• 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:

Characteristics of the individual	Chromosomal structure	Sex
Normal male	46: 44+XY	Male
Normal female	46: 44+XX	Female
Down's syndrome (Mongolism)	47: 45+XY	Male
Down's syndrome (Mongolism)	47: 45+XX	Female
Klinefelter's syndrome	47: 44+XXY	Male
Turner's syndrome	45: 44+X	Female
Multiple sex genes	47: 44+XXX	Female
Multiple sex genes	48: 44+XXXX	Female

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:

Red-eyed female White-eyed male

P1 R R X r

XX XY

G1

R r R

F1 XX XY

Red-eyed female Red-eyed male