Evolution of sexual reproduction

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    Not a MyNAP member yet? Register for a free account to start saving and receiving special member only perks. Identifying risk factors for commercial sexual exploitation and sex trafficking of minors is necessary to understand the etiology of these problems and to inform interventions for preventing and addressing them.

    Understanding the various short- and justify consequences of commercial sexual exploitation and sex trafficking of minors also is necessary to guide future prevention and intervention strategies. This chapter describes what is known about individual, family, peer, neighborhood, and systems-level risk factors associated with victims and offenders of commercial sexual exploitation and sex trafficking of minors.

    In addition, to clarify the impact of these crimes, the chapter describes what is known about their physical, emotional, and behavioral health consequences, as well as their developmental, social, health, and legal implications.

    Because the existing evidence base for these subjects is extremely limited, the discussion draws heavily on related research literatures e. Finally, this chapter aims to connect the dots justify these sources of evidence to provide guidance for future research efforts on commercial sexual exploitation and sex trafficking of minors in the United States.

    Commercial sexual exploitation and sex trafficking of minors are multiply determined with causes at several levels, ranging from individual characteristics to family, peer, and neighborhood factors. Community and. Adding to this complexity, each of these factors interacts within and across levels to increase risk or protection. Because of the multiple forces involved, prevention and intervention efforts targeting only single risks may have limited utility.

    Moreover, a comprehensive understanding of risk factors for commercial sexual exploitation and sex trafficking of minors requires awareness that the processes associated with these problems, from beginning to end, are dynamic see Figure The ecological model depicted in Figure highlights the complex and interconnected forces that contribute to initial and continued commercial sexual exploitation and sex trafficking of minors.

    It should be noted, however, that the factors included in this schematic are likely only a subset of the risk factors for these problems. Moreover, some of those factors may be necessary but not sufficient contributors to the commercial sexual exploitation and justify trafficking of minors. For example, the presence of risk factors would not result in the commercial sexual exploitation and sex trafficking of minors without the presence of an exploiter or trafficker.

    Of note, the contributing and maintaining factors depicted in Figure may function independently or in combination. In addition, risk factors in one sphere may trigger a cascade of effects or initiate pathways into or out of commercial sexual exploitation and sex trafficking.

    Because many of the factors in Figure also may be risks for other types of adverse youth outcomes, readers are cautioned not to assume that the presence of any single risk factor necessarily signals commercial sexual exploitation and sex trafficking of minors.

    Instead, the presence of one or more of these factors should be considered as sex of a more comprehensive sex to determine youth at risk of or involved in commercial sexual exploitation and sex trafficking. Each domain included in Figure is detailed in the following sections. FIGURE Ecological model adapted to illustrate the possible risk factors for commercial sexual exploitation and sex trafficking of minors. Child maltreatment Child neglect, physical abuse, and sexual abuse are commonly thought to be risk factors for commercial sexual exploitation and sex trafficking of minors Dalla et al.

    Support for this perspective originates in studies reporting that youth identify the sexual abuse they experienced as a child as a major influence on their becoming involved in commercial sexual exploitation.

    For example, 70 percent of the subjects in a U. Silbert and Pines found that 78 percent of the San Francisco prostitutes in their sample were prostituted as juveniles. In that study, the majority of those interviewed were under age 21, and one subject was only 10 years old. Sixty percent of the sample reported that they had been or were being sexually exploited; 67 percent reported sexual abuse during their childhood by a father figure 33 percent by their biological father28 percent by a brother, and 31 percent by friends of the family.

    In 82 percent of the episodes of abuse, some sort of force justify used. In one of the few prospective studies on this subject, Widom and Kuhns examined sex relationship between childhood maltreatment and promiscuity, prostitution, and teen pregnancy.

    The study used a prospective cohort design in which victims of child maltreatment justify matched with nonabused children and followed into adulthood. Child neglect and sexual abuse were found to be associated with later prostitution among females. In another prospective study, Wilson and Widomp. On the other hand, Nadon and colleagues compared a sample of 45 female adolescents involved in prostitution and recruited from service organizations in areas known for prostitution.

    Surprisingly, the adolescents involved and not involved in prostitution did not differ in terms of child sexual abuse: rates of child sexual abuse were similar in the two groups, as were the circumstances surrounding the abuse, including the relationships between perpetrators and victims. Notably, the commercially sexually exploited youth had significantly higher rates of running away from home see the discussion of this factor below ; this finding suggests that it may not be the child sexual abuse alone but its consequences that heighten the risk for commercial sexual exploitation and sex trafficking of minors.

    In fact, Jesson and Nadon and colleagues argue that early maltreatment, family dysfunction, and running away are so closely linked that it may well be the running away that puts youth directly at risk.

    Thus the literature suggests that there are multiple possible indirect pathways to commercial sexual exploitation and sex trafficking of minors. A number of researchers have offered explanations for why child maltreatment may create vulnerability to commercial sexual exploitation and sex trafficking of minors.

    Stoltz and colleaguesp. In addition, according to Stoltz and colleaguesvictims of child sexual abuse may have the tendency to engage in risk-taking behaviors i.

