 Male reproductive cells.  The human sperm cell is haploid, so that means it has 23 chromosomes. Structure of mature sperm  The head,  The neck,  The midpiece,  The tail and  The end piece all have their separate functions.

 Head The head contains the 23 chromosomes (genetic material) capable of merging with an egg to produce a new individual.  Acrosome The acrosome resembles a toque which covers the upper portion of the head of the sperm. The covering contains enzymes that help the sperm penetrate the egg.

 Midpiece The midpiece connects the head of the sperm to the tail. It contains many mitochondria that produce ATP. This ATP is used to move the microtubules in the tail and create propulsion for the sperm. Once in the female system, sperm will "swim" for up to 2 days looking for an egg.  Tail The tail of a sperm is really a flagellum with the common 9+2 arrangement of microtubules. The microtubules within the tail move past each other and move the tail in a whip-like motion. ATP provided by the midpiece is required for this movement.

 Female reproductive cell.  The human ovum is a haploid, i.e. 23 ‘C’. Structure of mature ovum:  Cell surface membrane.  Layer of jelly.  Cytoplasm containing yolk.  Nucleus containing chromosome.  Corona radiata.

 When the ovum is fused with sperm it under goes fertilization but if it doesn’t it dies in 12 to 24 hrs and gets discharged.

 Abnormalities of Form (structure)  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.

 The gametes may be abnormal in chromosomal content as follows: non-disjunction  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 instead 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. Trisomy  This is called a Trisomy.

 Depending upon the particular chromosomes involved, various abnormalities are produced.  Trisomy of chromosome 21 results in a condition called mongolism or Down's syndrome.  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; 'super females'.  XYY abnormal male.  XXY Klinefelter's syndrome: abnormal male;

 When both chromosomes of a pair go to one gamete 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 a monosomy.  The best known example of this is (a female with only one X chromosome)Turner's syndrome.

 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 XXXY, XXXXY, XXYY, or XXXX may be produced.  Sometimes a gamete may have the diploid number of chromosomes so that the zygote will have (i.e. 69) chromosomes. Aneuploidy  This is called Aneuploidy. Higher multiples of 23 may also be seen; such fetuses are generally born dead.

 Abnormalities in the process of crossing over can result in a number of chromosomal abnormalities as follows: translocation (a) Part of a chromosome may get attached to a chromosome of a different pair (translocation). deletion (b) Part of a chromosome may be lost (deletion). duplicated (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. inversion (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.

isochromosomes  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. mosaicism  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.

 Nondisjunction disjunction  The normal separation of chromosomes in Meiosis I or sister chromatids in meiosis II is termed disjunction. nondisjunction  When the separation is not normal, it is called nondisjunction. This results in the production of gametes which have either more or less of the usual amount of genetic material, and is a common mechanism for a trisomy or monosomy.  Nondisjunction can occur in the meiosis I or meiosis II phases of cellular reproduction, or during mitosis.

 This is a cause of several medical conditions in humans, including:  Down Syndrome  Down Syndrome - trisomy of chromosome 21  Patau Syndrome  Patau Syndrome - trisomy of chromosome 13  Edward Syndrome  Edward Syndrome - trisomy of chromosome 18  Klinefelter Syndrome  Klinefelter Syndrome - an extra X chromosome in males  Turner Syndrome  Turner Syndrome - only one X chromosome present in females  XYY Syndrome  XYY Syndrome - an extra Y chromosome in males  Triple X Syndrome  Triple X Syndrome - an extra X chromosome in females