Genetic Disorders
Genetic Disorders: They are of 2 types:
- Mendelian
- Chromosomal
(1)Mendelian disorders: These disorders are caused due to alteration or mutation in single gene and follow the Mendal’s principles of inheritance.
- Haemophilia: Also called bleeders disease.
i) This sex linked recessive disease, which shows its transmission from unaffected carrier female to some of the male progeny.
ii) In this disease, a single protein that is a part of the cascade of proteins involved in the clotting of blood is affected which results in non-stop bleeding.
iii) The family pedigree of Queen Victoria shows a number of haemophilic descendents as she was a carrier of the disease.
iv) The gene ‘h’ for this disease is located on ‘X’chromosome and disease is controlled by allele Xh:
Xh and Xh === effected female
Xh and X === carrier female
Xh and Y === effected male
b) Sickle-cell anaemia:
i) This is an autosome linked recessive trait that can be transmitted from parents to the offspring when both the partners are carrier for the gene.
ii) The defect is caused by the substitution of Glutamic acid (Glu) by Valine (Val) at the sixth position of the beta globin chain of the haemoglobin molecule.
iii) The mutant haemoglobin molecule undergoes polymerisation under low oxygen tension causing the change in the shape of the RBC from biconcave disc to elongated sickle like structure.
iv) These biconcave disc may stick to the blood vessels which may cause heart attack.
iii) The disease is controlled by a single pair of allele, HbA and HbS.
HbS. And HbS.=== effected person
HbS. And Hba ==== carrier person
c) Phenylketonuria:
i) It is an autosomal recessive trait and an inborn genetic disorder.
ii) The affected individual lacks an enzyme that converts the amino acid phenylalanine into tyrosine .Due to which,
this phenylalanine is accumulated and converted into phenylpyruvic acid and other derivatives
iii) Accumulation of these in brain results in mental retardation. These are also excreted through urine because of its poor absorption by kidney.It also cause hair and skin pigmentation change.
d) Colour blindness:
i) Sex linked recessive trait. In this disease effected person is not able to differentiate red and green colour.
ii) X chromosome carry gene for this disease. (Xc Y)
Xc and Y === effected male
Xc and Xc === effected female
XC and X === carrier female
iii) It is observed more in males (XcY) because os presence of only one X chromosomes as compared to two chromosomes of female.
e) Thalassemia:
i) It is an autosomal recessive trait.
ii) It occurs due to either mutation or deletion resulting in reduced rate of synthesis pf one of globin chains of haemoglobin.
iii) It also causes aneamia. Thalassemia is further os two types:
Alpha thalassemia: Here synthesis of alpha globin chain is affected.
Beta thalassemia: Here synthesis of beta globin chain is affected.
(2) Chromosomal disorders: The chromosomal disorders on the other hand are caused due to absence
or excess or abnormal arrangement of one or more chromosomes. Due to aneuploidy, polyploidy, trisomy and monosomy etc.
a) Down’s syndrome:
i) Occurs due to trisomy or additional copy of chromosome number 21.
ii) The affected individual is short statured with small round head, furrowed tongue and partially open mouth
Palm is broad with characteristic palm crease. Physical, psychomotor and mental development is retarded.
b) Klinefelter’s syndrome:
i) Presence of additional copy of X chromosome with a karyotype of 45 +XXY.
ii) Individuals have overall masculine development, however, the
feminine development (development of breast,
i.e., Gynaecomastia) is also expressed. Such individuals are sterile.
c) Turner’s syndrome:
i) Absence of one of the X chromosome with a karyotype of 45 + XO.
ii) Such females are sterile as ovaries are rudimentary besides other features including lack of other secondary
sexual characters.