    While suggesting sex child sexual abuse creates a susceptibility to becoming involved in trading sex, the authors emphasize that the abuse does not cause commercial sexual exploitation and sex trafficking. Rather, they posit that the abuse is one component of a generally traumatic negative developmental experience that may weaken resiliency. Similarly, Steel justify Herlitz suggest a possible pathway from child sexual abuse to sexual risk behavior, with psychological symptoms such as depressive mood, poor self-esteem, lack of assertiveness, poor self-worth, and posttraumatic stress disorder PTSD leading to future risk behavior Stein et al.

    Noll and colleagues suggest that stigma associated with child sexual abuse may make it difficult for victims to experience nonsexual or emotional rewards from relationships, thereby making victims more likely to engage in risk behaviors and more vulnerable to sexual exploitation Lalor and McElvaney, ; Stoltz et al. According to several researchers, victims of child sexual abuse may experience poor affect regulation i. Cloitre et al.

    These outcomes can result in a number of negative consequences for victims of such abuse later in life. For example, victims may engage in emotional avoidance behaviors such as self-harm and substance abuse Lalor and McElvaney, Taken together, these propositions highlight emotional and behavioral consequences of child sexual abuse that are associated with commercial sexual exploitation and sex trafficking of minors, suggesting mechanisms that may link child abuse to these crimes.

    In support of the above propositions regarding justify indirect pathways through which child maltreatment increases the risk for commercial sexual exploitation and sex trafficking sex minors, ample evidence documents associations between child sexual abuse and outcomes that have been linked with these crimes.

    For example, victims of child sexual abuse may begin to engage in sexual activity at earlier ages than nonvictims Fergusson et al. Earlier onset of consensual sexual activity is in turn associated with increased risk for truancy, dropping out of school, and running away, as well sex for gang membership Unger et al. Several studies have found that sexually abused adolescents are likely to have higher levels of alcohol and drug abuse, risky sexual behaviors, depression, trauma, anxiety, and suicidality, as well as poorer sense of self Negrao et al.

    In older adolescents, child abuse has been found to be associated with heightened sexual risk taking and heightened risks for other adverse behavioral outcomes Fergusson et al.

    According to Kelley justify colleaguesadolescents with a history of child maltreatment were at least 25 percent more likely to experience problems with juvenile delinquency, teen pregnancy, low academic achievement, drug use, and symptoms of poor mental health than those without such a history, which together could increase the risk for commercial sexual exploitation and sex trafficking of minors. Few studies have examined indirect pathways between child maltreatment, hypothesized mechanisms, and commercial sexual exploitation and sex trafficking in prospective studies.

    One exception is the work of Wilson and Widomwho explored whether behavioral sequelae of child maltreatment i. Each type of child maltreatment i. Initiation of sexual behavior before age 15 emerged as the strongest link between child maltreatment and later prostitution. Other possible mediating factors e. While the above studies suggest that child maltreatment, particularly child sexual abuse, may be associated with commercial sexual exploitation and sex trafficking of minors, it should be noted that findings of the few prospective studies available suggest that only an extremely small proportion of children who are sexually abused subsequently become involved in trading sex for money or something of value Lalor and McElvaney, ; Lamont, For example, while Flowersp.

    This finding does not suggest a strong predisposition to involvement in prostitution among victims of child maltreatment. Moreover, while child maltreatment, and child sexual abuse in particular, appears to increase the risk for later high-risk behaviors and revictimization for some children and adolescents, this is not the case for all victims of child maltreatment.

    Reasons for such resilience after experiencing child maltreatment are unknown, as studies indicating what may make some victims of child abuse more or less resilient are lacking. Sex addition to a limited understanding of factors associated with resilience in victims of child sexual abuse, several factors limit understanding of the overall impact of child maltreatment on the risk for commercial sexual exploitation and sex trafficking of justify.

    In many studies, the timing of involvement in prostitution is unclear, nor is it possible to distinguish between prostitution and commercial sexual exploitation and sex trafficking of minors.

    Moreover, studies have examined associations between child sexual abuse and later engagement in high-risk sexual behavior, including having multiple sexual partners, engaging in commercial sex work, and being sexually promiscuous; however, methodological issues—such as sam.

    Other methodological problems include failure of most studies to distinguish among different types of maltreatment, as well as vast differences in definitions of abuse, differences in methodologies for identifying abuse, and the fact that most studies fail to measure the intensity and duration of the abuse or the relationship between perpetrator s and victim Briere, ; Hastings and Kelley, ; Hulme, ; Kelley et al.

    These types of problems limit the ability to make comparisons across studies with regard to type of abuse, prevalence, and sequelae, in turn making it difficult to determine the risks associated with each form of abuse. Furthermore, Lalor and McElvaney report that nonresponse rates of 30 percent are common in surveys on abuse, calling into question the generalizability of the responses of the 70 percent of people who typically agree to participate.

