Sunday, July 26, 2020

Principles of Inheritance and Variation - Notes | Class 12 | Part 8: Genetic Disorders


The disorders due to change in genes or chromosomes.
2 types:
Mendelian disorders & Chromosomal disorders.

1. Mendelian Disorders

·   It is caused by alteration or mutation in the single gene.

·   E.g. Haemophilia, Colour blindness, Sickle-cell anaemia, Phenylketonuria, Thalassemia, Cystic fibrosis etc.

·   The pattern of inheritance of Mendelian disorders can be traced in a family by the pedigree analysis.

·   Mendelian disorders may be dominant or recessive.

·   Pedigree analysis helps to understand whether the trait is dominant or recessive.

Pedigree analysis of

(A) Autosomal dominant trait (E.g. Myotonic dystrophy)    

(B) Autosomal recessive trait (E.g. Sickle-cell anaemia)

Haemophilia (Royal disease):

·   It is a sex linked (X-linked) recessive disease.

·   In this, a protein involved in the blood clotting is affected.

·   A simple cut results in non-stop bleeding.

·  The disease is controlled by 2 alleles, H & h. H is normal allele and h is responsible for haemophilia.


Normal female


Heterozygous female (carrier). She may transmit the disease to sons.


Hemophilic female


Normal male


Hemophilic male

·   In females, haemophilia is very rare because it happens only when mother is at least carrier and father haemophilic (unviable in the later stage of life).

·   Queen Victoria was a carrier of hemophilia. So her family pedigree shows many haemophilic descendants.

Colour blindness:

·    It is a sex-linked (X-linked) recessive disorder due to defect in either red or green cone of eye. It results in failure to discriminate between red and green colour.

·    It is due to mutation in some genes in X chromosome.

·    It occurs in 8% of males and only about 0.4% of females. This is because the genes are X-linked.

·    Normal allele is dominant (C). Recessive allele (c) causes colour blindness.

·    The son of a heterozygous woman (carrier, XCXc) has a 50% chance of being colour blind.

·    A daughter will be colour blind only when her mother is at least a carrier and her father is colour blind (XcY).

Sickle-cell anaemia:

·   This is an autosome linked recessive disease.

·   It can be transmitted from parents to the offspring when both the partners are carrier (heterozygous) for the gene.

·   The disease is controlled by a pair of allele, HbA and HbS.

Homozygous dominant (HbAHbA): normal

Heterozygous (HbAHbS):  carrier; sickle cell trait

Homozygous recessive (HbSHbS): affected

·  The defect is caused by the substitution of Glutamic acid (Glu) by Valine (Val) at the sixth position of the β-globin chain of the haemoglobin (Hb).

·  This is due to the single base substitution at the sixth codon of the β-globin gene from GAG to GUG.

·  The mutant Hb molecule undergoes polymerization under low oxygen tension causing the change in shape of the RBC from biconcave disc to elongated sickle like structure.


·   An inborn error of metabolism.

·   Autosomal recessive disease.

·   It is due to mutation of a gene that codes for the enzyme phenyl alanine hydroxylase. This enzyme converts an amino acid phenylalanine into tyrosine.

·   The affected individual lacks this enzyme. As a result, phenylalanine accumulates and converts into phenyl pyruvic acid and other derivatives.

·   They accumulate in brain resulting in mental retardation. These are also excreted through urine because of poor absorption by kidney.


·    An autosome-linked recessive blood disease.

·    It is transmitted from unaffected carrier (heterozygous) parents to offspring.

·    It is due to mutation or deletion.

·  It results in reduced synthesis of a or b globin chains of haemoglobin. It forms abnormal haemoglobin and causes anaemia.

·    Based on the chain affected, thalassemia is 2 types:

o  a Thalassemia: Here, production of a globin chain is affected. It is controlled by two closely linked genes HBA1 & HBA2 on chromosome 16 of each parent. Mutation or deletion of one or more of the four genes causes the disease. The more genes affected, the less a globin molecules produced.

o  b Thalassemia: Here, production of b globin chain is affected. It is controlled by a single gene HBB on chromosome 11 of each parent. Mutation of one or both the genes causes the disease.

·    Thalassemia is a quantitative problem (synthesise very less globin molecules). Sickle-cell anaemia is a qualitative problem (synthesise incorrectly functioning globin).

2. Chromosomal disorders

They are caused due to absence or excess or abnormal arrangement of one or more chromosomes. 2 types:

a.  Aneuploidy: The gain or loss of chromosomes due to failure of segregation of chromatids during cell division.

b.  Polyploidy (Euploidy): It is an increase in a whole set of chromosomes due to failure of cytokinesis after telophase stage of cell division. This is very rare in human but often seen in plants.

Examples for chromosomal disorders

§ Down’s syndrome: It is the presence of an additional copy of chromosome number 21 (trisomy of 21).

Genetic constitution: 45 A + XX or 45 A + XY (i.e. 47 chromosomes).


o  They are short statured with small round head.

o  Broad flat face.

o  Furrowed big tongue and partially open mouth.

o  Many “loops” on finger tips.

o  Broad palm with characteristic palm simian crease.

o  Retarded physical, psychomotor & mental development.

o  Congenital heart disease.

§ Klinefelter’s Syndrome: It is the presence of an additional copy of X-chromosome in male (trisomy).

Genetic constitution: 44 A + XXY (i.e. 47 chromosomes).


o  Overall masculine development. However, the feminine development is also expressed. E.g. Development of breast (Gynaecomastia).

o  Sterile. 

o  Mentally retarded.

§ Turner’s syndrome: This is the absence of one X chromosome in female (monosomy).

Genetic constitution: 44 A + X0 (i.e. 45 chromosomes).


o  Sterile, Ovaries are rudimentary.

o  Lack of other secondary sexual characters.

o  Dwarf.

o  Mentally retarded.

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