1. ABNORMALITIES IN FORMATION OF GAMETES Abnormalities of Form Spermatozoa may be too large (giant) or too small (dwarf). The head, body or tail may be duplicated. The ovum may have an unusually large nucleus or two nuclei. Two oocytes may be seen in one follicle.

2. Chromosomal Abnormalities The gametes may be abnormal in chromosomal content as follows: During the first meiotic division, the two chromosomes of a pair, instead of separating at anaphase, may both go to the same pole. (This is called non-disjunction.) The resulting gamete then has 24 chromosomes in- stead of the normal 23. At fertilization by this gamete, the zygote will, therefore, have 47 chromosomes; there being three identical chromosomes instead of one of the normal pairs. This is called trisomy.

3. Depending upon the particular chro­ mosomes involved, various abnormalities are produced. Trisomy of chromosome 21 results in a condition called mongolism or Down's syndrome. In this condition the child has a broad face, obliquely placed palpebral fissures, epicanthus, a furrowed lower lip, and broad hands with a single transverse crease. Usually the patients are mentally retarded, and have anomalies of the heart.

4. The presence of an extra X or Y chromosome can give rise to various syndromes associated with abnormal genital development, mental retar­dation and abnormal growth. Some of these are: XXX (abnormal female); XXV (Klinefelter's syndrome: abnormal male); XYY (abnormal male). In Klinefelter's syndrome the subject is a male (because of the presence of a Y chromosome). However, the testes are poorly developed leading to sterility and gynaecomastia.

5. Patients with XXX chromosomes show two masses of sex chromatin in their cells and are sometimes referred to as 'super females'. However, there is nothing 'super' about them. In fact, their bodies show poor sexual devel­opment (i.e. they are infantile), and menstruation is scanty. Mental retardation is usual.

6. When both chromosomes of a pair go to one' gamete (as described above), the other gamete resulting from the division has only 22 chromosomes; and at fertilization the zygote has only 45 chromosomes. Hence one pair is represented by a single chromosome. This is called monosomy.

7. The best known example of this is a female with only one X chromosome (Turner's syndrome). In this syndrome the subject is always female (be­cause of absence of a Y chromosome). There is agenesis of ovaries. Asso­ciated deformities include mental retardation, skeletal abnormalities, and folds of skin on the sides of the neck (webbed neck).

8. Such anomalies may affect more than one pair of chromosomes. Alterna­tively, one pair may be represented by more than three chromosomes; when this happens with the sex chromosomes, individuals with the consti­tution XXXV, XXXXY, XXYY, or XXXX may be produced. Sometimes a gamete may have the diploid number of chromosomes so that the zygote will have 46 + 23 (i.e. 69) chromosomes. This is called triploidy. Higher multiples of 23 may also be seen; such fetuses are gen­erally born dead.

9. Abnormalities in the process of crossing over can result in a number of chromosomal abnormalities as follows: (a) Part of a chromosome may get attached to a chromosome of a dif­ferent pair (translocation). (b) Part of a chromosome may be lost (deletion). (c) The two chromosomes of a pair may break at unequal distances; when each piece joins the opposite chromosome, one chromosome is longer than normal and some of the genes are duplicated. The other chromosome will be shorter than normal, some genes being missing. (d) A piece separating from a chromosome may get inverted before join­ing the opposite chromosome (inversion). Although the same genes are present, their sequence is disturbed.

10. We have seen that during cell division the centromere splits longitudinally so that each chromatid becomes a separate chromosome. Sometimes the centromere splits transversely producing two dissimilar chromosomes; one chromosome is made up of the short arms of both chromatids, while the other is made up of the long arms. Such chromosomes are called isochromosomes. Chromosomal errors of the type described above may also occur during segmen­tation of the ovum. This results in a fetus having a mixture of cells with normal and abnormal chromosomes. This is called mosaicism. Such individuals may also show various abnormalities.