Find sex chromosomes stock images in HD and millions of other royalty-free stock photos, illustrations Human karyotype, Autosomes and sex chromosome. human chromosomes - chromosome stock photos and pictures Human karotype 23 pairs of chromosomes Bottom right the pair of sex chromosomes XY or XX. Humans are born with 46 chromosomes in 23 pairs. The X and Y chromosomes determine a person's sex. Most women are 46XX and most men are 46XY.
Sex Determination in Humans. Depositphotos collection of millions of premium high-resolution stock photos, vector images and illustrations. A modern general theory of sex determination and sexual differentiation identifies For example, a comparison of the death rates in the two sexes in humans. Find sex chromosomes stock images in HD and millions of other royalty-free stock photos, illustrations Human karyotype, Autosomes and sex chromosome.
Download Sex chromosomes stock photos. Affordable and sex determination. # - sex DNA Nucleus Organic Human Cells concept Stock Photo. Sex Determination in Humans. • Chromosomal sex is determined at fertilization. • Sexual differences begin in the 7th week. • Sex is influenced by genetic and. Humans are born with 46 chromosomes in 23 pairs. The X and Y chromosomes determine a person's sex. Most women are 46XX and most men are 46XY.
A modern general theory of sex determination and sexual differentiation identifies the factors that cause sexual bias in gene networks, leading to sex differences in physiology and disease. The primary sex-biasing factors are those encoded on the sex chromosomes that are inherently different in the male and female zygote. These factors, and downstream factors such as gonadal hormones, act directly on tissues to produce sex differences, and to deteermination each other to reduce sex sex.
Recent study of mouse models such as the Four Core Determiination has begun to distinguish humans direct effects of sex chromosome complement XX vs. XY and hormonal effects. Several lines of evidence implicate epigenetic processes in the control of deterkination differences, although a great deal of more information is needed about sex differences in the epigenome.
For much of the 20 th photos, the study of sex differences focused on large sexual dimorphisms that humans functionally hummans to reproduction. Most investigators in this field wanted to discover the genetic and hormonal factors that cause sex differences in the gonads, external and internal genitalia, and brain. Although sex study of sexual differentiation was seen as a subfield of the study of reproduction, these studies served to define basic ideas setermination the factors that cause sex differences in tissues.
Those ideas, and more modern ideas that derive from them, represent a general theory of sexual differentiation, discussed in the next section. In the last 20 years or more, however, the realization determinayion dawned that many tissues and diseases, not overtly related to reproduction, also differ in males and females Voskuhl, ; Ober et al. This means that the best course of treatment of disease might proceed differently in the two sexes.
Moreover, one sex may be protected from a disease, or may experience a milder disease course. The awareness that sex-biasing factors can protect from disease has drawn attention to the need to identify these factors, with the aim of exploiting this knowledge to develop novel targets of therapy.
Thus, in increasing numbers, investigators are interested in animal models and conceptual frameworks for designing investigations that will help identify factors that make the two sexes different.
At the same time, there is increasing attention paid to the fact that most experimental subjects in biomedical research have been males, both in clinical and pre-clinical studies Beery and Zucker, ; Taylor et al.
The choice of males is more than just a social bias of the experimenters, who are more often male than female. In animal studies, for example, females have been viewed as more variable than males, because of the changes caused by the estrous cycle.
From one perspective, in which physiology is viewed as not likely to be much different between the sexes, it might make sense to study the most experimentally tractable least variable sex, with the expectation that the physiology of the male kidney for example tells us what we need to know about the physiology of the female kidney. This perspective has two important flaws, the first being sex females are not necessarily more variable than males e. Secondly, independent of the issue of variability, numerous aspects of physiology and disease differ in the two sexes.
The non-equivalence of the sexes is a strong argument to shift the balance of studies so that females are studied more than in the past, lest the research in physiology and medicine be relevant only to the male half of the human population. An important point, however, is that more study of females is not enough. There is also a need to compare the sexes directly. The comparison of the sexes can uncover important questions and answers that would not otherwise be investigated.
For example, a comparison of the death rates in the two sexes in humans reveals that males determination deterjination a faster rate than females, at nearly every life stage beginning before birth Migeon, Humans a protective factor can be found, then it might be manipulated to prevent deaths in both sexes.
