The cause of mentally challenged in many cases is unknown but there are certain abnormalities known to be associated with mentally challenged. One of them is abnormality in the chromosomal pattern of the individual.

Abnormality in the chromosomes results in a large number of reasons for mentally challenged. Chromosomes are the structures in the nucleus of a cell, which carry the genetic material, which is transmitted from one generation to the next. The fertilized human egg cell normally has 23 pairs of chromosomes; one of each pair comes from each parent. One of these pairs determines the sex of the individual. Of this pair, one is an X chromosome and comes from the mother; the other sex chromosome comes from the father and may be an X or a Y. If it is an X chromosome the child will be a female with XX sex chromosomes and if it is a Y chromosome then a male child will be born with XY sex chromosomes. The other 22 pairs are called autosomes. 

Chromosomal disorders could be due to:
	Error in chromosome numbers
	Chromosomal translocations
	Deletion of chromosome
	Sex Chromosome Abnormalities
	Duplication of a chromosome

1.	Change/Error in the Chromosome Numbers:
	Change in the number of chromosomes produces conditions with medical problems and most of these conditions cause mentally challenged. The error may be in the number of chromosomes being too many, too few or the error may be in the structure of the chromosome. One common condition due to error in chromosome number is Down syndrome (trisomy 21), which is a leading cause of challenge.
2.	Chromosomal Translocations:
	The transfer of a segment of a chromosome to a chromosome of another pair i.e. non-homologous chromosome is called translocation. Translocation is the term used when, during the division of the germ cell, one chromosome joins on with another one. They increase the chances of chromosome abnormality in the next generation. Translocation can be diagnosed from the chromosomes in the white blood cells. Translocation in any pregnancy can be tested for trisomy by using amniotic fluid obtained during the 3rd to 4th month of pregnancy. There is then the possibility of therapeutic termination. An example of chromosomal translocation is Down syndrome.
3.	Deletion of Chromosome:
	Abnormalities occur when a part of a chromosome is missing. This leads to conditions, which cause mentally challenged in persons. An example of this is Cri-du-chat Syndrome where the short arm of a number 5 chromosome is deleted. It is also known as Cat Cry Syndrome .It is characterized at birth by a high-pitched cry, low birth weight, poor muscle tone, microcephaly and potential medical complications. Intellectual handicap is severe and speech is limited.
4.	Sex Chromosome Abnormalities:
	Only a small proportion of people with sex chromosome abnormalities are Mentally Challenged. One of the examples of sex chromosome abnormalities is Klinefelter's Syndrome, which is a chromosomal abnormality that affects only men and causes infertility. Many people with Klinefelter's are of normal intelligence. Those who are Mentally Challenged are in the mild and borderline groups. Secondary sex characteristics fail to develop. Testes are small, body hair is sparse and the body is feminine due to the distribution of fat.  Secondary sex characteristics are promoted by giving male hormone, such as testosterone. Support and care are especially important during adolescence. Another sex chromosome abnormality is Turner's Syndrome. Secondary sex characteristics are diminished. Here females may lack ovarian tissues and sex hormones. mentally challenged occurs in most of the cases.
5.	Duplication of Chromosome:
	Certain abnormalities can occur if a chromosome has been duplicated or a part of that chromosome has been duplicated. These abnormalities can occur in autosomes or the sex chromosomes.