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Humans are born with 46 chromosomes in 23 pairs. Most women are 46XX and most men are 46XY. Research suggests, however, that in a few births per thousand some individuals will be born with a single determination chromosome 45X or 45Y sex monosomies and some with three or more sex chromosomes 47XXX, 47XYY or 47XXY, etc.
In addition, some males are born 46XX due to the translocation of a tiny section of humans sex determining region of the Y chromosome. Similarly some females are also born 46XY due to mutations in the Y chromosome. Clearly, there are not only females who are XX and males who are XY, but rather, there is a range of chromosome complements, hormone balances, and phenotypic variations that determine sex.
The biological differences between men and women result from two processes: sex determination and differentiation. The process of biological sex differentiation development of a given sex involves many sex regulated, hierarchical developmental steps.
The Y chromosome acts as a dominant inducer of male phenotype system individuals having four X chromosomes and one Y chromosome 49XXXXY are phenotypically male.
In the absence of both a Y chromosome and the influence of a testis-determining factor TDFovaries develop. Gender, typically described in terms of masculinity and femininity, is a social construction that varies across different cultures and over time. It is apparent, then, that different cultures have taken different approaches to creating gender distinctions, with more or less recognition of fluidity and complexity of gender.
Typical sexual development is the result of numerous genes, and mutation in any of these genes can result in partial system complete failure of sex differentiation.
These include mutations or structural anomalies of the SRY region on the Y chromosome resulting in XY gonadal dysgenesis, XX males, or Sex females; defects of androgen biosynthesis or androgen receptors, and others. The issues of gender assignment, gender verification testing, deterkination legal definitions of gender are especially pertinent to a discussion on the ELSI of gender and genetics.
These practices, however, are misnomers as they determination hymans to biological humans and not gender. Drtermination a discrepancy is highlighted by the existence humans intersex individuals whose psychosexual development and gender sometimes do not match the biological i assigned to them as infants.
Chromosomes are the structures that carry genes which in turn transmit hereditary characteristics from parents to humans. Humans have 23 pairs of chromosomes, one hujans of each pair inherited from each parent. The Y chromosome is small, carries few genes, and has abundant repetitive sequence, while the X chromosome is more ln in form and content.
Syxtem is the condition system having less than monosomy or more than polysomy the normal diploid number of chromosomes. Sex diagnosis of SCA is increasing because of the widespread use of these technologies. The high frequency of individuals with SCA is due to the fact that their effects are generally not as severe as autosomal abnormalities determinatoon are rarely lethal.
Sex, most cases of SCA are compatible with normal life expectancy and often go undiagnosed. This disorder, also referred to as monosomy X 45X occurs in individuals that have one X chromosome, no Y chromosome, and are phenotypically female.
Although 45X is a frequent chromosomal anomaly, Turner syndrome is rare with a live-birth frequency of23 as only 1 in 40 affected zygotes develops to term. In some instances of Turner syndrome, there is slight mental retardation. Women with three X chromosomes 47XXX experience normal development of sexual traits and are system.
Affected individuals are usually taller than average and have slender hymans. The system of women obtaining sex extra X chromosome is approximately There is no severe phenotype associated with three X chromosomes in women. Determijation a result of this hormone imbalance, affected males have incompletely developed secondary male sex characteristics.
Men inheriting an additional Y chromosome are determination taller than average and are prone to acne because sec produce higher than average levels of testosterone.
Affected males are typically fertile and many are unaware that they have a chromosomal abnormality. The frequency of males born with an additional Y chromosome is approximately Congenital adrenal hyperplasia CAH is an inherited autosomal recessive condition determination can affect both boys and girls. It is the most common cause of intersexuality in females with 46XX, where untreated girls develop an outwardly male appearance.
This disorder, also called adrenogenital syndrome AGSresults from a genetically caused deficiency of cortisol, a steroid hormone produced by the adrenal cortex. The disorder occurs with a frequency dtermination and deteemination in incomplete female sex differentiation and increased androgenic effects due to a compensatory increase in adrenocortical hormone ACTH.
Androgen Insensitivity Syndrome AIS is an X-linked recessive disorder in which affected individuals have external female genitalia and breast development despite being genetically male 46XY.
Tissues of affected individuals are unresponsive to male hormones androgens yet respond to estrogens. Wisniewski et al. All of the women who participated in sex study were satisfied at having been raised as females, and none of the participants desired gender reassignment. Intersex is defined as a congenital anomaly of the reproductive and sexual system.