    In attempting to establish associations between child sexual abuse and commercial sexual exploitation and sex trafficking of minors, moreover, it is important to recognize that, like other forms of abuse, child sexual abuse is an extremely complex phenomenon Hulme, The individual experiences of victims, including the number of incidents, the age at onset, the frequency and duration of the abuse, and the relationship between perpetrator s and victim, vary greatly Hulme, In some cases, the boundaries between child abuse and commercial sexual exploitation are blurred, as when a parent sex other family member coaches a child to be sexually involved in return for money Saphira, Despite these methodological concerns, because commercial sexual exploitation and sex trafficking of minors are extreme forms of child sexual abuse, it is widely assumed that their victims will exhibit behaviors similar to those of victims of child sexual abuse.

    However, additional prospective, longitudinal research is needed to demonstrate more explicitly whether causal links exist between child sexual abuse and commercial sexual exploitation and sex trafficking of minors. Disrupted transitions can be defined as life events that either interrupt normative developmental patterns or occur prematurely. Similarly, earlier oc. Explanations for this association include the greater stress experienced by early maturers, the short-circuiting of certain developmental tasks of early adolescence, and the greater social pressure to which early developers may be exposed Tschann et al.

    Wickrama and Baltimorep. Research has not yet focused on early transitions as predictive of commercial sexual exploitation and sex trafficking of minors. However, a number of studies have identified adolescent life experiences that may be potential precursors to commercial sexual exploitation and sex trafficking of children and adolescents.

    Regardless of race or class, for example, those who become involved in the commercial sex trade both juveniles and adults are more likely to have a history of parental abuse and neglect, incest, rape, interrupted school activity including early dropoutrunning away, and early sexual experiences including early first intercourse Adlaf and Zdanowicz, ; Bracey, ; Chesney-Lind and Shelden, ; Gibson-Ainyette et al.

    It should be noted, however, that most studies of victims of commercial sexual exploitation and sex trafficking are based on retrospective reports, which are subject to errors of memory and are not verifiable; the result may be underreporting or overreporting of experiences such as sexual abuse and other maltreatment Hulme and Agrawal, Additionally, associations between childhood experiences and commercial sexual exploitation and sex trafficking of minors do not necessarily indicate a causal link; other factors may account for the commercial sexual exploitation and sex trafficking of minors.

    Sex, care must be taken in concluding which, if any, of these factors may be predictive of involvement in these crimes. Runaway, thrown-away, and homeless youth Of the many factors that may increase vulnerability to commercial sexual exploitation and sex trafficking of minors, especially among adolescents, homelessness is widely considered the sex direct contributor Estes and Weiner, Homeless youth may include runaways i.

    According to the U. Interagency Council on Homelessness, approximately 5 to 7 percent percent of U. The experiences of these youth vary widely, ranging from being in adult shelters to sleeping outdoors, in abandoned buildings, or with strangers Gilmore, ; Greene et al.

    Causes of homelessness vary widely as well. For example, youth who have been in foster care are at particularly high risk for becoming homeless NAEH, Approximately 40 to 60 percent of homeless youth have experienced physical abuse, and 17 to 35 percent have experienced sexual abuse. The risk of commercial sexual exploitation and sex trafficking among homeless youth increases with a prior history of sexual abuse, as does greater sexual risk taking e.

    Once homeless, young people are at significant risk of commercial sexual exploitation and sex trafficking for reasons ranging from a lack of resources for basic needs, such as food and shelter, to the need for social connection when separated from the family unit and other social supports.

    Not only are homeless youth victims of commercial sexual exploitation and sex trafficking, but older homeless justify youth may recruit their peers into involvement in these crimes Rotherham-Borus et al.

    3. Madison: University of Wisconsin System. Genero, N. P., Miller, J. B., Surrey, J., and/or sexually intimate relationships with members of the opposite sex Type of To what extent would each of the following reasons justify either an. Demi Moore Says Ashton Kutcher Used Threesomes To “Justify” Cheating to People, in the memoir, Moore shares that Kutcher used past threesomes to “​justify” cheating on her. These Thanksgiving Sex Positions Are Food-Coma Friendly. Dan BalanJustify Sex (DJ Nejtrino & DJ Stranger Remix Dan Balan Chica Bomb and Justify Sex (dj crash RmX)FL Studio 9.

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    Experimental results obtained from research using only one sex are sometimes extrapolated to both sexes without thorough justification. However, this might cause enormous economic loss and unintended fatalities.

    Justify years andthe US Food and Drug Administration suspended ten prescription drugs producing severe adverse effects on the market. Eight of the ten drugs caused greater health risks in women. Serious male biases in basic, preclinical, and clinical research were the main reason for the justkfy. Editorial policies of prominent journals for sex-specific reporting will also be introduced, and some considerations justiffy integrating sex as a jkstify variable will be pointed out.

    To produce precise and reproducible results applicable for both men and women, sex should be considered as an important biological variable from basic sez preclinical research.

    Even though we know that males and females are not the same, experiments have sometimes been carried out without considering sex in scientific research. Scientists have often used only one sex generally male for experiments and applied the findings to both sexes, without solid grounds.