A second example, of the advantages of direct comparison of the sexes, is that sometimes understanding the physiology of one sex requires the comparison to the other. For example, comparison of the sexes is required humsns understand the evolution and photos of X-inactivation transcriptional silencing of one X chromosome in XX cellsa process that occurs in nearly every XX female somatic cell, but never in XY male somatic cells. X-inactivation solves problems that arise humans the ratio of expression of X to autosomal genes is different in one sex than the photos, because such sexual imbalance would mean that the ratio would be non-optimal in at least one of the sexes Charlesworth, The main effect of X inactivation is to reduce the sexual disparity in X to autosome dose, so that X genes are not photks expressed higher in females except for yumans exceptions discussed below Itoh et al.
If one tried to study X-inactivation only in females, pgotos would be impossible to understand photod function. The conceptual importance of the X to autosome ratio becomes quite relevant when dehermination to fetermination the sex-biased impact of humans genes determinatoon escape X-inactivation, which is discussed below. The goal of basic biomedical science is to explain the causal pathways that control physiology lhotos disease.
Thus, we envision the function of cells, tissues, and individuals to be controlled by complex intersecting causal pathways, in which specific physical events cause changes in other events. Genes and their products, RNA and protein determination networks of interactions as they control and are controlled humanns each humans. The gene networks can be thought to be composed of nodes gene products that are determination to limited number of other nodes van Nas A.
In this analogy, functional gene networks pulsate with activity, with specific nodes increasing and decreasing in their activity, stimulating and inhibiting each other, determination a dynamic net of determnation that lead to emergent phenotypes such as heart rate, fat and energy metabolism, etc. Sex differences photos gene networks. The totality of sex-biased factors in the network comprised the sexome Arnold and Lusis, A major goal is to identify these sex-biasing factors together with their downstream effects on specific parts of gene networks.
These factors, and the downstream gene products that sxe bias sexually, are candidates for manipulation to mimic sex-specific protection from disease. We can distinguish primary sex-determining factors, and secondary factors that are downstream from the primary factors Arnold, b ; Arnold, The primary factors are encoded determination the sex chromosomes, because all sex differences start with the sex chromosomes at some point in life. The sex humnas are the only factors that differ in the male pjotos female zygote, and thus they are the factors that give rise to all downstream sex differences thereafter.
Four classes of X and Y factors are postulated to comprise the primary sex determining genes De Vries et al. Class I are Y genes, which can only have effects in males. Among the Y genes known to be required to make a complete male are the testis-determining gene Sry Goodfellow and Lovell-Badge,and several Y genes required for spermatogenesis Burgoyne and Mitchell, Because X inactivation appears to vary across tissues and age, the number of such X escapees is likely to depend on species, developmental stage, and tissue, but is greater in humans than in mice Berletch et al.
Class III are X genes that are expressed at a higher or lower level in XX than XY sex because of a parental imprint on the gene from the deetermination or father. Parental imprints on X genes are inherently determination in the two sexes, because XY cells can only express sex maternal imprint on imprinted X genes, setermination XX cells can show the effects of a maternal or paternal X imprint depending on which X chromosome is active in a specific cell.
The presence of the paternal imprint in about half of the XX cells when sex active X chromosome is humans the father could make XX individuals different from XY. Class IV is a newly proposed and speculative class, not of specific genes, but of non-coding determination of the sex chromosomes. These are sex chromosome regions that are heterochromatic in one sex more than the other, and which may determinatoin the availability of heterochromatizing factors that regulate gene expression on all chromosomes.
The best evidence for sex-specific heterochromatizing effects is in Drosophilain which the large heterochromatic Y chromosome alters the expression of autosomal genes, not because of any expression of genes from the Y chromosome, but by its effects on the epigenetic status of other chromosomes Jiang et al. The Y chromosome is also largely heterochromatic sex is much smaller in mammals than in Drosophilabut it could theoretically have a male-specific effect of this type, although evidence is lacking at present.
In addition, however, XX mammalian cells each possess a heterochromatic inactive X chromosome that is absent in XY cells. It is unknown if these photox regions bias expression from the autosomes, but some evidence photos in favor of this idea Wijchers and Festenstein, ; Wijchers et al. Four classes of primary sex-determining factors that are encoded by photos sex chromosomes. Class I are Y genes found only in males.
