The Y chromosome does not carry other genes than those of sex determination in mammals, while X chromosome carries other genes that have no relation to sex or sexual development.

Abnormal cases of sex determination in Humans:

Sometimes, during meiosis and gamete formation the sex chromosomes are not equally distributed. This leads to abnormal cases. In some rare cases, the two sex chromosomes adhere closely to each other during meiosis and do not separate. This leads to the presence of the two X chromosomes in one ovum and the absence of the X chromosome from the other.

· If the abnormal ovum having two X chromosomes was fertilized with a sperm having Y chromosome, the chromosomal structure of the zygote will be (44+XXY). This zygote develops into abnormal male having Klinefelter's syndrome where the genital organs are masculine but very small, his muscles are feminine and his breast may grow slightly. This individual is always sterile due to the absence of sperm generating cells in his testes. The extra X chromosome leads to a disturbance in the balance of the sex determining genes. Also some feminine genes manifest themselves to a certain extent.

· If an ovum lacking the X chromosome is fertilized with a sperm having X chromosome, the individual will have only one X chromosome. The chromosomal structure will be (44+X) and it is known as Turner's syndrome. This baby will be a female but mentally retarded. In this case, genes carried on only one sex chromosome determine the sex, the amount of hormones present is not sufficient for sexual maturity.

Barr body test:

It is possible to determine the number of X chromosomes of any person by scratching some of his mucous membrane lining his cheek. The nuclei of the cells of this tissue are stained by special stain during the interphase of mitosis. By examining this preparation under the microscope, an X chromosome appears in each cell in a loose form.

- In males, there is only one X chromosome, so, it is found in the loose form.

- In females, there are two X chromosomes. One of them is in the loose form, while the other is more compact and takes up the stain. This stained chromosome in the interphase of mitosis is called Barr body.

Thus, the presence of one Barr body indicates the presence of two X chromosomes, and the presence of two Barr bodies indicates the presence of three X chromosomes, and so on. So, it is easy to distinguish between males and females by the examination of the nucleus as mentioned before. This method is used to ensure the sex of some competitors at the Olympics.

N.B.:

Some cases are recorded known as Gynanders (double sex) appear. In these cases half the body is masculine, whereas the other half is feminine. This is found also in some insects, where some cells have XX, while the others have XY.

There are many abnormal cases in the number and kind of the sex chromosomes and autosomes in Man as shown in the following table:

Down's syndrome (Mongolism):

The abnormality is due to an extra autosome (no. 21) which leads to disturbance in the chromosome balance. The individual in this case is characterized by slant eyes with folded skin on the eyelids, and is therefore called Mongol. He (or she) grows with some abnormalities and idiocy or mental deficiency.

Sex-linked inheritance

Morgan studied the inheritance of Drosophila eye colour. He found that some white-eyed male insects appeared in his culture of the wild red-eyed insects. When he crossed a white-eyed male with a red-eyed female, the F1 generation was all red-eyed indicating that:

- The red colour of eyes is dominant over the white colour of eyes.

By inbreeding the F1 individuals, he got red-eyed and white-eyed insects in the ratio 3:1. However, the white-eyed insects were all males.

Morgan explained the appearance of white-eyed males in the F1 generation by assuming that the gene for the white colour of the eye is recessive and carried on the sex chromosome X. Due to the shortness of the chromosome Y; it doesn’t carry the other allele of the gene. Thus, the presence of one gene of the white colour of eyes is sufficient for males to be white-eyed. Morgan called this case sex-linked inheritance to refer to traits that are determined by the genes located on the sex chromosome X. The white colour of eyes rarely appears in female insects as they have two X chromosomes. Even if the white-eyed gene is found on one of the two X chromosomes, it will be masked by the dominant red-eyed gene found on the other X chromosome. Thus this female will be red-eyed. On the other hand, if both sex chromosomes of the female carry the gene for white-eyed colour, it will be white-eyed.

Morgan represented the crossing between white-eyed males, and red-eyed females as follows: