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Down Syndrome's

• It is one of the most familiar conditions in humans. Down's syndrome results from aneuploidy-trisomy 21.,.Hence the person with Down's syndrome has 47 instead of 46 chromosomes because of the presence of one extra chromosome number 21. This is the reason why Down's syndrome can also be represented as (47, +21). Originally studied by Langdon Down in 1866. it had been termed Mongoloid Idiocy or Mongolism because individuals with trisomy 21 were considered to have certain facial characteristics that resembled oriental features. The condition is now referred to as Down's syndrome which removes the nuances of associating mental retardation with any particular race of people.
• The presence of air extra copy of one of the smallest chromosomes in trisomy 21 results in a wide variety of abnormalities in almost every area of the body, differing in combinations and degree of severity from patient to patient. One of the most constant characteristics which allows doctors to identify Down's syndrome in very young children, even in new born babies, is the line found on the palms of both hands, which are known as dermatoglyphic patterns. The presence of a "simian crease" and a tendency for every finger to have a loop in the print region rather than whorls or arches, distinguish the Down's child from a normal child.
• Because of various defects, such as in the cardiovascular system, the life expectancy of an individual with Down's syndrome is shorter than normal. One study sets the life expectancy at 16 years.
• Other phenotypic characteristics of this condition are short stature (about 4 feet tall] and an epicanthic fold (thus the earlier name "mongolism"), broad short skulls, wide nostrils, large tongues with distinctive furrowing stubby hands (particularly the fifth digit) and a general loose jointedness, particularly in the ankles. The patients are characterized as low in mental ability, but they can Be trained in routine mechanical skills. Through the investigation of J. Lejeune in 1959 Down's syndrome became the first chromosomal disorder to be described human.
• The incidence of Down's syndrome increases dramatically with maternal age. This increase is thought to result from the age of the egg cell, that is fertilized to produce the zygote. The human female is born with an estimated 400,000 gametes and will not produce any new ones in her lifetime. Therefore, if a woman is 45 years old, her gametes are also 45 years old. Increasing age is believed to result in a tendency for a particular aberration of meiosis called Nondisjunction to occur. There has been little correlation found between the incidence of Down's syndrome and paternal age.

Nondisjunction

• Nondisjunction can occur before either the first or the second meiotic division. The basic event in Nondisjunction is just what the term implies: The chromosomes do not separate i.e., they, do not "disjoin". This results in aneuploidy in the daughter cells.
  Gamete containing the extra chromosome will produce an individual with Down syndrome.
• Nullisomic gametes (gametes that are missing a particular chromosomes altogether) are presumably nonfunctional or Iethal. If nondisjunction occurs in germ cells that are normal, it is called Primary nondisjunction. If however, a trisomic individual reproduces the nondisjunction resulting in aneuploidy in the germ cells is called Secondary nondisjunction.
• Down syndrome occurs once in about 700 live births in Europe alone. About one in six of the Down's syndrome children born alive die within the first year. The average expectation of life is 16.2 years. The cost of training and maintaining Down's syndrome cases in the United States is estimated at $1 Billion per yean Emotional stress in families with Down's syndrome children and adults is also a factor in their care. The need for effective counseling and prevention is readily apparent.

Amniocentesis for detecting Aneuploidy

• Chromosomal abnormalities are sufficiently well understood to permit genetic counseling. A fetus may be checked in early stages of development by karyotyping the cultured cells obtained by a process called amniocentesis. A sample of fluid is withdrawn with a needle from the amniotic sac. Fetal cells ate cultured and, after a period of two to three weeks, chromosomes in dividing cells can be stained and observed, if three number 21 chromosomes are present, Down’s syndrome is confirmed. The risk for mothers less than 25 years of age to have the trisomy is about 1 in 1500 births; at the age of 40,1 in 100 births; at 45,1 in 40 births. Pregnant women above 40 years are thus the special high risk groups.

Patau Syndrome

• This syndrome was described by K. Patau in 1960. It results when there is a trisomy of chromosome number 13. It can also be represented by its karyotype (47, +13). Patau syndrome occurs in about 1 in 20,000 newborns. It is rare in children and non-existing in adults because the severe symptoms result in early death. Most of the deaths occur within the first three months after birth, but a few victims have lived for as long as five years.
• The symptoms of the syndrome include small brain, apparent mental deficiency, deafness and numerous other external and internal abnormalities.

Edward Syndrome

• It is first described by J.H. Edwards and his colleagues in 1960. It is the result of trisomy of chromosome number 18. Thus its karyotype is (47, +18).
• The symptoms include mental deficiency and multiple congenital malformations involving virtually every organ system. Most infants with this syndrome die at an early age, some 90 percent within their first six months. Nearly all are deceased before they reach one year, but a few have been reported to be alive in their teen years.
• The incidence of Edward's syndrome is 1 in 8000 Thus far, only, a few cases have been observed, and it is not known whether racial or other population groups differ in incidence. In general, the incidence of this deformity is greater among infants of older women, as expected if the cause of this trisomy is primary nondisjunction in meiosis.

Crui-De-Chat Syndrome

• This is a syndrome resulting from the deletion chromosomal aberration. Chromosomal deletions are usually lethal resulting in zygotic loss, still births, or infant deaths. Sometimes infants with small chromosome deficiencies, however, survive long enough to permit observation of some of the abnormal phenotypes they express.
• L. Lejeune and his colleagues discovered a chromosome deficiency in humans that has been associated with the term Crui-de-chat syndrome. The name of this syndrome came from a plaintive catlike mewing cry from small weak infants with he disorder. Other characteristics are microcephaly (small head), broad face and saddle nose, widely spaced eyes with epicanthic folds, unique facial features, and physical and mental retardation. Is of Crui de chat children studied are in the range of 20-40. The chromosome deficiency is in the short arm of chromosome 5 and is designated 5g-,, A karyotype (46,XX,5p-) is represented for the patient.         
• Crui-de-chat patients die in infancy or early childhood and do not transfer the chromosome deletion to offspring, this chromosome deficiency however has been shown by Lejeune and others to become involved sometimes in a reciprocal translocation and thus to be transmitted. When the short arm of chromosome 5 became translocated to chromosome 15, the heterozygous translocation was carried in a normal healthy parent.

Conclusion

The text discusses various chromosomal disorders, including Down Syndrome, Patau Syndrome, Edward Syndrome, and Cri-du-Chat Syndrome. These disorders arise from abnormalities in chromosome numbers, such as trisomy (extra chromosome) or deletion (missing part of a chromosome). The severity and symptoms of these conditions vary, with many affected individuals having shorter life expectancies, mental and physical disabilities, and abnormal physical features. The occurrence of these disorders is often influenced by maternal age, with older women being at higher risk. Genetic counseling and amniocentesis can be used to detect chromosomal abnormalities during pregnancy, allowing for informed decisions and early intervention.