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Sex differences in human physiology are distinctions of physiological characteristics associated with either male or female humans. These can be of several types, including direct and indirect. Direct being the direct result of differences prescribed by the Y-chromosome, and indirect being a characteristic influenced indirectly (e.g. hormonally) by the Y-chromosome. Sexual dimorphism is a term for the phenotypic difference between males and females of the same species. Direct sex differences follow a bimodal distribution. Through the process of meiosis and fertilization (with rare exceptions), each individual is created with zero or one Y-chromosome. The complementary result for the X-chromosome follows, either a double or a single X. Therefore, direct sex differences are usually binary in expression (although the deviations in complex biological processes produce a menagerie of exceptions). These include, most conspicuously, male (vs female) gonads. Indirect sex differences are general differences as quantified by empirical data and statistical analysis. Most differing characteristics will conform to a bell-curve (i.e. normal) distribution which can be broadly described by the mean (peak distribution) and standard deviation (indicator of size of range). Often only the mean or mean difference between sexes is given. This may or may not preclude overlap in distributions. For example, most males are taller and stronger than females, but an individual female could be taller and/or stronger than an individual male. The most obvious differences between males and females include all the features related to reproductive role, notably the endocrine (hormonal) systems and their physiological and behavioural effects, including gonadal differentiation, internal and external genital and breast differentiation, and differentiation of muscle mass, height, and hair distribution. ==Sex determination and differentiation== : The human genome consists of two copies of each of 23 chromosomes (a total of 46). One set of 23 comes from the mother and one set comes from the father. Of these 23 pairs of chromosomes, 22 are autosomes, and one is a sex chromosome. There are two kinds of sex chromosomes–"X" and "Y". In humans and in almost all other mammals, females carry two X chromosomes, designated XX, and males carry one X and one Y, designated XY. A human egg contains only one set of chromosomes (23) and is said to be haploid. Sperm also have only one set of 23 chromosomes and are therefore haploid. When an egg and sperm fuse at fertilization, the two sets of chromosomes come together to form a unique "diploid" individual with 46 chromosomes. The sex chromosome in a human egg is always an X chromosome, since a female only has X sex chromosomes. In sperm, about half the sperm have an X chromosome and half have a Y chromosome. If an egg fuses with a sperm with a Y chromosome, the resulting individual is male. If an egg fuses with a sperm with an X chromosome, the resulting individual is female. There are rare exceptions to this rule in which, for example, XX individuals develop as males or XY individuals develop as females. Chromosomes are not the final determinant of sex. In some cases, for example, chromosomally female babies that have been exposed to high levels of androgens before birth can develop masculinized genitals by the time they are born.〔Birke, Lydia. The Gender and Science Reader ed. Muriel Lederman and Ingrid Bartsch. New York, Routledge, 2001. 310-311.〕 There are other variations of sex chromosomes that lead to a variety of different physical expressions.〔Fausto-Sterling, Anne "Of Gender and Genitals" from Sexing the body: gender politics and the construction of sexuality New York, NY: Basic Books, 2000, (3, pp. 44-77 )〕 The X-chromosome carries a larger number of genes in comparison to the Y-chromosome. In humans, X-chromosome inactivation enables males and females to have equal expression of the genes on the X-chromosome since females have two X-chromosomes while males have a single X and a Y chromosome. X-chromosome inactivation is random in the somatic cells of the body as either the maternal or paternal X-chromosome can become inactivated in each cell. Thusly, females are genetic mosaics.〔Carrel L and Willard HF. (2005). X-inactivation profile reveals extensive variability in X-linked gene expression in females " ''Nature'' 434; 400-404〕 This process is seen in all mammals and is also referred to as ''lyonisation'' —after the geneticist Mary F. Lyon who described the process in 1962. In the somatic cells of a developing female child, one of the X-chromosomes is shortened and condensed. The genes on this chromosome therefore can not be transcribed into an mRNA transcript and remain unread. These condensed structures can be seen as dark bodies under the microscope and are commonly referred to as Barr bodies. In individuals with Klinefelter's syndrome (females: XXX, males: XXY) the extra X-chromosome is inactivated, resulting in two bar bodies.〔Bird A. (2007). Perceptions of epigenetics" ''Nature'' 447; 396-8〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Sex differences in human physiology」の詳細全文を読む スポンサード リンク
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