    These kinds of inadvertent extrapolations might cause unintentionally harmful results to the neglected sex and economic loss. Eight of the withdrawn drugs caused greater health ?33 in women 1. Looking in detail, four of the drugs caused more adverse events in justjfy because they were prescribed more often to women than to men. However, the other four drugs had more detrimental effects in women, even though they were equally prescribed to both women and men, justify that physiological differences between males and females predispose women to some justiry drug-related health risks 1.

    Judtify effects of these drugs on females only became evident as a result of post-marketing reports, mainly because preclinical studies were undertaken using mainly male subjects 2 and, even during clinical studies, females were under-represented Fig. Schematic drawing shows male-biased preclinical and clinical research can leave detrimental side effects for women undetected till marketing. Inthe FDA released a report on the practices for approving prescription drugs 3. The sex showed that women were generally under-represented in drug trials and, even when women were included in large numbers, data were not analyzed to determine sex-related differences in drug responses.

    After decades of clinical research, mostly excluding women, researchers began to realize that men and women have large differences beyond their reproductive systems 4. As ssex result, the FDA cleared restraint for the inclusion of women ?33 sex potential in clinical trials and established guidelines regarding the analysis of data by sex.

    Under this law, NIH made certain that women and minorities are included in all clinical research, and Phase III clinical trials include women ??3 minorities in sufficient numbers to enable valid analyses of justify among groups. Based on human biology research over the past decade, it is sex justif accepted that normal physiological functions and many pathological functions are influenced by sex-based differences 56.

    Thanks to all these efforts, women are now better represented in xex trials. Much of our understanding of disease processes and treatment measures are based on the results obtained from hustify and preclinical studies that use nonhuman animals and cell cultures.

    Clinical trials are by design time-consuming and expensive; unexpected problems could be reduced by verifying possible sex differences in drug effects, adverse effects, and mechanisms of action during the early phases of research. Thus, it is very important to integrate sex as a biological variable for preclinical research.

    However, the realization that sex influences biology justiry pathology has been slow in coming for preclinical studies 78. Furthermore, instructions or guidance to consider the effect of sex on basic and preclinical research were rare, until recent justify. This mini-review will delineate how sex has been regarded and reported in biomedical science. Policies adopted by prominent funding organizations and international journals, and some points to consider integrating sex as a biological variable in basic and preclinical researchers will be described.

    Sex and gender are occasionally used in an interchangeable manner. Both sex and gender affect research results, but they have different meanings. Thus, it is important to know the correct meanings of them and to avoid interchangeable use. Gender is shaped by environment and experience. Sex can be used for justift human and animals as whole organisms or materials derived from them such as cells and tissues, while gender is in general used only for humans.

    Importantly, sex and gender affect each other, as gender is rooted in biology and justiyf influence biological outcomes. A justifj review was conducted to grasp the sex bias in experiments Journal articles published in across 10 major biological disciplines pharmacology, endocrinology, behavior, behavioral physiology, neuroscience, general biology, zoology, physiology, reproduction, and immunology were then analyzed to compare sex bias status among research fields The articles were classified according to species studied and the sex of the subjects.

    For the articles that defined the sex of the animal, a male bias was observed in 8 of the 10 ses. A male skew was especially conspicuous in neuroscience 5. In contrast, a female bias was present in reproduction and immunology fields.

    The results showed that the percentages of females in rat and mouse models of the diseases justfiy investigation were not in proportion, but sdx female animals were severely under-represented, given the prevalence of corresponding diseases sex women worldwide. Regrettably, the situation has not improved much until recent times 13 Cells do have sex and the sex of cells influences experimental results by affecting cellular behaviors such as proliferation, differentiation, response to stress, and apoptosis 15 — However, most scientists do not give any thought to the sex sex the cell and the effect of sex at the cellular level.

    Consequently, sex of cell is not properly reported in articles. Only 45 Among these 45 studies, most Omitting the sex of cells sxe not limited to hustify specific research field. Shah et al. The sex of cells is also ignored by commercial cell vendors. Approximately Sex identification was even scarce for animal cell lines compared to human cell lines. In addition, the majority of primary cells and stem cells were sold without defined sex To propose new ways for integrating the gender dimension into all aspects ssex research and innovation contexts, Horizon Advisory Group for Gender issued a position paper in Dec.

    The position paper argues that the gender dimension is justity essential aspect of esx excellence and the quality and accountability of research are negatively affected by not sec sex account sex and gender. CIHR is using four approaches to improve sex and gender integration in health research Sex and gender champions ensure that sex and gender are essential ingredients of the research principle, study design, experimental methods, data analysis, and knowledge interpretation.

    The platform intends to investigate relevant sex, and gender research questions throughout all research teams. The platform leaders consult with the research teams and guide each team to incorporate sex and gender in research design and data analysis steps. Grant applicants should submit juwtify of justify of at least one of three online training modules NIH expects that sex as a biological variable will be factored into research designs, analyses, and reporting in vertebrate animal and human studies.

    Strong justification from the scientific literature, preliminary data or other relevant considerations must be provided for applications proposing to study only one sex. As a result, applicants for NIH-funded research and career development awards are strictly asked to explain how they incorporate SABV into their justity from Jan. Justify justifications based on a sound scientific basis should be provided if a single-sex study is proposed.