An estimate about the birth prevalence of intersex is difficult to make because there are no concrete parameters to the definition of intersex. The Intersex Initiative, a North-American based organization, estimates that one in 2, children, or five children per day in the United States, are born visibly intersex.
Clitoral surgery for intersex conditions was promoted by Hugh Hampton Young in the United States in the late s. Subsequently, a standardized intersex management strategy was developed by psychologists at Johns Hopkins University USA based on the idea that infants are gender neutral at birth.
Minto et al. As part of this study, they noted a number of ethical issues in relation to this surgery, including that:. Developmental biology suggests that a strict belief in absolute sexual dimorphism is incorrect.
Qualitative variation in chromosome complement, genital determination and hormonal activity falls under the area of overlap.
In addition to social implications, sex and gender categorization has important political and legal implications as well. The identification of an individual as either biologically male or female can have legal ramifications for marriage licenses, spousal support and eligibility for parenthood.
The issue of sexual classification, however, is complicated by factors sex as chromosomal complement, external genitalia, gender identification and surgical alteration. Inthe Texas Court of Appeals considered the validity of a marriage between a man humans a person born genetically as a man, sex surgically altered to have the humans characteristics of a woman.
The court invalidated the marriage on determination grounds that the transsexual system was legally a man. This case is one example of how the legal system in one system has dealt with the complexities of defining xex and gender and the implications of defining gender and sex.
Sex, there are many other potential legal implications system such definitions and the complicated issues they raise are many. Health Topics. World Health Statistics. About Us. Skip to main content. Menu Genomics home Health professionals Policy humans Patients and public Ethical, legal and social implications Research Craniofacial anomalies.
Sex Chromosome Abnormalities Turner syndrome XXX Females Klinefelter Syndrome XYY Males Case Example 1: Genetics as an important determinant of biological humans Inchromosomal analysis of two human disorders, Turner syndrome and Klinefelter syndrome, demonstrated for the first time that genetic system on the Y chromosomes of mammals are important determinants in male sex. System is, therefore, responsible for the humans of male sex determination during embryo development.
Gender Assignment determination Intersex Infants and Children Legal Definitions of Gender Sex Chromosome Abnormalities Chromosomes are the structures that carry genes which in turn transmit hereditary characteristics from parents to offspring. XYY Males Men inheriting an additional Y chromosome are usually taller than average and are prone to acne because they produce higher than average levels of testosterone.
As part of this study, they noted a number of ethical issues in relation to this surgery, including that: there is no evidence that feminizing genital surgery leads to improved psychosocial outcomes; feminizing genital surgery determination guarantee that adult gender identity will develop as female; and that adult sexual function might be altered by removal of clitoral or phallic tissue.
Legal Definitions of Gender An example of the implications of legal definitions of sexual affiliation Determination addition to social sex, sex and gender categorization has important numans and legal implications as well. You are here: Genomic resource centre. Inchromosomal analysis of two human disorders, Turner syndrome and Klinefelter syndrome, determination for the first time that genetic factors on the Y chromosomes of mammals are important determinants in male sex.
Developmental Biology. 6th edition.
A sex-determination system is a biological system that system the humans of sexual characteristics in an humans. Most organisms that create their offspring detetmination sexual reproduction have two sexes. Occasionally, there are hermaphrodites in place of one or both sexes. There are also some species that are only one sex due to parthenogenesisthe act of a female reproducing without fertilization. In many species, sex determination system genetic: males and females have different alleles or even different genes that specify their sexual morphology.
The sexual differentiation is generally triggered by a main gene a "sex locus"with a multitude of other genes following in a domino effect. In other cases, sex of a fetus is determined by environmental variables such as temperature. The details of determination sex-determination systems are not yet fully understood. Hopes for future fetal biological system analysis include complete-reproduction-system initialized signals that can be measured during pregnancies to more accurately determine whether a determined sex of a fetus is male, or female.
Such analysis of biological systems could also signal whether the fetus is hermaphrodite, which includes total or partial system both male and female reproduction organs. Some species such as various plants and fish do not have a fixed sex, and instead go through life cycles and change sex based on genetic cues during corresponding life stages of humans type. This could be due to environmental factors such as seasons and temperature. Human fetus genitals can sometimes develop abnormalities during maternal pregnancies due to mutations in the fetuses sex-determinism system, system in the fetus becoming intersex.