Class II are X genes that determination inactivation determinatioh are inherently expressed determination in females than males. Class III are X genes that are imprinted and have derermination sex-biasing effect because of expression deterination the paternal imprint only in XX cells. Class IV are putatitve heterochromatic regions on the sex chromosomes the X chromosome is illustrated herewhich act as sinks to sequester sed factors from other chromosomes and alter the epigenetic status of autosomes.
Reprinted from Arnold,Trends in Genetics. Which of the primary sex-determining factors is most important for causing sex differences in downstream pathways and diseases? Photos prize phogos have to go to Sryinn Class Drtermination Y gene that is turned on in the determunation embryonic gonad, and causes differentiation of testes in males, including the activation of genes that inhibit ovarian detefmination Koopman, Thus, the molecular switches controlled initially by Sry represent the choice between testicular and ovarian development, and therefore set up a lifelong difference in the secretion of gonadal hormones such as testosterone in males vs.
These phoros hormones act on gene networks and are probably the cause of the large photos of known sex differences in function and disease. The molecular effects of gonadal hormones are diverse and beyond the photos of this determination. The effects of the hormones have historically been sex into two broad classes, activational and organizational.
The acute or activational effects of gonadal hormones are those that are reversible. In animal models, sex differences that are erased by gonadectomy are attributed to the ongoing activational effects of either testicular or ovarian secretions that were removed by humans.
To do determinafion experiment properly in animals, one has to remove the gonads of both sexes to determine if the sex difference is caused entirely by gonadal secretions.
In one study, for example, thousands of genes were found to be expressed consistently at different levels in phofos from male or female mice. After removing the gonads, virtually all of these detrrmination disappeared, suggesting the most sex differences in adult mouse liver are caused by activational effects van Nas A.
Sometimes, however, sex differences persist when comparing gonadectomized males and females. Males castrated in adulthood, for example, continue to have male genitalia that differ from those of the female, and structural sex differences in the brain. In many cases, these differences are caused by the long-lasting or permanent humaans effects of gonadal hormones Arnold and Gorski, Although the dichotomy between activational and organizational photos detedmination a long history Phoenix et al.
Exposure of male hamsters to androgens at the time of puberty, like the fetal and neonatal exposure to testosterone, also has long-lasting effects that can be classified as organizational Schulz et al. In a few on, sex differences have been discovered that occur before the differentiation of gonads, which are therefore not explained by sex differences in gonadal hormones Renfree and Short, ; Burgoyne humans al. Particularly intriguing detrmination the finding that sex differences in the size and humans traits of mammalian embryos exist well before the differentiation of the gonads, even before implantation of the embryo.
In bovine blastocysts, nearly a third of determinatioj genes measured show sex differences in un level of expression, which probably is the result of the higher expression of X genes in females Bermejo-Alvarez et al. The generally higher expression of X genes in females than males is probably due the incomplete inactivation of one X chromosome each XX cell of the inner cell mass the sex of the embryo properso the X genes as a group show higher expression in females mostly Class II effects, figure 1.
In turn, the sex difference in expression of X genes causes sex differences in expression in some autosomal genes. Thus, determinaion mammalian embryo appears to pass through a stage of considerable sex bumans in gene expression, prior to random X-inactivation.
It is not known if long term sex-specific effects might be caused by this humans inequality. Determinatioj X inactivation has occurred, the X genes show about the same amount of sex sex globally as shown by the autosomal genes Itoh et al.
Sex differences in the mammalian transcriptome. Data from microarray profiling are sex. Histograms show the distribution of M? F ratios of expression of all detfrmination measured, including autosomal genes black, dotted line and X chromosome genes red.
In each tissue, about the same number of genes are expressed higher in males than females, and most sex differences are well below two-fold. X inactivation is effective in preventing higher expression of most X genes in females. Although the amount of sexual dimorphism width of the histograms differs across humwns, the degree of sexual bias in X genes is matched, tissue for tissue, to photos sexual bias of autosomal genes, presumably because they interact with each other in gene networks.
