Twins are siblings carried together in the womb and born at the same time. Similarities and differences between twins can be used to answer questions about the role genes and the environment play in the development of traits such as personality, intelligence, and susceptibility to disease. Twins are classified as either dizygotic or monozygotic. Dizygotic twins also called fraternal twins arise from two separately fertilized eggs, or zygotes. In humans, usually only one egg is released at a time from a woman's ovaries.
You're not going to tell me what your kids twinns Gay brothers who showed this maternal pedigree were then tested for Anatomy twins through the ages youth chromosome linkage, Girl wakes up and masturbates twenty-two markers on the X chromosome to test for similar alleles. According to one version of the Egyptian creation myth, the earth god Geb and the. Such twins have the same genes but, presumably, different environments. Pearson Education. Women who are carrying twins or multiple babies should double that amount in order to meet the needs of their developing babies. Studies of twins suggest that there are likely genetic causes of transsexuality, although the precise genes involved are not fully understood. The Sharpes believe Kadyn and Zyler will know their gender preferences by the time they reach elementary Anagomy. Content on this website is twijs for information purposes only. Among these are sense of humor, which appears to be largely environmentally determined, as MZ and DZ pairs have similar concordance.
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Learn about pregnancy with twins or multiple babies, including diagnosis and effects during pregnancy. Care during pregnancy is also discussed. There are two types of twins: identical twins and fraternal, or non-identical, twins. The majority of twins are fraternal, and they result from the fertilization of two different egg cells by two different sperm cells. Identical twins are formed from one single fertilized egg cell that subsequently divides into two identical cells and forms two identical babies.
Either of these processes can lead to triplets, quadruplets, or other multiple babies. About one of every births is a set of identical twins. This rate is basically the same throughout the world and is largely independent of race, age, and heredity. Reproductive technology has led to an increased incidence of twins. The frequency of twins and multiple babies varies between races and ethnic groups.
In white women, the rate is one of every births; in black women, one of every 80 births. The rate increases dramatically in some parts of Africa and is less common in Asia. The likelihood of having twins runs in the family. If the pregnant mother has a family history of twins, they have a one-in chance of having twins themselves.
If her partner has a history of twins in the family, the chances are one in Infertility therapy and assisted reproduction have a major impact on the chances of having twins or multiple babies. During infertility therapy, ovulation is stimulated with hormones. These hormones are known to increase the incidence of twins. Assisted reproduction involves inducing ovulation, collecting all the egg cells, fertilizing them, and implanting a number of fertilized egg cells in the uterus.
Some of these might miscarry, and others will survive and thrive. A woman who is carrying twins or multiple babies will have a larger uterus than the average pregnant woman. However, a larger uterus could indicate other complications such as a pregnancy that is farther along than originally thought, too much amniotic fluid in the uterus, a large baby late in pregnancy, or other concerns. Therefore, the suspicion of twins or multiples in pregnancy should be properly diagnosed.
Ultrasound has become a useful tool in the diagnosis of twins and multiple babies. Very early in pregnancy, the separate embryo sacs can be seen. Soon after, the bodies of each baby can be seen. There are other techniques used to diagnose twins and multiple babies. Late in the first trimester, it may be possible for the doctor to hear the different heartbeats using Doppler ultrasound.
Twins and multiple babies have a number of effects on the woman during pregnancy. This blood volume is increased even further in women carrying twins or multiple babies, and there is a higher chance of anemia.
Also, the heart needs to work that much harder to pump the higher volume of blood, leading to an increased heart rate. Women who are carrying twins and multiple babies need to take extra folic acid and iron because of the increased blood volume and chance of anemia. The uterus can become very large, and excessive amounts of amniotic fluid may develop in the uterus. The size and weight of the uterus can cause some women carrying twins and multiple babies to lead a very sedentary existence.
