GENETIC DISORDERS

GENETIC DISORDERS:

Pedigree Analysis:

  • Analysis of traits in several of generations of a family is called the pedigree analysis.
  • In the pedigree analysis the inheritance of a particular trait is represented in the family tree over generations.

Autosomal Dominant:

  • Affected individuals have at least one affected parent
  • The phenotype generally appears every generation
  • Two unaffected parents only have unaffected offspring
  • Traits are controlled by dominant genes
  • Both males and females are equally affected
  • Traits do not skip generations
  • e.g. polydactyly, tongue rolling ability etc

Autosomal recessive:

  • Unaffected parents can have affected offspring
  • Traits controlled by recessive  genes and
  • Appear only when homozygous 
  • Both male and female equally affected
  • Traits may skip generations
  • 3:1 ratio between normal and affected. 
  • Appearance of affected children from normal parents (heterozygous)
  • All children of affected parents are also affected.
  • e.g.- Albinism, sickle cell anaemia etc

 

 

Mendelian Disorder:

  • Genetic disorders grouped into two categories –
    • Mendelian disorder
    • Chromosomal disorder
  • Mendelian disorders are mainly determined by alteration or mutation in the single gene.
  • Obey the principle of Mendelian inheritance during transmission from one generation to other.
  • Can be expressed in pedigree analysis.

E.g. Haemophilia, colorblindness, Cystic fibrosis, Sickle cell anemia, Phenylketonuria, Thalasemia etc.

Hemophilia:

In this disease a single protein that is a part of the cascade of proteins involved in the clotting of blood is affected. Due to this in an affected individual a simple cut will result in non-stop bleeding.

  • Sex linked recessive disease.
  • The diseases transmitted from unaffected carrier female to some of the male progeny.
  • Female becoming hemophilic is extremely rare because mother of such a female at least carrier and the father should be hemophilic.
  • Affected transmits the disease only to the son not to the daughter.
  • Daughter can receive the disease from both mother and father.

Sickle cell anaemia:

  • The defect is caused due to substitution of Glutamic acid (Glu) by Valine (Val) at the sixth position of the beta globin chain of the haemoglobin molecule.
  • Substitution of amino acid takes place due to the single base substitution at the sixth codon of the beta globin gene from GAG to GUG.
  • The mutant haemoglobin molecule undergoes polymerization under low oxygen tension causing the change in the shape of the RBC from biconcave disc to elongated sickle like structure.
  • This is an autosomes linked recessive trait.
  • Transmitted from parents to the offspring when both the parents are carrier for the gene (heterozygous).
  • This disease is controlled by single pair of allele, HbA, and HbS.
  • There are three possible genotypes (HbA HbA, HbA HbS, and HbSHbS.
  • Only homozygous individuals for HbS (HbS HbS) show the diseased phenotype.
  • Heterozygous (HbA HbS) individuals appear apparently unaffected but they are carrier of the disease as there is 50 percent probability of transmission of the mutant gene to the progeny.

Phenylketonuria:

  • Autosomal recessive trait.
  • Inborn error of metabolism.
  • The affected individual lack one enzyme called phenyl alanine hydroxylase that converts the amino acid phenyl alanine to tyrosine.
  • In the absence of the enzyme phenyl alanine accumulated and converted into phenylpyruvic acid and other derivatives.
  • Accumulation of these results in mental retardation.
  • These derivatives excreted through kidney.

Chromosomal disorders:

  • Caused due to absence or excess or abnormal arrangement of one or more chromosome.
  • Failure of segregation of chromatids during cell division cycle results in the gain or loss of chromosome(s), called Aneuploidy.
  • Failure of cytokinesis after telophase stage of cell division results in an increase in a whole set of chromosome in an organism and this phenomenon is called polyploidy.

Trisomy: additional copy of a chromosome may be included in an individual (2n+1).
Monosomy: an individual may lack one of any one pair of chromosomes (2n-1)

Down syndrome:

  • Caused due to presence of an additional copy of the chromosome number 21 (trisomy of 21).
  • This disorder was first described by Langdon Down (1866).
    • Short stature with small round head.
    • Furrowed tongue
    • Partially opened mouth
    • Palm is broad with characteristic palm crease.
    • Physical, psychomotor and mental development is retarded.

 Klinefelter’s syndrome:

  • Caused due to the presence of an additional copy of X-chromosome resulting into a karyotype of 47, (44+XXY).
    • Overall masculine development.
    • Also develop feminine character (development of breast i.e. Gynaecomastia)
    • Individuals are sterile.

Turner’s syndrome:

  • Caused due to the absence of one of the X- chromosomes i.e. 45 (44 + X0).
    • Such females are sterile as ovaries are rudimentary.
    • Lack of other secondary sexual characters.

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