressed male characters (supermale). Individuals with a sex index of 0.67 and 0.75 are marked by sexual characteristics intermediate between the male and female and are called intersexes. The phenomenon of intersexuality demonstrates the bisexual potential of genetic information transmitted to all offspring.
The genetic mechanism controlling the development of sex characters can be intracellular or intercellular. Intracellular sex determination does not result from the formation of sex hormones (for example, in insects), and the action of the sex-determining genes is limited by the cells in which the genes function. In such cases, parts of the body with female and male characters (gynandromorphism) may develop normally in the same organism, without influencing one another. With intercellular sex determination, which is characteristic of mammals and birds, the elaboration of sex hormones is controlled by genes. Upon entering all the cells of the body, the hormones stimulate phenotypic development of characters of the appropriate sex.
Sex determination may be progamic, syngamic, or epigamic. Progamic sex determination occurs before fertilization of the egg cell. The egg cells may differentiate into slow-growing and fast-growing cells. The latter become large and, after fertilization, produce females; the former are smaller in size and yield males. Such differentiation occurs even though both types of egg cells are genetically identical. Syngamic sex determination occurs during fertilization, but at different stages. In some species with male heterogamety and physiological polyspermy (fertilization of egg cells by several spermatozoa), sex is determined at the moment the nuclei of the sex cells fuse (karyogamy). If a male nucleus with a Y chromosome fuses with the nucleus of the egg cell, a male individual will develop; if a male nucleus with an X chromosome does so, a female will develop. With female heterogamety, the sex of the offspring depends on which of the sex chromosomes enters the nucleus of the egg cell during meiosis. If a Z chromosome enters the nucleus, a male individual will develop, but if a W chromosome enters, a female will develop. Thus, in this instance the sex of the zygote is established before karyogamy. Epigamic sex determination is observed in dioecious species with phenotypic sex determination, when the direction of development toward male or female sex is influenced by external conditions.
Sex-limited, sex-conditioned, and sex-linked characters. Characters that are limited or controlled by sex are said to be sex-limited, sex-conditioned, or sex-linked. Sex-limited characters, which appear by virtue of sexual differentiation, may be manifested in only one of the sexes. An example is the production of milk and eggs by only the female sex. Such characters appear only in one sex, even though polymeric genes of these characters are localized in the autosomes of both sexes.
Sex-conditioned characters controlled by sex are manifested either in both sexes (the different-sized horns of rams and ewes) or, more frequently, in only one of the sexes (the beard of billy goats). Such dissimilar development, which occurs although both sexes contain the genes of these characters in equal measure in their autosomes, is conditioned by the substantially different physiological processes of organisms of different sexes.
Genes that determine sex-linked characters are localized in paired and unpaired sex chromosomes and are therefore inherited differently from characters conditioned by paired genes localized in the autosomes of both sexes. If the genes are localized in the unpaired Y chromosome of a heterogametic male, the characters conditioned by them are inherited only by male offspring; if the genes are localized in the chromosome of a heterogametic female, the characters conditioned by them are transmitted only to female offspring. Characters heritable in this way are called holandric and hologynic, respectively. This type of heredity has been observed in some fish and insect species; its existence has not been fully substantiated in other animal species.
When genes are localized in homologous X or Z chromosomes, the characters conditioned by them are transmitted by criss-cross inheritance, in which a recessive character of the mother is transmitted to the sons, and a dominant one to the daughters. This type of inheritance, which was first described by T. H. Morgan, is found in many species of animals (for example, tricolor kittens, striped coloration of plumage and rapidity of its growth in chickens). Many sex-linked mutations have been discovered in Drosophila and the silkworm.
Lethal genes—genes that cause the death of the organism when it develops—may also be sex-linked. If a homogametic parent is heterozygotic for a lethal gene that is borne by one of the homologous sex chromosomes (X or Z), half of the heterogametic offspring will die, having received a lethal gene whose destructive effect in the genotype will not be opposed by the normal allele. With heterogamety of the female sex, half the daughters will die from the lethal genes; with heterogamety of the male, half the sons will die. Sometimes mutant genes in the X and Z chromosomes only partially decrease the viability of offspring or cause various diseases that are most often manifested in the heterogametic sex. In humans, more than 50 sex-linked mutations have been discovered, most of which bring about various disturbances.