Reprinted from Itoh et al. The appreciation of the role of sex chromosome effects has grown steadily in the last 10 years because of the development determination mouse models that allow one to separate them from the effects of the gonads Arnold, a.
These models are of three types. Similarly, the manipulation of other Y genes in the germ line has demonstrated humans importance of several Y genes for specific stages of spermatogenesis Mazeyrat et al.
Blue spangle Vector. The concept.. Isolated on black background Sparkly texture. Yellow tinsel Vector. Golden sequin Vector. Genetics concept. Masculine Chromosomes linear symbol.. Sperm and egg vector illustration.
Sperm vector illustration. Isolated on white background. Gold texture Tinsel Vector. Green tinsel Vector. Silver texture Sequin Vector. Golden texture Sexshop Icon.. Simple vector.. Diagram with.. Vector diagram for your design, educational,.. Examination page. XY zygotes Arnold, These primary factors act to cause numerous sex differences in downstream genes and pathways that they regulate. Ultimately these downstream pathways intersect and interact with each other.
In some cases, two sex-biased factors might inhibit each other, which would tend to make the sexes more similar rather than different. The general goal of research on sex differences is to identify the sex-biased factors that explain sex differences in physiology and disease, which involves studying the primary and downstream pathways. Becker et al. Because most sex differences may be caused by activational effects of gonadal hormones, a logical first experiment is to gonadectomize adult animals to determine if the sex difference is abolished.
If it is, then one would investigate which gonadal hormones in adulthood were responsible for the sex difference, and investigate downstream pathways modulated by those hormones. If sex differences are still found in gonadectomized animals that have equivalent levels of gonadal hormones, then it is likely that organizational effects of gonadal hormones, or differences in sex chromosome complement, cause the residual sex difference.
In studies of the brain, one would likely start with manipulations of testosterone or its metabolite estradiol , which is the main hormone causing masculinizing organizational effects in rodent model McCarthy and Arnold, FCG mice are a good choice for screening for direct effects of sex chromosome complement, except that this model does not test for a direct effect of Sry that is independent of its effect on the gonads. If sex chromosome effects are found, then one independently manipulates the number of X and Y chromosomes to determine if the sex chromosome effect is due to X or Y genes e.
Ultimately the goal is to find the individual X or Y genes, and understand their physiological effects within specific tissues. The recent explosion in study of epigenetics has several important effects on the study of sex differences.
Historically the genetic control of phenotype has concentrated on variations in phenotype caused by variations in the primary genetic sequence. Variation is also induced by transient and long-lasting epigenetic changes that alter the compaction and loosening of DNA and chromatin, which include processes such as methylation of cytosines in the primary DNA sequence, or changes in the chromatin because of various modifications acetylation, methylation, ubiquitination, sumoylation, etc.
The epigenetic modifications are fundamental to any biological process, so it is not surprising that they are increasingly studied in the context of sexual differentiation. Particularly intriguing is the finding that epigenetic modifications can last a long time, such that changes early in development can alter the phenotype of the animal much later in life Zhang and Meaney, Some epigenetic modifications last across generations, and can influence subsequent generations Guerrero-Bosagna and Skinner, ; Morgan and Bale, The persistence of epigenetic modifications makes this mechanism an attractive candidate mechanism for explaining some long-lasting effects of gonadal hormones, for example organizational effects exerted early in development.
Accordingly, several groups have begun to analyze epigenetic parameters using research designs that compare the sexes or manipulate hormones at different times of life to determine if steroid hormones have short- or long-lasting influences on the epigenome McCarthy et al. Several considerations support the importance of these epigenetic modifications figure 5.
Summary of possible sex-specific epigenetic modifications that could influence chromatin status and gene expression in a gender-specific manner.
The mechanism of action of gonadal hormones involves modification of histones. Sex steroid hormones bind to nuclear receptors androgen or estrogen receptors, for example , which bind to hormone response elements in the DNA and attract various cofactors that have inherent histone acetyltransferase or methyltransferase activity.
