Aneuploidy is the term applied for the chromosomal mutations involving only a part of a set, i.e., loss (hypoploidy) or addition (hyperploidy) of one or more chromosomes. Aneuploidy may result from non disjunction of chromosome during cell division.
(1) Monosomy : Diploid organism that are missing one chromosome of a single pair with genomic formula 2n � 1. Monosomics can form two kind of gametes, (n) and (n �1).
(2) Nullisomy : An organism that has lost a chromsome pair is nullisomic. The result is usually lethal to diploids (2n � 2).
(3) Trisomy : Diploids which have extra chromosome represented by the chromosomal formula 2n + 1. One of the pairs of chromosomes has an extra member, so that a trivalent may be formed during meiotic prophase.
(4) Tetrasomy : In tetrasomic individual particular chromosome of the haploid set is represented four times in a diploid chromosomal complement. The general chromosomal formula for tetrasomics is 2n + 2 rather than 2n + 1+ 1. The formula 2n + 1 + 1 represents a double trisomic.
# Aneuploidy may be of following types on the basis of chromosomes involved in non disjunction.
(a) Aneuploidy involving non-disjunction in sex chromosomes : This kind of aneuploidy is brought about due to non-disjunction in sex chromosomes. It may lead to following types of syndromes :
(1) Turner�s syndrome : Such persons are monosomic for sex chromosomes i.e. possess only one X and no Y chromosome (XO). In other words they have chromosome number 2n � 1 = 45. They are phenotypic females but are sterile because they have under developed reproductive organs. They are dwarf about 4 feet 10 inches and are flat chested with wide spread nipples of mammary glands which never enlarge like those in normal woman. They develop as normal female in childhood but at adolescence their ovaries remain under developed. They lack female hormone estrogen. About one out of every 5,000 female births results in Turner�s syndrome.
(2) Klinefelter�s syndrome : Since 1942, this abnormality of sex is known to geneticists and physicians. It occurs due to Trisomy of sex chromosomes which results in (XXY) sex chromosomes. Total chromosomes in such persons are 2n + 1 = 47 in place of 46. Klinefelter (1942) found that testes in such male remain under developed in adulthood. They develop secondary sex characters of female like large breasts and loss of facial hair. Characters of male develop due to Y chromosome and those like female due to XX chromosomes. About one male child out of every 5,000 born, develops Klinefelter�s syndrome.
Such children are born as a result of fertilization of abnormal eggs (XX) by normal sperms with (X) or (Y) chromosomes or by fertilization of normal eggs with (X) chromosomes by abnormal sperms with (XY) chromosome. They are sterile males mentally retarded and are eunuchs.
(b) Aneuploidy involving non-disjunction in autosomes : This type of aneuploidy occurs due to trisomy of autosomes. In any particular autosomal pair, having 3 instead of normal 2 chromosomes. Such persons may be males 45 + XY = 47(2n + 1) or females 45 + XX =47(2n + 1). On the basis of the number of the autosome pair affected by trisomy, they can be of following types.
(1) Down�s syndrome : This autosomal abnormality is also known as Mongolian idiocy or mongolism. In Langdon Down of England (1866) studied the Mongolian idiocy and described the trisomic condition of their chromosomes. Down�s syndrome, a very common congenital abnormality arises due to the failure of separation of 21st pair of autosomes during meiosis. Thus an egg is produced with 24 chromosomes instead of 23. A Down�s syndrome has 3 autosomes in 21st pair instead of 2. Total number of chromosomes in this case is 2n + 1 (21st) = 47.
The affected children have a very broad fore head, short neck, flat palms without crease, stubby fingers, permanently open mouth, projecting lower jaw and a long thick extending tongue. They have low intelligence and are short heighted. They have defective heart and other organs. They are born to mothers aged 40 year and above during first pregnancy. They may survive upto 20 years under medical care.
They are called mongolian idiots because of their round, dull face and upper eyelids stretched downwards similar to mongolian race.
(2) Edward�s syndrome : This autosomal abnormality occurs due to trisomy of eighteenth pair of autosomes in which the number of chromosomes are 2n + 1 = 47. The child with this defect survives only about 6 months. Such children have defective nervous system, malformed ears and a receding chin.
(3) Patau�s syndrome : This is trisomy of thirteenth pair of autosomal chromosome. This trisomic condition involves numerous malformations such as harelip, clefted palate and cerebral, ocular and cardiovascular defects. Such children usually survive for about 3 months only.