Ratio of the sexes. With phenotypic sex determination, the sex ratio depends on the number of developing organisms that fall under the influence of external factors that determine one or the other sex. With genetic sex determination the ratio of the sexes in most species is generally very close to 100 ♀:100 ♂ (100 females: 100 males). There are, however, deviations even with this kind of sex determination. For example, in some mammal species with male heterogamety 1–2 percent more male offspring are born with statistical certainty.
Regulation. A substantial shift in the ratio of organisms toward one of the sexes has theoretical and practical significance, since one of the sexes is usually more productive. Four principal methods of sex regulation are used. The method used is determined by the type of sex determination and by the biological and economic characteristics of the species.
PHENOTYPIC REDETERMINATION. If the action of the sex genes is realized by means of hormones, the sex characters change when the sex organs of one sex are transplanted to the other or when hormones of the opposite sex or certain amino acids are injected in the body. The degree of phenotypic changes of sex depends on the characteristics of the species and the dosage of the injected preparation. However, only in rare cases (in some fishes and amphibians) do individuals with phenotypi-cally redetermined sex produce gametes opposite to their genoty-pic sex. If the action of the hormones ceases, the genetic mechanism of sex determination comes into play in succeeding generations.
MANAGEMENT OF THE GENETIC MECHANISM OF SEX DETERMINATION, OR ARTIFICIAL COMBINATION OF SEX CHROMOSOMES IN THE EGG CELL. Changes in the sex ratio have been achieved in experiments with the silkworm, in which sex is strictly determined by the combination of sex chromosomes (ZW-♀; ZZ-♂). Unfertilized egg cells develop parthenogeneti-cally after heating; this occurs because the diploid nucleus has not completed reduction division. All the cells of the partheno-genetic embryo preserve the maternal structure, especially in respect to the ZW sex chromosomes, and consequently develop only into females (B. L. Astaurov). Through the action of ionizing radiation and heating it has become possible to suppress the female nucleus in a newly inseminated egg cell and to switch development to the male principle. The diploid nucleus of the male zygote is formed by merging two male nuclei and, therefore, has the male sex structure ZZ. These zygotes in the silkworm always yield caterpillars of the male sex (H. Hashimoto; B. L. Astaurov).
By the above methods the problem of arbitrary sex regulation was first solved in agricultural silkworm species. Scientists are attempting to separate according to morphological and physiological characteristics X- and Y-bearing spermatozoa in mammals for the purpose of subsequent insemination with one category of spermatozoa. However, this method has not yet been used successfully to change the ratio of the sexes.
Early diagnosis. Early sex diagnosis is used for sorting hatched chicks by sex. Such a diagnosis is determined by the coloration of the plumage, which is a sex-linked character. Early sex diagnosis is used for the “ultra-early” sorting of the silkworm by sex. Ionizing radiation may cause the autosome with the dominant gene that conditions dark coloration of eggs of the silkworm to be linked to a sex W chromosome. Chromosome linkage is stably transmitted by heredity. The egg cells whose chromosome is linked with a transformed dominant gene acquire dark coloration and develop into females, whereas the egg cells of the male sex that have not received a dominant gene remain unpigmented. Automatic photoelectric machines very rapidly divide variously colored eggs by sex. Breeds of silkworm thus developed (V. A. Strunnikov and L. M. Gulamova) and tagged according to sex find practical use in Soviet sericulture.
In the 1960’s the British scientists R. Edwards and R. Gardner discovered a means of ensuring the birth of offspring of only one sex in mammals. Early embryos were removed from the pregnant rabbits, and their sex was determined by cytological method. The embryos of the undesired sex were culled out, and those of the desired sex were returned to the uterus. About 20 percent of the returned embryos became embedded and developed into rabbits of the sex predicted by the scientists.
In almost all animals with genetic sex determination, change in the sex ratio may be the result of the death of half the embryos of heterogametic sex, owing to sex-linked lethal genes. However, such an approach to sex regulation is not economically justified for many agricultural animals. An exception is the silkworm. In the USSR a genetically special breed of the silkworm has been developed by a radiation method (V. A. Strunnikov). Both Z chromosomes of the males always have one nonhomologous lethal gene (balanced lethal genes). If the males are crossed with females of ordinary breeds, one half the females will die in the egg stage from the first lethal gene, and the other half from the second lethal gene. Normal caterpillars will hatch from eggs of the male sex. This method makes it possible to obtain only the most productive males in unlimited numbers.
