Any change from the normal number or structure of chromosomes causes abnormalities.

1. Mongolism or Down’s Syndrome

The individual has 47 chromosomes because of one extra chromosome in the 21st pair (Trisomy of chromosome 21). The outcome of this defect are the following characters or features:

  • mentally retarded
  • have a thick tongue
  • drooping (false expression of pleasure) face

The possibility of giving birth to a mongolian child is far greater in pregnant mothers above the age of forty.

2. Klinefelter’s Syndrome

Individual is a male with 47 chromosome with one extra X chromosome (44 autosomes + XXY). Typical features of Klinefelter’s syndrome are:

  • Tall, mentally retarded male
  • Sterile and shows breast development or gynaecomastia

3. Turner’s Syndrome

Individual is a female with 45 chromosomes and with only one X chromosome (22 pairs of autosomes + XO). The characteristic features of this syndrome are:

  • Mentally retarded
  • Web like skin on neck
  • Incompletely developed breasts

4. Colour blindness and Haemophilia (Bleeder’s diseases)

Both these defects are sex linked disorders. In male, the single X-chromosome is received from the mother. Hence a defective, gene (for colour blindness or haemophilia) on X chromosome of the mother, is passed on to the son and expressed as a defect.

The daughter receives one X-chromosome from the mother and the other X from the father. In a carrier daughter the defective gene received from the mother is masked chromosome received from normal father by normal allele on the other X.

Colour blind males are unable to distinguish between red-green colours.

In haemophilia afflicted male, blood does not clot easily and the patient may bleed to death. Its mode of inheritance is exactly like that of colour blindness.

5. Thalassemia

It is an autosomal disorder in which normal haemoglobin is not synthesized. So, frequent blood transfusions are required for survival.

The defective gene is recessive and present on an autosome in the heterozygous. Parents may not show the disorder. The child who gets the defective genes from both the parents (homozygous recessive) suffers from Thalassemia.

6. Sickle Cell Anemia

This is another hereditary abnormality due to mutation of a single autosomal gene in which red blood corpuscles lose their shape and become sickle shaped because of defective Haemoglobin. Individuals possessing two defective genes (homozygous recessive), cannot survive. In the heterozygous individuals, one gene is normal and so half the number of total red blood corpuscles are normal containing normal haemoglobin while the others are defective.

For heterozygous individuals with sickle cell gene, it is a boon in disguise against malaria for children with one defective haemoglobin gene can survive as they are less affected by malarial because the malarial parasite cannot thrive inside the defective RBCs.

7. Rh factor

Rh factor is an antigen (a protein) present on the surface of red blood corpuscles. About 15% of all women do not have the gene for Rh antigen. They are Rh-negative.

Men can also be Rh-negative. But the problem which this trait creates is in Rh negative women. A pregnant Rh-negative woman whose husband is Rh+ may bear a the child who may have inherited the Rh+ gene from the father.

If the foetal blood of the Rh+ foetus enters mother’s body stream, her immune system produces antibodies against Rh antigen which may cause minor problems in first pregnancy. Antibodies remain in the mother’s blood and in the subsequent pregnancies, the mother’s antibodies against foetal Rh antigen may enter the foetal blood stream and destroy its red blood corpuscles causing severe anemia which may even be fatal for the foetus.