Mind the X: Sex Chromosomes and Sex Differences
Men and women often have various differences in disease vulnerability, behavior, and other health related predispositions. However, scientists are now realizing that this disparity may be explained not by the differences between hormonal production but by the sex of the individual cells within the body. As a result, several scientists are now encouraging other researchers to conduct their research on the basis of studying sex differences, a field of study that could potentially provide implications regarding issues such as disease vulnerability and other health related issues.
The “sex” of the individual cell in the human body is determined by the set of sex chromosomes within the cell. Sex chromosomes are bundles of DNAs within the human cell that determines the biological sex of humans. In humans, women normally have two copies of the X chromosome (XX), while males have one copy of X and one copy of Y (XY). These sex chromosomes were thought solely to determine the development of the human reproductive systems, testes or ovaries. Furthermore, the differences between male and female, including the differences in disease susceptibility, would be the results of varying amounts of sex hormones produced by the two different reproductive systems. However, according to recent research, researchers, led by Art Arnold of the University of California, Los Angeles, may now be doubtful of the simplicity of this existing model.
The “sex” of the individual cell in the human body is determined by the set of sex chromosomes within the cell. Sex chromosomes are bundles of DNAs within the human cell that determines the biological sex of humans. In humans, women normally have two copies of the X chromosome (XX), while males have one copy of X and one copy of Y (XY). These sex chromosomes were thought solely to determine the development of the human reproductive systems, testes or ovaries. Furthermore, the differences between male and female, including the differences in disease susceptibility, would be the results of varying amounts of sex hormones produced by the two different reproductive systems. However, according to recent research, researchers, led by Art Arnold of the University of California, Los Angeles, may now be doubtful of the simplicity of this existing model.
Scientists found that the sex chromosomes have the ability to affect differences between each sex on their own; the number of X chromosomes in the cell can affect the cell's behavior throughout life.
Image Source: PublicDomainPictures
As part of the effort in investigating sex differences determined by chromosomal sex, the scientists in the Arnold lab compared mice with the same sex chromosome pairs but different gonads to determine the hormonal effect. Like humans, mice also use X and Y chromosomes for sex determination. In the experiments, scientists were able to manipulate the genes to produce genetically female (XX) mice with testes, and genetically male (XY) mice with ovaries. In addition, the team also wanted to determine whether the sex chromosome complements (XX or XY) themselves caused differences in sex by removing the gonads of the mice.
The mechanism in which the chromosome can affect differences between sex can be explained by the production of proteins. Like other chromosomes, the sex chromosome can be transcribed to make essential proteins that the body uses to function. The males, having one copy of the X chromosome, would receive a single “dose” of the X chromosome. While the female, in order to avoid having an extra “dose” than males do, shuts down one of the copies of the X chromosomes, a process known as “X-inactivation”. The scientists originally thought that this inactivation is a complete shutdown of the other copy of the X chromosome. However, this became puzzling as the scientists began to observe Klinefelter syndrome males who have XXY. If one copy of the X were inactivated in each of the cell, the XXY male would have been indistinguishable from the normal, healthy male. However, males with XXY often displayed cognitive disabilities, which suggests that the inactivation may not be a complete shutdown of the X chromosome.
The incomplete inactivation of the X-chromosome, which gives the person a higher “dose” of X than the scientists have previously thought, may be one of the reasons why women exhibit several differences compared to men. However, this field of study has been largely ignored. As a result, researchers are currently being encouraged to include sex differences as one of the parameters in future research investigations. Further research can be conducted to investigate the genes with the incomplete X inactivation process and its implications to other health related topics.
The mechanism in which the chromosome can affect differences between sex can be explained by the production of proteins. Like other chromosomes, the sex chromosome can be transcribed to make essential proteins that the body uses to function. The males, having one copy of the X chromosome, would receive a single “dose” of the X chromosome. While the female, in order to avoid having an extra “dose” than males do, shuts down one of the copies of the X chromosomes, a process known as “X-inactivation”. The scientists originally thought that this inactivation is a complete shutdown of the other copy of the X chromosome. However, this became puzzling as the scientists began to observe Klinefelter syndrome males who have XXY. If one copy of the X were inactivated in each of the cell, the XXY male would have been indistinguishable from the normal, healthy male. However, males with XXY often displayed cognitive disabilities, which suggests that the inactivation may not be a complete shutdown of the X chromosome.
The incomplete inactivation of the X-chromosome, which gives the person a higher “dose” of X than the scientists have previously thought, may be one of the reasons why women exhibit several differences compared to men. However, this field of study has been largely ignored. As a result, researchers are currently being encouraged to include sex differences as one of the parameters in future research investigations. Further research can be conducted to investigate the genes with the incomplete X inactivation process and its implications to other health related topics.
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