Chromosomal
Abnormalities
Each human diploid (2n)
body cell has 46 chromosomes (23 pairs). Chromosomal disorders are caused by
errors in the number or structure of chromosomes. 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 non-disjunction of
chromosomes. Group of signs and symptoms that occur together and characterize a
particular abnormality is called a syndrome. In humans, Down’s syndrome,
Turner’s syndrome, Klinefelter's syndrome, Patau’s syndrome are some of the
examples of chromosomal disorders.
Several autosomal
aneuploidies have been reported in human beings. eg. Down’s syndrome
(21-Trisomy), Patau’s syndrome (13-Trisomy).
Trisomic condition of
chromosome - 21 results in Down’s syndrome. It is characterized by severe
mental retardation, defective development of the central nervous system,
increased separation between the eyes, flattened nose, ears are malformed,
mouth is constantly open and the tongue protrudes.
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.
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.
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.
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.
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