The formation of sex depends on the set in the fertilized ovum (zygote) of sex chromosomes. In each gamete (egg cell and sperm) there are 22 somatic chromosomes (auto-soma) and one sex chromosome. X chromosome is found in egg cell, and the Y-chromosome – in the sperm. The zygote contains 46 chromosomes live (22 pairs of autosomes and 2 sex chromosomes).
The content in the zygote of the twentieth chromosomes determines the development of the female organism, and XY-chromosomes – male. With further division cells containing the twentieth chromosome, inactivation occurs one of them. Inactivated X chromosome is located on the periphery of the cell nucleus and is called sex chromatin (little body B and r r a). Sexual chromatin is detected (positive)
only if there are at least two X chromosomes in the karyotype.
From this it follows that sex chromatin can be a positive only in women. In the study of 100 nuclei of cells in health out women can be found from 20 to 8 0% of cells containing sex chromatin, and in healthy men – only 0-5% of these cells.
Sex chromatin allows to detect discrepancy between the genetic floor (genotype) of the individual and its external the type (phenotype) of what happens during sexual abnormalities
Different abnormalities of sexual development may occur or due to impaired chromosome division during meiosis (formation of germ cells), or in the early stages of mitotic zygote. Chromosomal abnormalities occur in changing the shape of chromosomes, reducing or increasing them numbers Thus, with the karyotype 45X, Turner’s syndrome develops in Turner, with the karyotype 47XXY, Klinefel syndrome tera, etc. In some cases, violations of sexual differentiation are caused by damage to the genome composition of the genital chromosomes. The shape and number of sex chromosomes in the karyo ne may be normal.
Sometimes the cause of various genital anomalies is a mosaic.
icism It results from uneven separation.
(or loss) of sex chromosomes during the first and subsequent
zygote divisions. As a result, one part of the cells has one
a set of sex chromosomes, and the other – a different (mosaicism).
In women, mosaicism can be represented as
45X / 47XXX, and for men – 45X / 47XXY. With the loss of the X chromoso-
We mosaicism in women will be 45X / 46XX, and in men –
45X / 46XY.