Various genetic diseases are known which are caused by abnormality of the autosomal chromosomes (not connected with determination of sex). Autosomal abnormality may be due to polyploidy (more than normal diploid set of chromosomes) or may be due to aneuploidy (one chromosomes may be more or less than the normal). Aneuploidy has several forms. Sometime 21st pair of 18th pair or 13th pair of chromosomes fail to separate during meiosis.
Some inherited diseases have been briefly mentioned below:
(i) Down’s Syndrome: This syndrome is also called mongolism. In the middle aged female the 21st pair of chromosome may not be able to separate during oogenesis. Thus a fertilized egg may contain 3 sets of 21st chromosome. Thereby total chromosomes in the diploid nucleus becomes 47 instead of normal 46. A child having 3 sets of chromosome number 21 (trisomy-21) may suffer from physical and mental defects. Mental retardation is severe in such case. They may have below average height, widely separated eyes, flattened nose, short but broad feet etc.
(ii) Monosomy-21: If the fertilization egg is devoid of complete set of 21st chromosome the body cell will contain 45 chromosomes. Loss of one chromosome will produce a monosomic child with large ears and closely placed eyes.
(iii) 18-Trisomy: A Child having 3 sets of 18th chromosome possesses laterally flas head and the helix of the car scarcely develops. The hands are short and show little development of the second phalanx.
(iv) 13-Trisomy: A child having 3 sets of 13th chromosome shows severe body and organ malformation. Such children are also mentally retarded. Head is small and eyes are often very small or absent.
Genetic Disorders:
(1) Mendelian Disorders:
(a) These are mainly due to alternation or mutation in single gene.
(b) The disorder may be dominant or recessive.
(c) These may be metabolic abnormalities.
(i) autosomal (eg. sickle-cell anaemia, phenylketonuria)
(ii) sex-linked (eg. haemophilia, colour blindness)
(2) Chromosomal Disorder:
(a) These may be due to excess of certain chromosome or abnormal arrangement or structural defect of certain chromosome.
(b) Some of the more serious disorders-Down’s syndrome, Klinefelter’s syndrome, Turner’s syndrome.