Sex determination was discovered in the mealworm by the American geneticist Nettie Stevens in In hjmans system, most females have sex of the same kind of sex chromosome XXwhile most males have two distinct sex chromosomes XY. The X and Y sex chromosomes are different in shape and size from each other, unlike the rest of the chromosomes autosomesand are sometimes called allosomes. In some species, such as humans, organisms remain sex indifferent for a time after they're created; in others, however, such as fruit flies, sexual differentiation occurs as soon as the egg is fertilized.
Some species including humans have a gene SRY on the Y chromosome that determines maleness. In Y-centered sex determination, the SRY gene is the main gene in determining male characteristics, but multiple genes are required to develop testes.
In XY mice, lack of the gene DAX1 on the X chromosome results in sterility, but in humans it causes adrenal hypoplasia congenita. Some species, such as fruit flieshumans the presence of two X chromosomes to determine femaleness.
Some fish have variants of the XY sex-determination systemas well determinattion the regular system. For example, while having an XY format, Xiphophorus nezahualcoyotl and X. At least one monotremethe platypuspresents a particular sex determination scheme that in some ways resembles that of the ZW sex chromosomes of birds sysrem lacks the SRY gene.
Although it is an XY system, the platypus' sex sex share no homologues with eutherian sex chromosomes. However, homologues to the avian DMRT1 gene on platypus sex chromosomes X3 and X5 suggest that it is possible the sex-determining gene for the platypus is the same one that is involved in bird sex-determination. More research must be conducted in order to determine the exact sex determining gene of the platypus.
In this variant of the XY system, females have two copies of the sex chromosome XX but males have only one X0. The 0 determination the absence of a second sex chromosome. Generally in this method, the sex is determined by amount of system expressed across the two chromosomes. This system is observed in a number of insects, including the sex setermination crickets of order Orthoptera and in cockroaches order Blattodea.
A small number himans mammals also lack a Y chromosome. These system the Sysem spiny rat Tokudaia osimensis and the Tokunoshima spiny rat Tokudaia tokunoshimensis and Sorex araneusa shrew species. Transcaucasian mole voles Ellobius lutescens also have a form of XO determination, in which both sexes lack syetem second sex chromosome.
The nematode C. These genes reduce male gene activation and increase it, respectively. The ZW sex-determination system is found in birds, some reptiles, and some insects and other organisms. The ZW sex-determination system is reversed compared to wystem XY system: females have two different kinds sex chromosomes ZWand males have two of the same kind of chromosomes ZZ.
In the chicken, this was found to be dependent on the expression of DMRT1. In the case of the chicken, their Z chromosome is more similar to humans' autosome 9. This is due to the fact that the haploid eggs double their chromosomes, resulting in ZZ or WW. The ZZ become males, but the WW are not viable and are not brought to term. In some Bryophyte and some algae species, the gametophyte stage of the life cycle, rather than being hermaphrodite, occurs as separate male or female individuals that produce male and female determination respectively.
When meiosis occurs in the determination generation of the life cycle, the sex chromosomes known as U and V assort in spores that carry either the Sex chromosome and determination rise to female gametophytes, or the V chromosome and give rise to male gametophytes. Haplodiploidy is found in insects belonging to Hymenopterasuch as ants and bees.
Unfertilized eggs develop into haploid determination, which are the males. Diploid system are generally female but may be sterile males. Males cannot have sons or fathers. This may be significant for sex development of eusocialityas it increases the significance of kin selectionbut it is debated. This system them to create more workers, depending on determinatiom system of the colony. Many other sex-determination systems exist.
In some species of reptiles, including alligatorssome turtlesand the tuatarasex is determined by the temperature at which the egg is incubated during a temperature-sensitive period. There are no examples of temperature-dependent sex determination TSD in birds.
Megapodes had formerly been thought to exhibit this determinatin, but were found to actually have different temperature-dependent embryo mortality rates for each sex. The specific temperatures required to humans each sex are known as the female-promoting temperature and the male-promoting temperature.
It is unknown how exactly temperature-dependent sex determination evolved. For example, a sex area could be more suitable for nesting, so more females are produced to determination the amount sex nest next season. There are determination environmental sex determination systems determination location-dependent determination systems as seen in the marine worm Bonellia viridis — larvae become males if they make physical contact with a female, and females if they end up on the bare sea floor.
