Genetic Disorders - Chromosomal Abnormalities

Chromosomal Abnormalities:

• Chromosomal disorders are caused due to absence or excess than a normal number of chromosomes; or due to abnormal arrangement of the chromosomes.
• Each human diploid (2n) body cell has 46 chromosomes (23 pairs).
• Chromosomal anomalies usually occur when there is an error in cell division.
• Failure of chromatids to segregate during cell division resulting in the gain or loss of one or more chromosomes is called aneuploidy.
• It is caused by the non-disjunction of chromosomes.
• Down’s syndrome results in the gain of extra copy of chromosome 21. Similarly, Turner’s syndrome results due to loss of an X chromosome in human females. 
• Group of signs and symptoms that occur together and characterize a particular abnormality is called a syndrome.
• The failure of cytokinesis after telophase stage of cell division results in an increase in a whole set of chromosomes in an organism and, this phenomenon is known as polyploidy. This condition is often seen in plants.
• Additional pair of chromosomes (2n + 1) is called trisomy. The lack of one chromosome (2n - 1) is called monosomy.
• In humans, Down’s syndrome, Turner’s syndrome, Klinefelter's syndrome, and Patau’s syndrome are some of the examples of chromosomal disorders.

A) Autosomal aneuploidy in human beings:

• Several autosomal aneuploidies have been reported in human beings.
• Example - Down’s syndrome (21-Trisomy), Patau’s syndrome (13-Trisomy).

1) Down’s Syndrome/Trisomy – 21:

• Trisomic condition of chromosome - 21 results in Down’s syndrome.
• This disorder was first described by Langdon Down (1866).
• It is characterized by severe mental retardation, defective development of the central nervous system, increased separation between the eyes, flattened nose, ears are malformed, the mouth is constantly open and the tongue protrudes.

2) Patau’s Syndrome/Trisomy-13:

• Trisomic condition of chromosome 13 results in Patau’s syndrome.
• Meiotic non-disjunction is thought to be the cause for this chromosomal abnormality.
• It is characterized by multiple and severe body malformations as well as profound mental deficiency.
• Small head with small eyes, cleft palate, malformation of the brain and internal organs are some of the symptoms of this syndrome.

B) Allosomal abnormalities in 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) Klinefelter’s Syndrome (XXY Males):

This genetic disorder is due to the presence of an additional copy of the X chromosome resulting in a karyotype of 47, XXY. Persons with this syndrome have 47 chromosomes (44AA+XXY). They are usually 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):

This genetic disorder is due to the loss of a X chromosome resulting in a karyotype of 45, X. Persons with this syndrome have 45 chromosomes (44 autosomes and one X chromosome) (44AA+XO) and are sterile females. Low stature, webbed neck, under developed breast, rudimentary gonads lack of menstrual cycle during puberty, are the main symptoms of this syndrome.