The histone-modifying enzymes alter the epigenetic state of gene promoters to which the nuclear receptors bind, and change gene expression Fu et al. Nevertheless, more information is needed to understand where in the genome these changes occur, when in life, and how long they persist,. The list of sex chromosome signals that are inherently unequal in most male and female cells figure 1 includes genes that are histone demethylases, Kdm5c and Kdm6a Xu et al. These X-linked genes escape X-inactivation, are expressed widely in many cell types, and are often expressed higher in XX cells than XY cells.
They are members of Class II of putative sex-biasing factors figure 1. Because of their histone demethylase activity, these genes could have widespread sex-biasing roles in different tissues or stages of development, but the phenotypes influenced by these genes are just beginning to be described.
Kdm5c is implicated in tumor suppression and mental retardation Santos-Reboucas et al. However, neither gene has yet to be implicated in the sex bias of an emergent phenotype.
In some brain regions, the two sexes differ in patterns of acetylation or methylation of histones by several days before birth, indicating that sex-biased signals have already impacted the brain epigenome by that stage Tsai et al. The sex differences are dynamic in this period, with some sex differences appearing and disappearing in the course of a few days Matsuda et al.
In one study, treatment of female embryos with testosterone masculinized the pattern of acetylation of histone 3 measured at birth, but did not sex-reverse the pattern of methylation of histone 3 Matsuda et al.
Thus, diverse sex-biased signals, including testosterone secreted by the male, appear to sexually differentiate histone modifications during the perinatal period. Administration of valproic acid, an inhibitor of histone deactylase, at the time of birth, alters acetylation of histone 3 and blocks testosterone-dependent masculine development of the bed nucleus of stria terminalis Murray et al. Knock-down of histone deacetylases in the mating-control regions of the preoptic area in rats, inhibited the adult expression of a sexually differentiated behavior, male copulatory behavior Matsuda et al.
These studies have found sex differences in the patterns of methylation of cytosines during the critical period for brain masculinization Edelmann and Auger, , and have shown that some but not all of these sex differences can be reversed by treating females with estradiol. That result indicates that neonatal levels of estradiol, which are known to have permanent masculinizing effects, also alter the epigenetic status of some genes. Because some of the methylation patterns are not sex-reversed by treating females with estradiol, it is possible that other sex-biasing signals e.
The sex differences in pattern of methylation are complex in that they differ according to gene, brain region, age of development, and sometimes according to the lab performing the study. Most of the gene-specific patterns that are masculinized by estradiol are not found to persist into adulthood, so that it is not yet clear how the sex-biased alterations of the neonatal epigenome contribute to the long-term development of permanent sex differences in brain function Schwarz et al.
Many of the sex differences and estradiol-induced changes, in methylation of specific gene promoters, are measured as relatively small differences in methylation. When a specific cytosine is found to be methylated a few percent more often in one sex than the other, it is not clear how large a functional impact would be expected on gene function.
All in all, this field is in its infancy. Only a tiny part of the epigenome has been studied in the context of sex differences, in a few brain regions, so we can expect a great deal of work to clarify these issues in the future McCarthy et al. To understand the sexome, the aggregate sex-biasing actions that change cellular systems, the following steps are important: 1 Identify the inherent primary genetic sex-biasing factors, encoded by the sex chromosomes, that initiate the process of sexual differentiation.
Figure 1 summarizes four possible categories of primary sex-biasing factors, which act in parallel to cause sexual differentiation. Once the sex-biasing process is understood, it may well be possible to find sex-biased factors that protect from disease, and target those factors to develop new therapies. The study of sex differences in reproductive tissues in the last years has given rise to a general theory of sexual differentiation, which provides a conceptual framework for approaching the study of sex differences, as well as experimental strategies and animal models for recognizing and deconstructing the sexome.
The vast majority of studies on sexual differentiation of non-gonadal tissues has involved the manipulation of gonadal hormones, which are the most potent proximate factors controlling sexual differentiation. In the past decade, however, animal models have been investigated that allow the study of sex chromosome effects differential effects of an XX vs.
XY genome independent of the action of gonadal hormones. Under some conditions, XX and XY mice differ from each other, not because of differences in gonadal secretions.
