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Transcript
Sex determination Lec/6 Sex Determination in Man • In man XX-XY type sex determining mechanism occurs but here the Y chromosome contains potent male sex-determining genes which can almost completely overcome the feminizing action of the rest of the genotype. The conclusive evidences that Y chromosome is a determiner of fertility and sex of male individual came from certain abnormal conditions (called syndromes) which contained aneuploidic sex-chromosomal abnormalities. • For instance, Turner’s syndromes (XO) are sterile female individuals. Similarly, Klinefelter’s syndromes (XX Y) are males, despite the presence of two X chromosomes. A person with extra one X and Y chromosome display true hermaphroditism having both ovarian and testicular tissues and variable degrees of intersexual development of the genitalia. Sex differentiation • 1. Genetic Sex • Normal females ordinarily have two X chromosomes; normal males have one X and one Y. The genes on these sex chromosomes determine femaleness or maleness. • Further, since the X-chromosome carries much more genetic information in striking contrast to Y chromosome, one might wonder how it is that the female can carry a double dose of many vital X-linked genes, whereas the male has only a single dose of these X-linked genes. Such inequality in fact cannot be tolerated and so female seem to have developed their owntypes of dosage compensation mechanisms. Dosage Compensation of Genes • (i) X-chromosome inactivation in mammals. • It has been demonstrated that in homogametic XX female individuals, one X chromosome gets characteristically condensed and inactivated. • Since it becomes inactive in certain part of the life cycle and resumes activity before entering the germ line • The phenomenon of inactivation of X chromosome was confirmed by the observation of the Barr body. It has also been observed in most of the body cells (e.g., skin, oral epithelium and blood cells) of man . • The sex chromatin appears in the interphase nucleus as a small chromocentre, heavily stained • with basic dyes.It can be found in four position: (i) attached to the nucleus as in nerve cells of certain species; (ii)attached to the nuclear membrane as in cells of epidermis or of the oral mucosa; (iii)free in the nucleoplasm as in neurons after electric stimulation, (iv)as a nuclear expansion. • The best known example of nuclear expansion is that of the neutrophil leukocyte of female in which the sex chromatin (Barr body) appears as a small rod called the drumstick. • Lyon’s hypothesis. • The number of X chromosomes was two or more than two, the number of Barr bodies was one less than the number of X chromosomes (nX-1; i.e., one Barr body in XX females and XXY males; two Barr bodies in XXXY males and XXX metafemales). Thus, in normal female only one active X chromosome is present. Which of the two X chromosomes remains active in female individuals, is determined at the early stages of development. • In other words, the inactivation of X chromosome is a random phenomenon. This fact has been demonstrated in human diseases linked to X chromosome. The Lesch-Nyhan syndrome, in which a deficiency of one enzyme of the purine metabolism (i.e., hypoxanthineguanine phosphoribosyl transferase) produces mental retardation and • increased uric acid levels results, from a recessive mutation in the X chromosome. • if fibroblasts of these patients are cultured in vitro, two types of cell clones are obtained. Half the clones contain the enzyme, whereas the other half (in which the X carrying the normal gene is condensed) lack the enzyme. • In the human embryo X chromosome inactivation starts in the late blastocyst about the 16th day of life(100 cell). Once the inactivation is established, it is irreversibly maintained in somatic cells, however, in germ cell line reactivation occurs at a specific stage of germ cell development . • The good illustration of X chromosome inactivation is seen with calico cats, where • the coat is a mosaic patchwork of black and yellow hair. Black hair is produced by the dominant allele B, and yellow by its recessive allele b. This gene is Xlinked, so if one X chromosome contains the dominant allele B and other X chromosome the recessive allele b, random inactivation will allow both coat colours to be expressed. Male calico cats are understandably rare, since it has only one X chromosome. 2. Gonadal Sex • In the human embryos until the six weeks the gonads and primordia of the urinogenital tract are identical in males and females. At this stage (time) the gonad has already been invaded by the primary XX or XY cells. At this point, a gene or set of genes, called testis determining factor or TDF, present in the Y chromosome causes the undifferentiated gonad to differentiate into a testis and the absence of this gene allows the gonad to become an ovary • . The development into a testis starts as soon as the gonocytes (i.e., primordial germ cells) from the yolk sac have finished their migration into the gonadal ridge. Gonocytes of the male (XY) migrate deeper into the gonadal blastema forming the medulla and female gonocytes (XX) remain at the periphery, forming a thick cortical layer. Hence, the XX genetic sex is ordinarily associated with ovarian gonadal sex, and XY is associated with testicular gonadal sex. Sex determination • In human beings, the presence of Y chromosome determines maleness and its absence determines femaleness. So, males are XY and females are XX in human beings. However, in 1986, certain peculiar cases have been reported which were found to be males with XX chromosomes and females with XY chromosomes. These can be due to any one of the following two reasons : (i) A sex reversal gene SRY located on the Y chromosome leads to XX males and XY females. (ii) Translocation of a small segment of the Y chromosome to an X chromosome in XX males and its deletion from the Y chromosome results in the XY females. • Thanks