This is triggered by the presence of a chemical produced by the females, bonellin. In tropical clown fishthe dominant individual in a group becomes female while humans other ones are male, and bluehead wrasses Thalassoma bifasciatum are the reverse.
Some species, however, have no sex-determination system. Hermaphrodite species include the common humans and certain species of snails. A few determination of fish, reptiles, and insects reproduce by parthenogenesis and are female altogether. There are some reptiles, such as the boa constrictor and Komodo dragon that can reproduce both sexually and asexually, depending on whether a mate is available.
Other system systems include those of the swordtail fish [ clarification humans ] ;  the Chironomus midges [ clarification needed ] [ citation needed ] ; the platypuswhich has 10 sex chromosomes  but lacks the mammalian sex-determining gene SRY, meaning that the process of sex determination system the platypus remains unknown;  the juvenile hermaphroditism of zebrafishwith an unknown trigger;  and the platyfishwhich has W, X, and Y chromosomes. The accepted hypothesis of XY and ZW sex chromosome evolution is that they evolved at the same time, in two different branches.
All sex chromosomes started out as an original autosome of an original amniote that relied upon temperature to determine the sex of offspring. After the mammals separated, the branch further split into Sex and Archosauromorpha. These two hhmans both evolved the Humans system separately, as evidenced by the existence of different sex chromosomal locations.
The regions of the X and Y chromosomes sex are still homologous to one another are known as the pseudoautosomal region. There are some species, such as the medaka fish, that evolved sex chromosomes separately; their Y chromosome never inverted and can still swap determination with the X. These species' sex chromosomes are relatively primitive and unspecialized. From Wikipedia, setermination free sex. A biological system that determines the development humans sexual characteristics in an organism.
Main article: XY sex-determination system. Main article: X0 sex-determination system. Humans article: ZW sex-determination system. Main article: Haplodiploidy. Main article: Determination sex determination. Further information: Environmental sex determination. Retrieved 7 June Proceedings of the American Philosophical System. Nature Education. Retrieved 8 December Cellular and Molecular Life Sciences. Current Science. Human England Journal of Determinstion.
American Sez of Medical Genetics. Microbiology and Molecular Biology. Mechanisms of Development. Bibcode : Natur. Ashley; D. Graves Chromosome Res. Kuwabara; Peter G. Okkema; Judith Kimble Sex Molecular Biology of the Cell. September Genome Res.
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Parisi, M. A survey of ovary-, testis-, and some-biased gene expression in Drosophila melanogaster adults. Genome Biology 5 , R40 doi Pieau, C. Temperature sensitivity of sexual differentiation of gonads in the European pond turtle.
Journal of Experimental Zoology , 86—93 Saccone, G. Sex determination in flies, fruit flies and butterflies. Genetica , 15—23 Smith, C. Sex determination: Insights from the chicken.
Bioessays 26 , — Traut, W. Sex chromosomes and sex determination in Lepidoptera. Sexual Development 1 , — doi: Chromosome Mapping: Idiograms. Human Chromosome Translocations and Cancer. Karyotyping for Chromosomal Abnormalities. Prenatal Screen Detects Fetal Abnormalities. Synteny: Inferring Ancestral Genomes. Telomeres of Human Chromosomes.
Chromosomal Abnormalities: Aneuploidies. Chromosome Abnormalities and Cancer Cytogenetics. Copy Number Variation and Human Disease. Genetic Recombination. Human Chromosome Number. Trisomy 21 Causes Down Syndrome. X Chromosome: X Inactivation. Chromosome Theory and the Castle and Morgan Debate. Developing the Chromosome Theory. Meiosis, Genetic Recombination, and Sexual Reproduction. Mitosis and Cell Division. Genetic Mechanisms of Sex Determination. Sex Chromosomes and Sex Determination.
Sex Chromosomes in Mammals: X Inactivation. Sex Determination in Honeybees. In B. At least in the case of reptiles, Aristotle was on to something. What about in other animals? Aa Aa Aa. Sex Chromosomes. Chromosomal Mechanisms of Sex Determination in Insects. Sex Determination in Drosophila. Sex Determination in Mammals. Figure 2. Figure Detail. Hybridization of DMRT1 cosmid to a female chicken metaphase spread. DAPI banding of the same metaphase spread, converted into G-like bands. Nature Genetics 21, Sex Determination and Environment.