Too much amniotic fluid can also cause the pregnant woman to develop problems with her kidneys. In severe cases, some amniotic fluid can be removed by amniocentesis if required. Also, there may be differences in the birth weights of each twin or multiple baby. About half of all twins deliver at 36 weeks or less. Half of triplets deliver before 32 weeks. Premature birth is the major cause of newborn death and illness in twins.
Many mothers of twins and multiple babies go into premature labour spontaneously. Others may need to have premature labour induced because of fetal growth restriction, hypertension in the mother, or a condition called placental abruption where the placenta comes away from the walls of the uterus. In a pregnancy with twins or multiple babies, the goals of care are to prevent very premature delivery, identify and deliver any babies that are failing to thrive in the womb, eliminate trauma to the babies during labour and delivery, and provide expert care for the newborn babies.
Pregnant women are advised to take in an extra kcal kilocalories per day compared with women who are not pregnant. Note that kcal are often referred to as "calories" on food product labels.
Women who are carrying twins or multiple babies should double that amount in order to meet the needs of their developing babies. They should receive iron supplements in a dose of 60 mg to mg per day, and folic acid 1 mg per day.
Special measures may need to be taken if a woman develops hypertension during pregnancy. Fetal growth is slower in multiple pregnancies, and the growth of one twin may be faster than the other.
Because of this, a number of ultrasounds may be done throughout the third trimester to make sure that the babies are growing properly. The volume of amniotic fluid in the womb may also be checked regularly. An ultrasound may also be used to evaluate how well the babies are doing.
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Twins and multiple babies
The relationship between biology and sexual orientation is a subject of research. While scientists do not know what determines an individual's sexual orientation , they theorize that it is caused by a complex interplay of genetic , hormonal , and environmental influences.
Biological theories for explaining the causes of sexual orientation are favored by scientists. A number of twin studies have attempted to compare the relative importance of genetics and environment in the determination of sexual orientation. Self reported zygosity , sexual attraction, fantasy and behaviours were assessed by questionnaire and zygosity was serologically checked when in doubt. Other researchers support biological causes for both men and women's sexual orientation.
Women showed a statistically non-significant trend to weaker influence of hereditary effects, while men showed no effect of shared environmental effects. Biometric modeling revealed that, in men, genetic effects explained. Corresponding estimates among women were. Nonetheless, it is possible to conclude that, given the difference in sexuality in so many sets of identical twins, sexual orientation cannot be attributed solely to genetic factors.
Another issue is the finding that even monozygotic twins can be different and there is a mechanism which might account for monozygotic twins being discordant for homosexuality. Monoamniotic twins share a hormonal environment, but can suffer from the 'twin to twin transfusion syndrome' in which one twin is "relatively stuffed with blood and the other exsanguinated".
Sanders et al. Chromosome linkage studies of sexual orientation have indicated the presence of multiple contributing genetic factors throughout the genome.
In Dean Hamer and colleagues published findings from a linkage analysis of a sample of 76 gay brothers and their families. Gay brothers who showed this maternal pedigree were then tested for X chromosome linkage, using twenty-two markers on the X chromosome to test for similar alleles.
This was popularly dubbed the " gay gene " in the media, causing significant controversy. A later analysis by Hu et al. Mustanski et al. In the full sample they did not find linkage to Xq Significant linkage was also detected in the pericentromeric region of chromosome 8, overlapping with one of the regions detected in the Hamer lab's previous genomewide study. The authors concluded that "our findings, taken in context with previous work, suggest that genetic variation in each of these regions contributes to development of the important psychological trait of male sexual orientation".
Female sexual orientation does not seem to be linked to Xq28,   though it does appear moderately heritable. In addition to sex chromosomal contribution, a potential autosomal genetic contribution to the development of homosexual orientation has also been suggested.
They also found that "unusually high" proportions of homosexual males and homosexual females were Rh negative in comparison to heterosexuals. As both blood type and Rh factor are genetically inherited traits controlled by alleles located on chromosome 9 and chromosome 1 respectively, the study indicates a potential link between genes on autosomes and homosexuality.
