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Chapter: 11th 12th standard bio zoology Human Body higher secondary school

Genetic Diseases

Genetic Diseases : 1. Sickle cell anaemia 2. Thalassemia 3. Agammaglobulinemia 4. Albinism 5. Huntington's chorea 6. Severe Combined Immunodeficiency (SCID)

Genetic Diseases


1. Sickle cell anaemia :


Sickle cell anaemia is a genetic syndrome caused by an autosomal mutant allele Hbs. In homozygous condition (Hbs Hbs), it causes the production of an abnormal haemoglobin called haemoglobin S. The normal haemolobin is designated as HbA (HbAHbA). Sickle cell persons with the genotype HbsHbs suffer from a fatal haemolytic anaemia. The patient dies due to damaged heart, kidney, spleen and brain as a result of clogged blood vessels or vascular obstruction. Persons with heterozygous genotype HbA Hbs are said to be carriers and they survive.

2. Thalassemia :


Thalassemia is an erythroblastic anaemia due to homozygous recessive gene expression in children. Two types of this disease viz., thalassemia major and thalassemia minor exist. The former is the severe form while the latter is its milder form. The homozygotes suffer from severe thalassemia while all heterozygotes suffer from milder thalassemia. The clinical manifestations of thalassemia include I) decrease in the bone marrow activity, ii) peripheral haemolysis, iii) splenomegaly (enlarged spleen) and hepatomegaly, (enlarged liver) etc. The thalassemic children die at the age of seventeen.


3. Agammaglobulinemia :


Agammaglobulinaemia is a recessive gene disease, wherein r-globulin synthesis fails to occur. In this disease, the patient shows a great deficiency or total absence of plasma cells and unusual lymph nodes with fewer lymphocytes than normal. The failure of antibody synthesis in this disease, makes the subjects more prone to viral and bacterial infections especially of the chest .This disease mostly affects boys.


4. Albinism :


It is an inherited disorder of melanin metabolism characterized by the absence of melanin in the skin, hairs and eyes. The clinical characteristics of this disease are the milk-white coloured skin and marked photophobia. Albinism is an inborn error metabolic disease, In this, the genes by undergoing mutation do not produce particular enzymes, which take part in the metabolic pathways. The metabolism of one amino acid phenylalanine proceeds in chains of enzyme-mediated reactions. The change or absence of enzyme due to defective genes, results in physiological abnormalities. In albinism, complete lack of melanin pigment (a dark brown pigment) causes the albino to suffer. The incidence of albinism in human has been reported to be from 1:5000 to 1:25000. The albinism may be generalized albinism, localized albinism of the eye (ocular albinism) or partial albinism (skin and hair). The recessive genes 'aa' do not produce the tyrosinase enzyme, which converts DOPA (3,4 - dihydroxy phenyl alanine) into melanin in the melanocytes.


5. Huntington's chorea :


This is a fatal disease caused by an autosomal dominant gene in hu-man. The onset of the disease is between 35 and 40 years of age. It is characterized by uncontrolled jerking of the body due to involuntary twitching of voluntary muscles. It leads to progressive degeneration of the central nervous system accompanied by gradual mental and physical deterioration. Huntington's disease was the first completely dominant human genetic disease to come to light. The affected gene is located on chro-mosome 4. Other characteristics of this disease are deterioration of intellec-tual faculty, depression, occasional hallucination and delusions and other psy-chological problems. This disease is incurable.


6. Severe Combined Immunodeficiency (SCID) :


This is an extremely rare inherited disease affecting children. The gene for the disease called ADA (Adenine deaminase) is located on chromosome 20. The children suffering from the syndrome completely lack the immune defense mechanism against infection due to rapid death of all white blood cells. SCID is also called Bubble Boy Syndrome. The child is kept in a sterile bubble. Unless given bone marrow transplant, the child's life span is short lived.

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