* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Depat.Anato Genetic/lec 5 Dr.sarab H. 2015 Sex Determination in Man
Survey
Document related concepts
Polymorphism (biology) wikipedia , lookup
Dominance (genetics) wikipedia , lookup
Sexual dimorphism wikipedia , lookup
Artificial gene synthesis wikipedia , lookup
Gene expression programming wikipedia , lookup
Genomic imprinting wikipedia , lookup
Designer baby wikipedia , lookup
Polycomb Group Proteins and Cancer wikipedia , lookup
Microevolution wikipedia , lookup
Epigenetics of human development wikipedia , lookup
Genome (book) wikipedia , lookup
Skewed X-inactivation wikipedia , lookup
Y chromosome wikipedia , lookup
Transcript
Depat.Anato Genetic/lec 5 Dr.sarab H. 2015 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 In human beings sex differentiation occurs in the following steps : 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 Dosage compensation of the genes is done either by hypoproduction due to inactivation of one X chromosome in homogametic female sex. (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. 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. 1 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. This is shown as follows : 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. 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 X-linked, 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 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 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. 3