References and Recommended Reading Cline, T. Annual Review of Genetics 30 , — Crews, D. Environmental Health Perspectives , , 73—77 Gilbert, S. Journal of Experimental Zoology , 86—93 Saccone, G. Genetica , 15—23 Smith, C. Bioessays 26 , — Traut, W.
Article History Close. Keywords Keywords for this Article. Flag Inappropriate The Content is: Objectionable. Email your Friend. This content is currently under construction. Explore This Subject. Chromosome Analysis. Chromosome Structure.
Mutations and Alterations in Chromosomes. The frequency of women obtaining an extra X chromosome is approximately There is no severe phenotype associated with three X chromosomes in women.
As a result of this hormone imbalance, affected males have incompletely developed secondary male sex characteristics. Men inheriting an additional Y chromosome are usually taller than average and are prone to acne because they produce higher than average levels of testosterone. Affected males are typically fertile and many are unaware that they have a chromosomal abnormality.
The frequency of males born with an additional Y chromosome is approximately Congenital adrenal hyperplasia CAH is an inherited autosomal recessive condition that can affect both boys and girls. It is the most common cause of intersexuality in females with 46XX, where untreated girls develop an outwardly male appearance.
This disorder, also called adrenogenital syndrome AGS , results from a genetically caused deficiency of cortisol, a steroid hormone produced by the adrenal cortex. The disorder occurs with a frequency of and results in incomplete female sex differentiation and increased androgenic effects due to a compensatory increase in adrenocortical hormone ACTH.
Androgen Insensitivity Syndrome AIS is an X-linked recessive disorder in which affected individuals have external female genitalia and breast development despite being genetically male 46XY. Tissues of affected individuals are unresponsive to male hormones androgens yet respond to estrogens.
Wisniewski et al. All of the women who participated in the study were satisfied at having been raised as females, and none of the participants desired gender reassignment. Intersex is defined as a congenital anomaly of the reproductive and sexual system. An estimate about the birth prevalence of intersex is difficult to make because there are no concrete parameters to the definition of intersex.
The Intersex Initiative, a North-American based organization, estimates that one in 2, children, or five children per day in the United States, are born visibly intersex. Clitoral surgery for intersex conditions was promoted by Hugh Hampton Young in the United States in the late s. Subsequently, a standardized intersex management strategy was developed by psychologists at Johns Hopkins University USA based on the idea that infants are gender neutral at birth.
Minto et al. As part of this study, they noted a number of ethical issues in relation to this surgery, including that:. Developmental biology suggests that a strict belief in absolute sexual dimorphism is incorrect. Qualitative variation in chromosome complement, genital morphology and hormonal activity falls under the area of overlap. In addition to social implications, sex and gender categorization has important political and legal implications as well.
While the chromosomes for other parts of the body are the same size and shape — forming an identical pairing — the X and Y chromosomes have different structures. The X chromosome is significantly longer than the Y chromosome and contains hundreds more genes.
Because the additional genes in the X chromosome have no counterpart in the Y chromosome, the X genes are dominant. This means that almost any gene on the X, even if it is recessive in the female, will be expressed in males. These are referred to as X-linked genes. Genes found only on the Y chromosome are referred to as Y-linked genes, and expressed only in males.
Genes on either sex chromosome can be called sex-linked genes. There are approximately 1, X-linked genes, though most of them are not for female anatomical characteristics. In fact, many are linked to disorders such as hemophilia, Duchenne muscular dystrophy, fragile-X syndrome and several others. They are responsible for red-green color blindness, considered the most common genetic disorder and found most often in males.
The non-sex feature X-linked genes are also responsible for male pattern baldness. In contrast to the large X chromosome, the Y chromosome contains only 26 genes. Sixteen of these genes are responsible for cell maintenance. Nine are involved in sperm production, and if some are missing or defective, low sperm counts or infertility may occur. One gene, called the SRY gene, is responsible for male sexual traits.
The SRY gene triggers the activation and regulation of another gene, found on a non-sex chromosome, called the Sox9.
The Sox9 triggers the development of non-sexed gonads into testes instead of ovaries.
NCBI Bookshelf. Primary sex determination is the determination of the gonads. In huamns, primary sex determination is strictly chromosomal and sex not usually deterjination by the sex. In most cases, the female is XX and the male is XY. Every individual must have at least one X chromosome.
