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 α or β globin chains of haemoglobin. It forms abnormal haemoglobin and causes anaemia.

Based on the chain affected, thalassemia is 2 types:
  • α Thalassemia: Here, production of α 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 α globin molecules produced.
  • β Thalassemia: Here, production of β 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:
  1. Aneuploidy: The gain or loss of chromosomes due to failure of segregation of chromatids during cell division.
  2. 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).

  • They are short statured with small round head.
  • Broad flat face.
  • Furrowed big tongue and partially open mouth.
  • Many “loops” on finger tips.
  • Broad palm with characteristic palm simian crease.
  • Retarded physical, psychomotor & mental development.
  • 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).

  • Overall masculine development. However, the feminine development is also expressed. E.g. Development of breast (Gynaecomastia).
  • Sterile.
  • Mentally retarded.
  • Turner’s syndrome: This is the absence of one X chromosome in female (monosomy).
Turner’s syndrome: 

This is the absence of one X chromosome in female (monosomy). 

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

  • Sterile, Ovaries are rudimentary.
  • Lack of other secondary sexual characters.
  • Dwarf.
  • Mentally retarded.

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