03 Inheritance and Variation - part 12 - Genetic Disorders

03 Inheritance and Variation - part 12 - Genetic Disorders


Genetic Disorders : 
  • Genetic Disorders are broadly grouped into two categories as -
  1. Mendelian disorders and 
  2. Chromosomal disorders
Mendelian discorders  :
  • Mainly caused due to alteration or mutation in the gene. 
e.g. thalassemia, sicklecell anaemia, colourblindness, haemophilia, phenylketonuria, etc.

Chromosomal disorders :
  • Caused due to absence or excess of one or more chromosomes or their abnormal arrangment. 
  • For eg, Down's syndrome, Turner's syndrome, Klinefelter's syndrome etc.
Thalassemia : 
  • Thalassemia is an autosomal, inherited recessive disease. 
  • Haemoglobin molecule is made of four polypeptide chains- 2 alpha (a) and 2 beta (b)chains. 
  • The synthesis of alpha chains are controlled by two closely linked genes (HBA1 and HBA2) on chromosome 16.
  • Synthesis of beta chain is controlled by a single gene (HBB) on chromosome 11.
  • Depending upon which chain of haemoglobin is affected, thalassemia is classified as - 
  1. Alpha-thalassemia and 
  2. Betathalassemia. 
  • It is caused due to deletion or mutation of gene which codes for alpha (a) and beta (b) globin chains that result in abnormal synthesis of haemoglobin. 
  • In Thalassemia, person shows symptoms like anaemia, pale yellow skin, change in size and shape of RBCs, slow growth and development, dark urine, etc. 
  • Massive blood transfusion is needed to these patients. 
  • Thalassemia differs from sickle-cell anaemia. 
  • The former is a qualitative problem of synthesising few globin molecule, while the latter is a qualitative problem of synthesising an incorrectly functional globin.
 Down's Syndrome (21st trisomy) : 

  • Down's syndrome is named after the physician John Langdon Downwho first described this autosomal chromosomal disorder in 1866.
  • This Syndrome is caused due to an extra copy of chromosome number 21st. It shows presence of three copies of 21st chromosome instead of homologous pair. 
  • These individuals will have 47 chromosomesinstead of the normal number 46. 
  • 21st Trisomy occurs due to non-disjuction or failure of separation of chromosomes (autosomes) during gamete formation. 
  • The incidence of non-disjunction is distinctly higher in mothers who are over 45 years old.
  • These patients have mild or moderate mental retardation and skeletal development is poor. 
  • Distinct facial features like small head, ears and mouth, face is typically flat and rounded with flat nose, open mouth and protruding tongue.
  • Eyes slant up and out with internal epicanthal folds, flat hands and stubby fingers and palm is broad with single palmer crease.
Turner's Syndrome (X monosomy / XO females) : 
  • It is sex chromosomal disorder caused due to non-disjunction of chromosome during gamete formation. 
  • Individual born with Turner's syndrome has 44 autosomes with XO. They are phenotypically female. 
  • They have a short stature (height) and webbed neck, lower posterior hair line, broad shield-shaped chest, poorly developed ovaries and breast, and low intelligence. 
Klinefelter's syndrome (XXY males) : 
  • It is chromosomal disorder caused due to extra X chromosome in males. Thus genotype of individuals is 44 + XXY. 
  • They are described as feminized males. 
  • Extra chromosome is a result of non-disjunction of X-chromosome during meiosis. 
  • Individual is male and has over all masculine development. 
  • Voice pitch is harsh and have under developed testis. 
  • They are tall with long arms, feminine development (development of breast i.e. Gynaecomastia) and no spermatogenesis, therefore, individuals are sterile.

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