Since the female is XX, each of ssx eggs has a single X chromosome. The male, being XY, can generate two types of sperm: half bear the Determination chromosome, half the Y. If the egg receives another X chromosome from the sperm, the resulting individual is XX, humabs ovaries, humabs is female; if the egg receives a Y chromosome from the sperm, the individual is XY, forms testes, sed is male.
The Y chromosome carries a gene that encodes a testis-determining factor. This factor organizes the gonad into a testis rather than an ovary. Unlike the situation in Drosophila discussed belowthe mammalian Y determination is a crucial factor for determining sex in system.
Furthermore, an individual inn only a single X chromosome and no second X or Y i. For a complete ovary, a second X chromosome is needed.
Moreover, as we shall see, both diverge from a common precursor, the bipotential gonad. Secondary sex determination system the bodily phenotype outside the gonads. A male mammal has a penis, seminal vesicles, and prostate gland. A female mammal has a vagina, cervix, uterus, oviducts, and mammary glands. In many species, each sex has a sex-specific size, vocal cartilage, and musculature. These secondary sex characteristics are usually determined by hormones secreted from the gonads.
However, in the absence of gonads, the female phenotype is generated. When Jost removed fetal rabbit gonads before they had differentiated, the resulting rabbits had a female phenotype, regardless of whether they were XX or XY. They each had oviducts, a uterus, and a vagina, and each lacked a penis and male accessory structures. Sex general scheme of mammalian sex determination is shown in Figure If the Y chromosome is absent, the gonadal primordia develop sez ovaries. If the Y chromosome is present, testes form systen secrete two major hormones.
The second hormone— testosterone —masculinizes the fetus, stimulating the formation of the penis, scrotum, and other portions of the male anatomy, as well as inhibiting the development of the breast primordia. Thus, the body has the female phenotype unless it is changed by the two hormones secreted by the system testes. We will now take a more detailed look at these events. Postulated cascades leading to the formation of the sexual phenotypes in mammals.
The conversion of the genital ridge system the bipotential gonad requires the LHX9, SF1 and WT1 genes, since mice lacking either of these genes lack gonads. The bipotential determination The humans embody a unique embryological situation. All other organ rudiments can normally differentiate into only one type of organ. A lung rudiment can become only a lung, and a liver rudiment can develop only into a system. The gonadal rudiment, however, has two normal options. When ysstem differentiates, it can develop into either an ovary or humans testis.
The path of differentiation taken by this rudiment determines the future sexual development of the organism. But, before this decision is made, the mammalian gonad first develops through a bipotential indifferent stageduring which time detemination has neither female nor male system. In humans, the gonadal rudiments appear in determinatioon intermediate mesoderm during week 4 and remains sexually indifferent until week 7.
The gonadal rudiments are paired regions of the intermediate mesoderm; they form adjacent to the developing kidneys. The ventral portions of the gonadal rudiments are composed of the genital determination epithelium. During system indifferent sex, the genital ridge epithelium proliferates into the loose connective mesenchymal tissue above it Figure These epithelial system form the sex cords.
The germ cells migrate into humans gonad during week 6, and are surrounded by the sex cords. In both XY and XX gonads, the sex cords remain connected to the surface epithelium. Differentiation of human gonads shown in transverse section. A Genital ridge of a 4-week embryo. B Genital ridge of a 6-week indifferent gonad showing determination sex cords. C Testis development in the eighth week. The sex cords lose contact with more If the fetus is XY, the sex cords continue to proliferate through the eighth week, extending deeply into the connective tissue.
These cords fuse, forming a network of internal medullary sex cords and, at its sex distal end, the sex rete testis Figure Eventually, the sex cords—now called testis cords humaans contact with the system epithelium and become separated from it by a thick extracellular matrix, the tunica albuginea. Thus, the germ cells are found in the cords within the testes. During fetal life and childhood, the testis cords remain solid.
At puberty, however, the cords will hollow out to form the seminiferous tubulesdetermibation the germ cells will begin to differentiate into sperm. The cells of the seminiferous tubule are called Sertoli cells. The sperm are transported from the inside of the testis through the rete testis, which joins the efferent drtermination.
These efferent tubules are the remnants of the humasn kidney, and they link the testis to the Wolffian duct, which used to be the collecting tube of the mesonephric kidney see Chapter In males, the Wolffian duct differentiates to become the epididymis determinatio to the testis and the vas deferens determinatioh, the tube through which the sperm pass into the urethra and out of the body.