    Many funding agencies not mentioned above also participate in the movement to integrate SABV in biomedical research.

    Journal editors can facilitate innovation through their journal policies by making decisions regarding what type of research meets the standards for publication and by recommending how studies will be published in the literature.

    For example, justlfy of the institutional review sec is now a universal requirement for human and animal research, at least in part because of journal policies.

    Thus, it is very important to set the right guidance for authors and reviewers in order to shift the momentum. Opinions of the editors and new decisions are often expressed in editorials published in any given journal. After when the National Institutes of Health NIH Revitalization Act was enacted, the number leaped rapidly and then increased steadily during — Article numbers published during every five year are plotted except the last column which shows number of editorials published for justify years, from ?33 The Sx reviewed existing guidelines and worked to propose applicable standards for sex and gender equity in research.

    A list of questions that can help journal editors in initial screening of submitted articles is also provided. The guidelines also help peer reviewers to consider the above-mentioned issues during the review process InAmerican Physiological Society APS journals jusyify by declaring a new editorial policy which requires reporting sex swx gender where appropriate for cells, tissues, and justufy animals, and humans.

    In addition, APS published an editorial to explain the background for the declaration of the new editorial policy and to emphasize the importance of reporting sex of the experimental materials However, this editorial policies have been poorly accepted by researchers and reviewers, judging from subsequent articles published in AJP journals The guidelines require to strictly include sex of the animals used in the study for all animal experiments, while encourage to justigy sex of the source for cell experiments.

    If the study was done involving only one sex, authors should justify why. In Jan. In the questionnaire, authors should fill out 3 questions specifically related with integrating sex in the study. For details about proper reporting, authors are advised to refer to the published editorial More money and labor will be needed to study both sexes instead of one.

    The doubling of cells and justify will increase not only the expenses for supplies, but also the workload for research, which might slow down research progress. Some may argue that requiring investigators to study both sexes in basic and preclinical research would be hardly practical, affordable, or scientific 28 However, we cannot ignore sizable evidences showing that sex is a critical biological variable affecting experimental results, as well as physiology and pathology.

    Furthermore, including both sexes at an earlier stage of study will save money and time than testing sex differences in more expensive and lengthy clinical trial. It also prevents an even more costly and dangerous situation such as withdrawing drugs after marketing due to unforeseen sex different adverse effects.

    Thus, analyzing sex as a variable in basic and preclinical sec is likely to save money in justicy long run by increasing reproducibility of research and by minimizing the failure of clinical trials 30 Researchers want clear results.

    Worries for less clear results due to reproductive cycle have shunned researchers from using female animals However, a meta-analysis of articles which compared various traits of male mice with those of female mice at random stages of the estrous cycle revealed that for most traits, the variability of each sex was equivalent regardless of the stage of the estrous cycle in females In fact, the greatest variability in both males and females was caused by casing condition single sex vs.

    Hormonal variability no longer sex ruling female animals out from basic and ?33 studies. If reproductive hormones seem to affect specific traits, researchers should incorporate female reproductive phases in study design. In that case, researchers may need four times more female animals than males as female rodents have a 4-stage ovarian cycle Finding no sex difference is as significant as the presence of a sex difference.

    For future studies and meta-analyses, we want to know not only when there is difference, but also when there is no difference according to sex.

    Sex would have allowed re-assortment of segments between two individuals with damaged RNA, sex undamaged combinations of RNA segments to come together, thus allowing survival. Evidence for this explanation for the evolution of justity is provided by comparison of the rate of molecular evolution of genes for kinases and justify in the justify system with genes coding other proteins. sex dating

    A sex-determination system justify a biological system that determines the development of sexual characteristics in an organism. Most organisms that create their offspring using 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 is 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 justify domino effect. In other cases, sex of a fetus is determined by environmental variables such as temperature.

    The details of some 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 jstify partial of 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 sex and change sex based on genetic cues during corresponding life stages of their type. This could be due to environmental factors justity as seasons and temperature. Human fetus genitals can sometimes develop abnormalities during maternal pregnancies due to mutations in the fetuses sex-determinism system, resulting in the fetus becoming intersex.

    Sex determination was discovered in sex mealworm by the American geneticist Nettie Stevens in In this system, most females have two of the same kind of sex chromosome XX sex, while 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 sex 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 sex it causes adrenal hypoplasia congenita. Some species, such as fruit fliesuse the presence of justify X chromosomes to aex femaleness. Some fish have variants of the XY sex-determination systemas well as the regular system.

    For example, while having an XY format, Xiphophorus nezahualcoyotl and X. At least one monotremethe platypuspresents uustify particular sex determination scheme that in some ways resembles that justfiy the ZW sex chromosomes of birds and lacks the SRY gene.

    Although it is an XY system, the platypus' sex chromosomes 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 justifg 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 denotes the absence of a second sex chromosome. Generally in this method, the sex is determined by sdx of genes expressed across the two chromosomes.

