(i) It helps in gender identification.
(ii) It is used to detect the chromosomal aberrations like deletion, duplication, translocation, invasion and nondisjunction of chromosomes.  
(iii) It helps to identify the abnormalities of chromosomes like aneuploidy.
(iv) It is also used in predicting the evolutionary relationships between species.
(v) Genetic diseases in human beings can be detected by this technique.

In human beings the diploid (2n) body cell has 46 chromosomes (23 pairs).
i) chromosomal abnormalities are caused by errors is the number or structure of chromosome.
ii) Failure of chromatids to segregate during cell division resulting in the gain or loss of of one or more choromosomes called aneuploidy. It is caused by the nondisjunction of chromosomes.
iii) There abnormalities causes various syndromes in human beings. They are

1. Down's Syndrome/ Trisomy-21:
Trisomic condition of chromosome - 21 
Symptoms :
(i) severe mental retardation,
(ii) Defective development of the central nervous system,
(iii) increased separation between the eyes,
(iv) flattened nose, ears are malformed,
(v) mouth is constantly open and the tongue protrudes.
2. Patau's Syndrome/ Trisomy-13
i) Trisomic condition of chromosome l3 results in Patau's syndrome.
ii) Meiotic non disjunction is thought to be the cause for this chromosomal abnormality.
ii) It is characterized by
symptoms
(i) multiple and severe body malformations as well as profound mental deficiency.
(ii) Small head with small eyes,
(iii) cleft palate,
(iv) malformation of the brain
(v) internal organs 
b. Allosomal abnormalities is human beings:
Mitotic or meiotic non-disjunction of sex chromosomes causes allosomal abnormalities. Several sex chromosomal abnormalities have been detected. Eg. Klinefelter's syndrome and Turner's syndrome.
1. Klineletter's Syndrome(XXY Males):
(i) It is due to the presence of an additional copy of the X chromosome resulting in a karyotype of 47, XXY.
(ii) Persons with this syndrome have 47 chromosomes (44AA+XXY).
symptoms
sterile males, tall, obese, with long limbs, high pitched voice, under developed genitalia and have feeble breast (gynaecomastia) development.
2. Turner's Syndrome (XO Females):
(i) It is due to the loss of a X chromosome resulting in a karyotype of 45, X.
(ii) Persons with this syndrome have 45 chromosomes (44 autosomes and one X chromosome) (44AA + XO) and are sterile females.
symptoms
Low stature, webbed neck, under developed breast, rudimentary gonads lack of menstrual cycle during puberty, are the main symptoms of this syndrome.

The Mendelian disorders in human beings are
(a) Thalassemia
(b) Sickle cell anaemia
(c) Huntington chorea
(d) Phenylketonuria
(e) Albinism
(a) Thalassemia
(i) Thalassemia is an autosomal recessive disorder. It is caused by gene mutation resulting in excessive destruction of RBC's due to the formation of abnormal haemoglobin molecules.
Normally haemoglobin is composed of four polypeptide chains, two alpha and two beta globin chains.
(ii) Thalassemia patients have defects in either the alpha or beta globin chain causing the production of abnormal haemoglobin molecules resulting in anaemia.
Classified based on heaemoglobin
Alpha Thalassemia:
(i) Mutation or deletion of one or more of the four alpha gene alleles.
(ii) It is controlled by two closely linked genes HBAI and HBA2 on chromosome 16.
Beta Thalassemia:
(i) Production of beta globin chain is affected
(ii) It is controlled by a single gene (HBB) on chromosome 11.
(iii) It's is also known as Cocley's anemia.
(iv) It increases the alpha chain production and damages the membranes of RBC.
Huntington's chorea:
(i) It is inherited as an autosomal dominant lethal gene in man.
(ii) It is characterized by involuntary jerking of the body and progressive degeneration of the nervous system, accompanied by gradual mental and physical deterioration.
(iii) The patients with this disease usually die between the age of 35 and 40.

(i) Thalassemia is an autosomal recessive disorder. It is caused by gene mutation resulting in excessive destruction of RBC's due to the formation of abnormal haemoglobin molecules. Normally haemoglobin is composed of four polypeptide chains, two ph and two bet globin chains. Thalassemia patients have defects in either the alpha or beta globin chain causing the production of abnormal haemoglobin molecules resulting in anaemia.
(ii) Thalassemia. is classified into alpha and beta based on which chain of haemoglobin molecule is affected. It is controlled by two closely linked genes HBAI and HBA2 on chromosome 16.Mutation or deletion of one or more of the four alpha gene alleles causes Alpha Thalassemia. In Beta Thalassemia, production of beta globin chain is affected. It is controlled by a single gene (HBB) on chromosome 11. It is the most common type of Thalassemia and is also known as Cooley's anaemia. In this disorder the alpha chain production is increased and damages the membranes of RBC

Application of the laws of genetics for the improvement of human race is called eugenics.
Two methods of Eugenics are:
(i) Constructive method or Positive eugenics
(ii) Restrictive method or Negative eugenics
(i) Positive eugenics: Positive eugenics attempts to increase consistently better or desirable germplasm and to preserve the best germplasm of the society. The desirable traits can be increased by adopting the following measures:
(a) Early marriage of those having desirable traits
(b) Subsiding the fit and establishing sperm and egg banks of precious germ plasm.
(c) Educating the basic principles of genetics and eugenics.
(d) Improvement of environmental conditions.
(e) Promotion of genetic research.
(ii) Negative eugenics: Negative eugenics attempts to eliminate the defective germplasm of the society by adopting the following measures:
(a) Sexual separation of the defectives
(b) Sterilization of the defectives
(c Control of immigration and
(d) Regulation of marriages