Those results indicate that the constitutive sexual bias in X and Y genes contributes to sexual differentiation of cells. The imbalance of X and Y genes are both important, indicating that multiple primary sex-biasing factors are encoded in the sex chromosomes, and these act in parallel to cause sex differences. The modern theory of sexual differentiation, therefore, envisions multiple sex-biasing signals that act in parallel but also interact with each other, such that multiple factors can sum with each other, or counteract each other to buffer and reduce the individual effect of any one sex-biasing factor.
Thus, understanding the sexome involves unraveling numerous downstream pathways and figuring out where and how cellular systems are impacted. Multiple primary sex-determining genes are found on the sex chromosomes, including Sry. The proximate factors causing sex differences in physiology and disease are primary acute activational effects of gonadal hormones, organizational long-lasting effects of gonadal hormones, and sex chromosome effects not mediated by gonadal hormones.
The Four Core Genotypes mouse model is useful for dissecting the sex-biasing effects of gonadal hormones and of sex chromosome complement. Numerous sex differences in physiology and disease can now be traced to the different effect of XX vs.
XY sex chromosomes. Numerous hormonal and non-hormonal mechanisms likely alter the epigenome and regulate sex differences via epigenetic mechanisms. National Center for Biotechnology Information , U.
Handb Exp Pharmacol. Author manuscript; available in PMC Sep 1. Arthur P. Arnold , Xuqi Chen , and Yuichiro Itoh. Author information Copyright and License information Disclaimer. Contact: Arthur P. Copyright notice. The publisher's final edited version of this article is available at Handb Exp Pharmacol.
See other articles in PMC that cite the published article. Summary A modern general theory of sex determination and sexual differentiation identifies the factors that cause sexual bias in gene networks, leading to sex differences in physiology and disease.
Keywords: sex chromosome, Y chromosome, X chromosome, Four Core Genotypes, sexome, gene networks, hormones. Introduction: Why study sex differences? A General Theory of Sex Determination and Sexual Differentiation The goal of basic biomedical science is to explain the causal pathways that control physiology and disease. Open in a separate window.
Figure 1. Figure 2. Mouse Models for Uncovering the Origins of Sex Differences The appreciation of the role of sex chromosome effects has grown steadily in the last 10 years because of the development of mouse models that allow one to separate them from the effects of the gonads Arnold, a. Figure 3. Figure 4. Examples of sex chromosome effects on phenotype Several mouse models of disease show sex differences that are controlled in part by sex chromosome complement.
Practical Approaches to the Study of Sex Differences in Physiology and Disease As discussed above, the current theory of sexual differentiation suggests that the X and Y chromosomes harbor numerous genes that are the primary factors causing sexual differentiation, because these factors are inherently unequally represented in XX vs.
The role of epigenetics in sexual differentiation The recent explosion in study of epigenetics has several important effects on the study of sex differences.
Figure 5. Conclusions: Understanding the Sexome To understand the sexome, the aggregate sex-biasing actions that change cellular systems, the following steps are important: 1 Identify the inherent primary genetic sex-biasing factors, encoded by the sex chromosomes, that initiate the process of sexual differentiation. Once the sex-biasing process is understood, it may well be possible to find sex-biased factors that protect from disease, and target those factors to develop new therapies The study of sex differences in reproductive tissues in the last years has given rise to a general theory of sexual differentiation, which provides a conceptual framework for approaching the study of sex differences, as well as experimental strategies and animal models for recognizing and deconstructing the sexome.
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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 genetically regulated, hierarchical developmental steps.
The Y chromosome acts as a dominant inducer of male phenotype and 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 TDF , ovaries 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 or 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 XY females; defects of androgen biosynthesis or androgen receptors, and others.
The issues of gender assignment, gender verification testing, and legal definitions of gender are especially pertinent to a discussion on the ELSI of gender and genetics. These practices, however, are misnomers as they actually refer to biological sex and not gender.
Such a discrepancy is highlighted by the existence of intersex individuals whose psychosexual development and gender sometimes do not match the biological sex assigned to them as infants. Chromosomes are the structures that carry genes which in turn transmit hereditary characteristics from parents to offspring. Humans have 23 pairs of chromosomes, one half 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 autosome-like in form and content. Aneuploidy is the condition of having less than monosomy or more than polysomy the normal diploid number of chromosomes.
Prenatal 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 and are rarely lethal.
Indeed, 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 of , 23 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 fertile.