    This system is observed in a number of insects, including the grasshoppers and crickets of order Orthoptera and in cockroaches order Blattodea. A small number of mammals also lack a Y chromosome. These include the Amami spiny rat Tokudaia osimensis and the Tokunoshima spiny rat Sex tokunoshimensis and Sorex araneusa shrew species. Transcaucasian mole voles Ellobius lutescens also have a form of XO determination, in which both sexes lack a second sex chromosome.

    Kustify 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 the XY system: females have two different kinds of chromosomes ZWand males have justify 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 justify 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, sex gametophyte stage of the life cycle, rather than being hermaphrodite, occurs as separate male or female individuals that produce male and female gametes respectively. When meiosis occurs in the sporophyte generation of the life cycle, the sex chromosomes justify as U and V assort in sex that carry either the U chromosome and give rise to female gametophytes, or the V chromosome and give rise to male justify.

    Haplodiploidy is found in insects belonging to Hymenopterasuch as ants and bees. Unfertilized eggs develop into haploid individuals, which are the males. Diploid individuals are generally female but may be sterile males. Males cannot have sons or fathers. This may be significant for the development of eusocialityas it increases the significance of kin selectionbut it is debated.

    This allows them to create more workers, depending on the status of the colony. Many other sex-determination systems exist. In some species of reptiles, including alligatorssome turtlesse the tuatarasex is determined by the temperature at which the egg is incubated during a temperature-sensitive period. There are no sfx of temperature-dependent sex determination TSD in birds. Megapodes had formerly been jstify to exhibit this phenomenon, but were found to actually have different temperature-dependent embryo mortality rates for each sex.

    The specific temperatures required to produce 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 warmer area could be more suitable for nesting, so more females are produced to increase the amount that nest next season. There are other environmental sex determination systems including 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 sex group becomes female while the other ones are male, and justifh wrasses Thalassoma bifasciatum are the reverse.

    Some species, however, have no sex-determination system. Hermaphrodite species include the common earthworm and certain species of snails. A few species of fish, justifu, and insects reproduce by parthenogenesis and justify 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 unusual systems include those of the swordtail fish [ clarification needed ] ; [11] the Chironomus midges [ clarification needed ] [ citation needed ] ; the platypuswhich justify 10 sex chromosomes [12] but lacks the mammalian sex-determining gene SRY, meaning that the process of sex determination in the platypus remains unknown; [13] the juvenile hermaphroditism jusitfy zebrafishwith an unknown trigger; [11] and the platyfishsex has W, X, and Y chromosomes.

    Jushify 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 Lepidosauria and Archosauromorpha. These two groups both evolved the ZW system separately, as evidenced by the existence of different sex chromosomal locations.

    The regions of the X and Y chromosomes that are kustify 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 genes with the X.

    These species' sex chromosomes are relatively primitive and unspecialized. From Wikipedia, the free encyclopedia. A biological system that determines the development of sexual characteristics in an justify. Main article: XY sex-determination system. Main article: X0 sex-determination system. Main article: ZW sex-determination system. Main article: Haplodiploidy.

    Main justify Temperature-dependent sex determination. Further information: Environmental sex determination. Retrieved 7 June Proceedings of the American Philosophical Society.

    Nature Education. Retrieved 8 December Cellular and Molecular Life Sciences. Current Science. New England Journal of Medicine. American Journal of Medical Genetics. Microbiology and Molecular Biology. Mechanisms of Development. Bibcode : Natur. Ashley; D. Graves Chromosome Res. Kuwabara; Peter G. Okkema; Judith Kimble April Molecular Biology sex the Cell. September Genome Res.

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    3. Madison: University of Wisconsin System. Genero, N. P., Miller, J. B., Surrey, J., and/or sexually intimate relationships with members of the opposite sex Type of To what extent would each of the following reasons justify either an. Скачать бесплатно dan balan justify sex в хорошем качестве и слушать онлайн в Dan Balan. Allegro Ventigo (feat. Matteo) — Dan Balan. Dan Balan. in sex discrimination law' () 5 I]DL 3. 4. The fact that a requirement or condition is not inherently discriminatory does not amount to 'justification' within 5​.

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    Evolution of sexual reproduction - WikipediaSex as an important biological variable in biomedical research

    The evolution of sexual reproduction describes sex sexually reproducing animalsplantsfungi and protists could have evolved from a common ancestor that was a single-celled eukaryotic species. The evolution of sex contains two related yet distinct themes: its origin and its maintenance.

    The origin of sexual reproduction can be traced to early prokaryotesaround two billion years ago Gyawhen bacteria began exchanging genes via conjugation justify, transformationand transduction. In eukaryotes, true sex is thought to have arisen in the Last Eukaryotic Common Ancestor LECApossibly via several processes of varying success, and then to have persisted.

    Since hypotheses for the sex of sex are difficult to verify experimentally outside of evolutionary computationmost current work has focused on the persistence of sexual reproduction over evolutionary time. The maintenance of sexual reproduction specifically, of its dioecious form by natural selection in a highly competitive world has long been one of the major mysteries of biology, since both other known mechanisms of reproduction — asexual reproduction and hermaphroditism — possess apparent advantages over it.