Evolution. Dioecism, which is characteristic even of many unicellular organisms (algae, protozoans), derived from monoe-cism. Only in a few instances, for example with parasitism, could monoecism arise secondarily from dioecism. Thus, in parasitic crustaceans one observes all the transitions from monoecism to dioecism. Dioecious crustacean species with well-developed females and dwarf males are examples of an obvious shift toward hermaphroditism.
Phenotypic sex determination is an older phenomenon than genetic, since in the early stages of evolution a special apparatus of sex chromosomes did not exist. The special sex chromosomes of fishes and amphibians that arose in certain stages of evolution were initially morphologically indistinguishable from autosomes, and their presence may be judged only from sex-linked characters. After morphological differences between sex chromosomes and autosomes, differentiation between the X and Y chromosomes occurred, which caused ever rarer conjugation between them and inhibited exchange of their parts during crossing-over. All this promoted the performance of specific functions by the sex chromosomes, that is, the chromosomes became the determiners of female or male sex. Complete disappearance of the Y chromosome made genetic sex determination even more complete: in such cases, sex is determined by the equilibrium between the numbers of autosomes and chromosomes.
REFERENCESAstaurov, B. L. “Genetika pola.” In the collection Aktual’nye voprosy sovremennoi genetiki.[Moscow] 1966.
Breslavets, L. P. “Opredelenie i nasledstvennost’ pola u vysshikh rastenii.” Trudypoprikladnoi botanike, genetike i selektsii: Seriia 2; 1934, no. 6.
Ryzhkov, V. L. Genetika pola. [Kharkov] 1936.
Lobashev, M. E. Genetika, 2nd ed. Leningrad, 1967.
V. A. STRUNNIKOV
the aggregate of mental reactions, feelings, attitudes, and behavior associated with the manifestation and gratification of sexual desire.
What does it mean when you dream about sex?
Having sexual relations in a dream or seeing others having sex may indicate repressed desires for physical or emotional love, as well as the urge to “bond” and create new life. Sexuality is too complex and confused an area of modern life to capture here its broad range of possible meanings.
2. The mnemonic often used for Sign EXtend, a machine instruction found in the PDP-11 and many other architectures. The RCA 1802 chip used in the early Elf and SuperElf personal computers had a "SEt X register" SEX instruction, but this seems to have had little folkloric impact.
DEC's engineers nearly got a PDP-11 assembler that used the "SEX" mnemonic out the door at one time, but (for once) marketing wasn't asleep and forced a change. That wasn't the last time this happened, either. The author of "The Intel 8086 Primer", who was one of the original designers of the Intel 8086, noted that there was originally a "SEX" instruction on that processor, too. He says that Intel management got cold feet and decreed that it be changed, and thus the instruction was renamed "CBW" and "CWD" (depending on what was being extended). The Intel 8048 (the microcontroller used in IBM PC keyboards) is also missing straight "SEX" but has logical-or and logical-and instructions "ORL" and "ANL".
The Motorola 6809, used in the UK's "Dragon 32" personal computer, actually had an official "SEX" instruction; the 6502 in the Apple II with which it competed did not. British hackers thought this made perfect mythic sense; after all, it was commonly observed, you could (on some theoretical level) have sex with a dragon, but you can't have sex with an apple.
byte sexThe order of the bits in a byte. See byte order.
cybersexAn erotic communication between two people online via text, audio or video. The earliest cybersex was chat room text, and role playing was easy because nobody saw the other party. A 50-year-old man could pretend to be an 18-year-old boy or girl. Cybersex evolved to texting explicit images and ultimately to "virtual sex" with participants watching each other online. See sexting and teledildonics.
Cybersex in Games
Cybersex can be a part of multiplayer, online games. Using an avatar, people communicate with each other erotically, and there are games that specialize in cybersex role playing. See parental control software.
gender changerA coupler that reverses the gender of one of the connectors in order that two male connectors or two female connectors can be joined together.
|The unit in the middle is the gender changer.|