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Chapter 5 Sexual Differentiation
Figure 5-1 Human chromosomes (top), which have been arranged in homologous pairs (bottom).These are from a cell of one of the authors’ son (Evan Pryor Jones), which was obtained while he wasstill a fetus by amniocentesis (see Chapter 10). Note the X and Y chromosomes at lower right.
Figure 5-2 Possible variations in number of sex chromosomes in males and females and the resultant number of sex chromatins, or Barr bodies. Note that the number of Barrbodies is one less than the number of X chromosomes
Figure 5-3 Location of genes involved in gonadal sex differentiation. The sex-determining region of the Y (SRY) gene codes for the production of testis-determining factor (TDF), which in turn causes testis differentiation. Absence of this gene in an individual lacking the Y chromosome results in the formation of ovaries. The DAX-1 gene on the X chromosome suppresses SRY gene expression, but the normal interaction of DAX-1 and SRY has not been fully discovered.
Figure 5-4 Inheritance of a sex-linked trait (red–green color blindness). A normal visionfather (XCY) and a normal vision, carrier mother (XCXc) will have normal vision daughters,half of whom will be carriers. Half of their sons will be color blind. Xc represents the colorblind allele carried on the X chromosome; XC represents the normal allele.
Figure 5-5 Diagram illustrating differentiation of indifferent gonads into testes or ovaries: (a) Indifferent gonads from a 6-week-old embryo; (b) at 7 weeks, showing testes developing under the influence of TDF; (c) at 12 weeks, showing ovaries developing in the absence of TDF; (d) testis at 20 weeks, showing the rete testis and seminiferous tubules derived from medullary cords; (e) ovary at 20 weeks, showing primordial follicles; (f) section of a seminiferous tubule from a 20-week fetus; and (g) section from the ovarian cortex of a 20-week fetus, showing two primordial follicles
Figure 5-6 Differentiation of sex accessory structures in male and female human embryos.
Figure 5-7 Differentiation of the indifferent stage into male and female external genitalia.
Figure 5-9 A person with testicular feminization syndrome, a form of male pseudohermaphroditism. (Photograph courtesy of Dr. Howard W. Jones.)
Figure 5-10 External genitalia of female pseudohermaphrodites with adrenogenital syndrome: (a) A newborn female exhibiting enlargement of the clitoris and fusion of the labia majora; (b) a female infant showing considerable enlargement of the clitoris; and (c and d) a 6-year-old girl showing an enlarged clitoris and fusion of the labia majora to form a scrotum. (d) Note the clitoral glans and the opening of the urogenitalsinus (arrow).
Figure 5-11 Nondisjunction is the failure of a homologous pair of chromosomes to separate during meiosis. (a) Nondisjunction of sex chromosomes during sperm formation. The fertilized ova will have two X chromosomes and one Y chromosome (Klinefelter’s syndrome) or only an X chromosome (Turner’s syndrome). (b) Nondisjunction of sex chromosomes during meiosis in the ovary results in ova with either two X chromosomes or none. The fertilized ova therefore can exhibit one of four chromosomal aberrations,three of which is viable
Figure 5-12 Illustrations of various types of congenital uterine abnormalities: (a) doubleuterus and double vagina, (b) double uterus with single vagina, (c) branched (bicornuate)uterus, (d) bicornuate uterus with a small left branch, (e) septate uterus (divided in the middle), and (f) uterus formed from müllerian duct on one side only