The biology of sexual orientation has been studied in detail in several animal model systems. In the common fruit fly Drosophila melanogaster , the complete pathway of sexual differentiation of the brain and the behaviors it controls is well established in both males and females, providing a concise model of biologically controlled courtship.
Without the gene, the mice exhibited masculine sexual behavior and attraction toward urine of other female mice. Those mice who retained the gene fucose mutarotase FucM were attracted to male mice. In interviews to the press, researchers have pointed that the evidence of genetic influences should not be equated with genetic determinism. According to Dean Hamer and Michael Bailey, genetic aspects are only one of the multiple causes of homosexuality.
In , Scientific Reports published an article with a genome wide association study on male sexual orientation. The research consisted of 1, homosexual men and 1, heterosexual men. LeVay's research suggested that the hypothalamus of gay men is different from straight men. The researchers found that the thyroid stimulating hormone receptor TSHR on chromosome 14 shows sequence differences between gay and straight men.
It had been suggested that the overactive TSHR hormone lowered body weight in gay people, though this remains unproven. In , Ganna et al. The data in the study was meta-analyzed and obtained from the UK Biobank study and 23andMe. The variants on chromosomes 11 and 15 were specific to men, with the variant on chromosome 11 located in an olfactory gene and the variant on chromosome 15 having previously been linked to male-pattern baldness.
The four variants were also correlated with mood and mental health disorders; major depressive disorder and schizophrenia in men and women, and bipolar disorder in women. However, none of the four variants could reliably predict sexual orientation. In August , a genome-wide association study of , individuals concluded that hundreds or thousands of genetic variants underlie homosexual behavior in both sexes, with 5 variants in particular being significantly associated.
Some of these variants had sex-specific effects, and two of these variants suggested links to biological pathways that involve sex hormone regulation and olfaction.
These genes partly overlap with those for several other traits, including openness to experience and risk-taking behavior. Additional analyses suggested that sexual behavior, attraction, identity, and fantasies are influenced by a similar set of genetic variants. They also found that the genetic effects that differentiate heterosexual from homosexual behavior are not the same as those that differ among nonheterosexuals with lower versus higher proportions of same-sex partners, which suggests that there is no single continuum from heterosexual to homosexual preference, as suggested by the Kinsey scale.
A study suggests linkage between a mother's genetic make-up and homosexuality of her sons. Women have two X chromosomes, one of which is "switched off". The inactivation of the X chromosome occurs randomly throughout the embryo, resulting in cells that are mosaic with respect to which chromosome is active.
In some cases though, it appears that this switching off can occur in a non-random fashion. Bocklandt et al. This maternal immunization hypothesis MIH begins when cells from a male fetus enter the mother's circulation during pregnancy or while giving birth. These Y-linked proteins would not be recognized in the mother's immune system because she is female, causing her to develop antibodies which would travel through the placental barrier into the fetal compartment.
Successive male fetuses are then attacked by H-Y antibodies which somehow decrease the ability of H-Y antigens to perform their usual function in brain masculinization. However, the maternal immune hypothesis has been criticized because the prevalence of the type of immune attack proposed is rare compared with the prevalence of homosexuality. In , researchers discovered a biological mechanism of gay people who tend to have older brothers.
They think Neuroligin 4 Y-linked protein is responsible for a later son being gay. The result also indicates that number of pregnancies, mothers of gay sons, particularly those with older brothers, had significantly higher anti-NLGN4Y levels than did the control samples of women, including mothers of heterosexual sons.
In , Italian researchers conducted a study of about 4, people who were the relatives of 98 homosexual and heterosexual men. The researchers concluded that there was genetic material being passed down on the X chromosome which both promotes fertility in the mother and homosexuality in her male offspring. Research conducted in Sweden  has suggested that gay and straight men respond differently to two odors that are believed to be involved in sexual arousal.