Aneuploidy is the term applied for the chromosomal mutations involving only a part of a set, i.e., loss (hypoploidy) or addition (hyperploidy) of one or more chromosomes. Aneuploidy may result from non disjunction of chromosome during cell division.
(1) Monosomy : Diploid organism that are missing one chromosome of a single pair with genomic formula 2n � 1. Monosomics can form two kind of gametes, (n) and (n �1).
(2) Nullisomy : An organism that has lost a chromsome pair is nullisomic. The result is usually lethal to diploids (2n � 2).
(3) Trisomy : Diploids which have extra chromosome represented by the chromosomal formula 2n + 1. One of the pairs of chromosomes has an extra member, so that a trivalent may be formed during meiotic prophase.
(4) Tetrasomy : In tetrasomic individual particular chromosome of the haploid set is represented four times in a diploid chromosomal complement. The general chromosomal formula for tetrasomics is 2n + 2 rather than 2n + 1+ 1. The formula 2n + 1 + 1 represents a double trisomic.
# Aneuploidy may be of following types on the basis of chromosomes involved in non disjunction.
(a) Aneuploidy involving non-disjunction in sex chromosomes : This kind of aneuploidy is brought about due to non-disjunction in sex chromosomes. It may lead to following types of syndromes :
(1) Turner�s syndrome : Such persons are monosomic for sex chromosomes i.e. possess only one X and no Y chromosome (XO). In other words they have chromosome number 2n � 1 = 45. They are phenotypic females but are sterile because they have under developed reproductive organs. They are dwarf about 4 feet 10 inches and are flat chested with wide spread nipples of mammary glands which never enlarge like those in normal woman. They develop as normal female in childhood but at adolescence their ovaries remain under developed. They lack female hormone estrogen. About one out of every 5,000 female births results in Turner�s syndrome.
(2) Klinefelter�s syndrome : Since 1942, this abnormality of sex is known to geneticists and physicians. It occurs due to Trisomy of sex chromosomes which results in (XXY) sex chromosomes. Total chromosomes in such persons are 2n + 1 = 47 in place of 46. Klinefelter (1942) found that testes in such male remain under developed in adulthood. They develop secondary sex characters of female like large breasts and loss of facial hair. Characters of male develop due to Y chromosome and those like female due to XX chromosomes. About one male child out of every 5,000 born, develops Klinefelter�s syndrome.
Such children are born as a result of fertilization of abnormal eggs (XX) by normal sperms with (X) or (Y) chromosomes or by fertilization of normal eggs with (X) chromosomes by abnormal sperms with (XY) chromosome. They are sterile males mentally retarded and are eunuchs.
(b) Aneuploidy involving non-disjunction in autosomes : This type of aneuploidy occurs due to trisomy of autosomes. In any particular autosomal pair, having 3 instead of normal 2 chromosomes. Such persons may be males 45 + XY = 47(2n + 1) or females 45 + XX =47(2n + 1). On the basis of the number of the autosome pair affected by trisomy, they can be of following types.
(1) Down�s syndrome : This autosomal abnormality is also known as Mongolian idiocy or mongolism. In Langdon Down of England (1866) studied the Mongolian idiocy and described the trisomic condition of their chromosomes. Down�s syndrome, a very common congenital abnormality arises due to the failure of separation of 21st pair of autosomes during meiosis. Thus an egg is produced with 24 chromosomes instead of 23. A Down�s syndrome has 3 autosomes in 21st pair instead of 2. Total number of chromosomes in this case is 2n + 1 (21st) = 47.
The affected children have a very broad fore head, short neck, flat palms without crease, stubby fingers, permanently open mouth, projecting lower jaw and a long thick extending tongue. They have low intelligence and are short heighted. They have defective heart and other organs. They are born to mothers aged 40 year and above during first pregnancy. They may survive upto 20 years under medical care.
They are called mongolian idiots because of their round, dull face and upper eyelids stretched downwards similar to mongolian race.
(2) Edward�s syndrome : This autosomal abnormality occurs due to trisomy of eighteenth pair of autosomes in which the number of chromosomes are 2n + 1 = 47. The child with this defect survives only about 6 months. Such children have defective nervous system, malformed ears and a receding chin.
(3) Patau�s syndrome : This is trisomy of thirteenth pair of autosomal chromosome. This trisomic condition involves numerous malformations such as harelip, clefted palate and cerebral, ocular and cardiovascular defects. Such children usually survive for about 3 months only.