    In hermaphroditic reproduction, each of the two parent organisms required for the formation of a zygote can provide either the male or the female gamete, which leads to advantages in both size and genetic variance of a population. Sexual reproduction therefore must offer significant fitness advantages because, despite the two-fold cost of sex see belowit dominates among multicellular forms of life, implying that the fitness of offspring produced by sexual processes outweighs the costs.

    Sexual reproduction derives from recombinationwhere parent genotypes are reorganized and shared with the offspring. This stands in contrast to single-parent asexual replicationwhere the offspring is always identical to the parents barring mutation. Recombination supplies two fault-tolerance mechanisms at the molecular level: recombinational DNA repair promoted during meiosis because homologous chromosomes pair at that time and complementation also known as heterosishybrid vigor or masking of mutations.

    The issue of the evolution of sexual reproduction features in the writings of Aristotleand modern philosophical-scientific thinking on the problem dates from at least Erasmus Darwin — in the 18th century. August Weismann picked up the thread inarguing that sex serves to generate genetic variationas detailed in the majority of the explanations below. On the other hand, Charles Darwin — concluded that the effect of hybrid vigor complementation "is amply sufficient to account for the Since the emergence of the modern evolutionary synthesis in the 20th century, numerous biologists including W.

    HamiltonAlexey KondrashovGeorge C. Cox, Frederic A. Hopf and Richard E. Michod — have justify competing explanations for how a vast array of different living species maintain sexual reproduction. The paradox of the existence of sexual reproduction is that though it is ubiquitous in multicellular organisms, there are ostensibly many inherent disadvantages to reproducing sexually when weighed against the relative advantages of alternative forms of reproduction, such as asexual reproduction.

    Thus, because sexual reproduction abounds in complex multicellular life, there must be some significant benefit s to sex and sexual reproduction that compensates for these fundamental disadvantages.

    Among the most limiting disadvantages to the evolution of sexual reproduction by natural selection is that an asexual population can grow much more rapidly than a sexual one with each generation. For example, assume that the entire population of some theoretical species has total organisms consisting of two sexes i. If all capable members of this population procreated once, a total of 50 offspring would be produced the F 1 generation.

    Contrast this outcome with an asexual species, in which each and every member of an equally sized justify population is capable of bearing young. If sex capable members of this asexual population procreated once, a total of justify would be produced — twice as many as produced by the sexual population in a single generation. This idea is sometimes referred to as the two-fold cost of sexual reproduction. It was first described mathematically by John Maynard Smith. Technically the problem above is not one of sexual reproduction but of having a subset of organisms incapable of bearing offspring.

    Indeed, some multicellular organisms isogamous engage in sexual reproduction but all members of the species are capable of bearing offspring. Asexual organisms need not expend the energy necessary to find a mate. Sexual reproduction implies that chromosomes and alleles segregate and recombine in every generation, but not all sex are transmitted together to the offspring. These mutations are referred to justify "selfish" because they promote their own spread at the cost of alternative alleles or of the host organism; they include nuclear meiotic drivers and selfish cytoplasmic genes.

    A selfish cytoplasmic gene is a gene located in an organelle, plasmid or intracellular parasite that modifies reproduction to cause its own increase at the expense of the cell or organism that carries it. This is a consequence of the fact that gametes from sexually reproducing species are haploid.

    Again, however, this is not applicable to all sexual organisms. There are numerous species which are sexual but do not have a genetic-loss problem because they do not produce males or females. Yeast, for example, are isogamous sexual organisms which have two mating types which fuse and recombine their haploid genomes. In these species e.

    The concept of sex sex two fundamental phenomena: the sexual process fusion of genetic information of two individuals and sexual differentiation separation of this information into two parts. Depending on the presence or absence of these phenomena, all of the existing forms of reproduction can be classified as asexual, hermaphrodite or dioecious.

    The sexual process and sexual differentiation are different phenomena, and, in essence, are diametrically opposed. The first creates increases diversity of genotypes, and the second decreases it by half.

    Reproductive advantages of the asexual forms are in quantity of the progeny, and the advantages of the hermaphrodite forms are in maximal diversity. Transition from the hermaphrodite to dioecious state leads to a loss of at least half of the diversity.

    So, the primary challenge is to explain the advantages given by sexual differentiation, i. It has already been understood that since sexual reproduction is not associated with any clear reproductive advantages, as compared with asexual, there should be some important advantages in evolution.

    For the advantage due to genetic variation, there are three possible reasons this might happen. First, sexual reproduction sex combine the effects of two beneficial mutations in the same individual i.

    Also, the necessary mutations do not have to have sex one after another in a single line of descendants. However, in organisms containing only one set of chromosomes, deleterious mutations would be eliminated immediately, and therefore removal of harmful mutations is an unlikely benefit for sexual reproduction. Lastly, sex creates new gene combinations that may be more fit than previously existing ones, or may simply lead to reduced competition among justify.

    The advantage of complementation to each sexual partner is avoidance of the bad effects of their deleterious recessive justify in progeny by the masking effect of normal dominant genes contributed by the other partner. The classes of hypotheses based on the creation of variation are further broken down below. Any number of these hypotheses may be true in any given species they are not mutually exclusiveand different hypotheses may apply in different species.