The research showed that when both heterosexual women and gay men are exposed to a testosterone derivative found in men's sweat, a region in the hypothalamus is activated. Heterosexual men, on the other hand, have a similar response to an estrogen-like compound found in women's urine. Researchers have suggested that this possibility could be further explored by studying young subjects to see if similar responses in the hypothalamus are found and then correlating these data with adult sexual orientation.
A number of sections of the brain have been reported to be sexually dimorphic; that is, they vary between men and women.
There have also been reports of variations in brain structure corresponding to sexual orientation. In , Dick Swaab and Michel A. Hofman reported a difference in the size of the suprachiasmatic nucleus between homosexual and heterosexual men.
Research on the physiologic differences between male and female brains are based on the idea that people have male or a female brain, and this mirrors the behavioral differences between the two sexes. Some researchers state that solid scientific support for this is lacking. Although consistent differences have been identified, including the size of the brain and of specific brain regions, male and female brains are very similar.
Simon LeVay , too, conducted some of these early researches. This was a relevant area of the brain to study, because of evidence that it played a role in the regulation of sexual behaviour in animals , and because INAH2 and INAH3 had previously been reported to differ in size between men and women. He obtained brains from 41 deceased hospital patients.
The subjects were classified into three groups. The first group comprised 19 gay men who had died of AIDS -related illnesses. The second group comprised 16 men whose sexual orientation was unknown, but whom the researchers presumed to be heterosexual.
Six of these men had died of AIDS-related illnesses. The third group was of six women whom the researchers presumed to be heterosexual. One of the women had died of an AIDS-related illness. The HIV-positive people in the presumably heterosexual patient groups were all identified from medical records as either intravenous drug abusers or recipients of blood transfusions.
Two of the men who identified as heterosexual specifically denied ever engaging in a homosexual sex act. The records of the remaining heterosexual subjects contained no information about their sexual orientation; they were assumed to have been primarily or exclusively heterosexual "on the basis of the numerical preponderance of heterosexual men in the population".
However, the INAH3 group appeared to be twice as big in the heterosexual male group as in the gay male group; the difference was highly significant, and remained significant when only the six AIDS patients were included in the heterosexual group. However, other studies have shown that the sexually dimorphic nucleus of the preoptic area, which include the INAH3, are of similar size in homosexual males who died of AIDS to heterosexual males, and therefore larger than female.
This clearly contradicts the hypothesis that homosexual males have a female hypothalamus. These areas of the hypothalamus have not yet been explored in homosexual females nor bisexual males nor females. William Byne and colleagues attempted to identify the size differences reported in INAH 1—4 by replicating the experiment using brain sample from other subjects: 14 HIV-positive homosexual males, 34 presumed heterosexual males 10 HIV-positive , and 34 presumed heterosexual females 9 HIV-positive.
The researchers found a significant difference in INAH3 size between heterosexual men and heterosexual women. The INAH3 size of the homosexual men was apparently smaller than that of the heterosexual men, and larger than that of the heterosexual women, though neither difference quite reached statistical significance.
The neuron count also found a male-female difference in INAH3, but found no trend related to sexual orientation. A study, Garcia-Falgueras and Swaab asserted that "the fetal brain develops during the intrauterine period in the male direction through a direct action of testosterone on the developing nerve cells, or in the female direction through the absence of this hormone surge.
In this way, our gender identity the conviction of belonging to the male or female gender and sexual orientation are programmed or organized into our brain structures when we are still in the womb. There is no indication that social environment after birth has an effect on gender identity or sexual orientation. In many species, a prominent feature of sexual differentiation is the presence of a sexually dimorphic nucleus SDN in the preoptic hypothalamus, which is larger in males than in females.
Roselli et al. Neurons of the oSDN show aromatase expression which is also smaller in male-oriented rams versus female-oriented rams, suggesting that sexual orientation is neurologically hard-wired and may be influenced by hormones. Although the exact function of the oSDN is not fully known, its volume, length, and cell number seem to correlate with sexual orientation, and a dimorphism in its volume and of cells could bias the processing cues involved in partner selection.