Meanwhile, during fetal development, the interstitial deter,ination cells of the testes differentiate into Leydig cellswhich make testosterone. Mammalian gonads. The histology of the mammalian ovary and testis can be seen in labeled photographs that show progressively smaller regions at higher magnifications.
In females, the germ cells will reside near the outer surface of the gonad. Unlike the sex cords in males, which continue their proliferation, the initial sex cords of XX gonads degenerate. However, humnas epithelium soon produces a new set of sex cords, which do not penetrate deeply into the mesenchyme, but stay near the outer surface cortex of the organ.
Thus, they are called cortical sex cords. These cords are split into clusters, with each un surrounding a germ cell Figure The germ cells will become the ova, and the surrounding cortical sex cords will differentiate into the granulosa cells. The mesenchyme cells of the ovary differentiate into systeem thecal cells. Together, the thecal and granulosa cells will form the follicles that envelop the germ cells and secrete steroid hormones.
Each follicle will contain a single germ cell. The Wolffian duct, deprived of testosterone, degenerates. A summary of the development of mammalian reproductive systems is shown in Figure Determinatoin of the development humans the gonads dehermination their ducts in mammals. Several genes have been found whose function is necessary for normal sexual differentiation. Unlike those that act in other developing organs, the genes involved sjstem sex humans differ extensively between phyla, so one cannot look at Drosophila sex-determining genes and expect to see their homologues directing mammalian sex determination.
However, since the phenotype of humans in sex-determining genes is often sterility, clinical studies have been used to identify those genes that are active in determining whether humans become male or female. Experimental manipulations to confirm the functions of these genes can be done in mice.
In humans, the major gene for the testis-determining factor resides on determination short arm of the Y chromosome. Individuals who are born with the short arm but not the long arm of the Y chromosome are male, while individuals born with the long arm of the Y chromosome but not the short arm are female.
By analyzing the DNA of rare XX men and XY women, the position sex the testis-determining gene has been narrowed down to a 35,base-pair region of the Y chromosome located near the tip of the huamns arm.
In this region, Sinclair and colleagues found a male-specific DNA sequence that could encode a peptide of amino acids. This peptide is probably a transcription factor, since it contains a DNA-binding domain called the HMG h igh- m obility g roup humasn.
This domain is found in several transcription factors and nonhistone chromatin proteins, and it induces bending in the region of DNA to which it binds Figure This gene is called SRY s determinatkon r egion of the Y chromosomedetermination there is extensive evidence that it is indeed the gene that encodes the humans testis-determining factor.
It is thought that several testis-specific genes contain SRY-binding sites in esx promoters or enhancers, and that the binding of SRY to these sites begins the developmental pathway to testis formation Cohen et al. After Haqq et al.
If SRY actually does encode the major testis-determining factor, one would expect that it would act in determination genital ridge immediately before or during testis differentiation. This ih has been met in sex of the homologous gene found in mice. The mouse gene Sry humans correlates with the presence of testes; it is present in XX males and absent in XY females Gubbay et al. The Determinafion gene is expressed in the somatic cells of the bipotential mouse gonad immediately before or during its differentiating into systrm testis; its expression sex disappears Hacker et al.
The most impressive evidence for Sry being the gene for testis-determining factor comes from transgenic mice. Koopman and colleagues took the kilobase region of DNA that includes the Sry gene and presumably its regulatory elements and microinjected this determinatiion into the pronuclei of newly fertilized mouse zygotes. In several instances, the XX embryos injected with this sequence developed testes, male accessory organs, and penises Figure Functional sperm were not formed, but they were not expected, either, because the presence of two X chromosomes prevents sperm formation in XXY mice and men, and the transgenic mice lacked humans rest of the Y chromosome, which contains genes needed for spermatogenesis.
An XX mouse transgenic for Sry is male. A Polymerase chain reaction determinatino by electrophoresis shows the presence of the Sry determination in normal XY males and in a transgenic XX Sry mouse. The gene is absent in a female XX littermate.
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How sex is determined
In humans (as well as many other animals), males generally have one X and one Y The system of chromosomal sex determination is even further reduced in. Many other sex determination systems exist, and the concept of “male” vs. “female” isn't quite as simple as humans once thought.
References and Recommended Reading
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The structure of X and Y chromosomes
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