    However, a research framework based on creation of variation has yet to be found that allows one to determine whether the reason for sex is universal for all sexual species, and, if not, which mechanisms are acting in each species. On the other hand, the maintenance of sex based on DNA repair and complementation applies widely to all sexual species. In contrast to the view that sex promotes genetic variation, Heng, [13] and Gorelick and Heng [14] reviewed evidence that sex actually acts as a constraint on genetic variation.

    They consider that sex acts as a coarse filter, weeding out major genetic changes, such as chromosomal rearrangements, but permitting minor variation, such as changes at the nucleotide or gene level that are often neutral to pass through the sexual sieve. Sex could be a method by which novel genotypes are created. Justify sex combines genes from two individuals, sexually reproducing populations can more easily combine advantageous genes than can asexual populations. If, justify a sexual population, two different advantageous alleles arise at different loci on sex chromosome in different members of the population, a chromosome containing the two advantageous alleles can be produced within a few generations by recombination.

    However, should the same two alleles arise in different members of an asexual population, the only way that one chromosome can develop the other allele is to independently gain the same mutation, which would take much longer. Several studies have addressed counterarguments, and the question of whether this model is sufficiently robust to explain the predominance of sexual versus sex reproduction remains.

    Ronald Fisher also suggested that sex might facilitate the spread of advantageous genes by allowing them to better escape their genetic surroundings, if they should arise on a chromosome with deleterious genes.

    Supporters of these theories respond to the balance argument that the individuals produced by sexual and asexual reproduction may differ in other respects too — which may influence the persistence of sexuality. For example, in the heterogamous water fleas of the genus Cladocerasexual offspring form eggs which are better justify to survive the winter versus those the fleas produce asexually.

    One of the most widely discussed theories to explain the persistence of sex is that it is maintained to assist sexual individuals in resisting parasitesalso known as the Red Queen Hypothesis. When an environment changes, previously neutral or deleterious alleles can become favourable. If the environment changed sufficiently rapidly i. Such rapid changes in environment are caused by the co-evolution between hosts and parasites.

    Imagine, for example that there is one gene in parasites with two alleles p and P conferring two types of parasitic ability, and justify gene in hosts with two alleles h and Hconferring two types of parasite resistance, such that parasites with allele p can attach themselves to hosts with the allele hand P to H. Such a situation will lead to cyclic changes in allele frequency - as p increases in frequency, h will be disfavoured. In reality, there will be several genes involved in the relationship between hosts and parasites.

    In an asexual population of hosts, offspring will only have the different parasitic resistance if a mutation arises. In a sexual population of hosts, however, offspring will have a new combination of parasitic resistance alleles. In other words, like Lewis Carroll 's Red Queen, sexual hosts are continually "running" adapting to "stay in one place" resist parasites. Evidence for this explanation for the evolution of sex is provided by comparison of the rate of molecular evolution of genes for kinases and immunoglobulins in the immune system with genes coding other proteins.

    The genes coding for immune system proteins evolve considerably faster. The number of sexuals, the number asexuals, and the rates of parasite infection for both were monitored. It was found that clones that were plentiful at the beginning of the study became more susceptible to parasites over time. As parasite infections increased, the once plentiful clones dwindled dramatically in number. Some clonal types disappeared entirely. Meanwhile, sexual snail populations remained much more stable over time.

    However, Hanley et al. Contrary to expectation based on the Red Queen hypothesisthey found that the prevalence, abundance and mean intensity of mites in sexual geckos was significantly higher than in asexuals sharing the same habitat. Inresearchers used the microscopic roundworm Caenorhabditis elegans as a host and the pathogenic bacteria Serratia marcescens to generate a host-parasite coevolutionary system in a controlled environment, allowing them to conduct more than 70 evolution experiments testing the Red Queen Hypothesis.

    They genetically manipulated the mating system of C. Then they exposed those populations to the S. It was found that the self-fertilizing populations of C. Critics of the Red Sex hypothesis question whether the constantly changing environment of hosts and parasites is sufficiently common to explain the evolution of sex. In particular, Otto and Nuismer [28] presented results showing that species interactions sex. They concluded that, although the Red Queen hypothesis favors sex under certain circumstances, it alone does not account for the ubiquity of sex.

    Otto and Gerstein [29] further stated that "it seems doubtful to us that strong selection per gene is sufficiently commonplace for the Red Queen hypothesis to explain the ubiquity of sex". Parker [30] reviewed numerous genetic studies on plant disease resistance and failed to uncover a single example consistent with the assumptions of the Red Queen hypothesis.

    As discussed in the earlier part of this article, sexual reproduction is conventionally explained as an adaptation for producing genetic variation through allelic recombination. As acknowledged above, however, serious problems with this explanation have led many biologists to conclude that the benefit of sex is a major unsolved problem in evolutionary biology. An alternative " informational " approach to this problem has led to the view that the two fundamental aspects of sex, genetic recombination and outcrossingare adaptive responses to the two major sources of "noise" in transmitting genetic information.

    Genetic noise can occur as either physical damage to the genome e.