Aneupioidy and structural changes
Trisomics (2n = 47) and monosomics (2n = 45) are known in human beings. Trisomy or monosomy may involve a sex chromosome or an autosome.
Aneupioidy involving sex chromosomes. Aneuploid chromosome numbers involving X-chromosome and the resulting phenotypes are listed in Table 24.2.
(a) Turner's syndrome.Turner's syndromes are characterized by monosomy of XO type. These are immature females (sterile) with webbed neck.
(b) Klinefelter's syndrome. Klinefelter's syndromes were characterized by trisomy (XXY).
These are male individuals, who are phenotypically fairly normal but have a very low sperm count and, are therefore sterile. Chromosome constitutions of other Klinefelter's syndromes are given in Table 24.2.
Autosomal aneupioidy. Trisomy for different autosomes is known, which includes the following : (i) trisomy 21, (ii) trisomy 17, (iii) trisomy 18 and (iv) trisomy D (13-15). Mosl frequent autosomal trisomics are those for chromosome 21, perhaps because chromosome 21 is a small chromosome and its addition does not cause lethality. Before trisomy was known, syndromes were described in 1866 by Langdon-Down of England and were popularly called mongolian idiots or Down's syndromes. Phenotypic abnormalities associated with these syndromes included slant of eyes, thick tongue, sagging mouth, unusual palm and feet, obesity and slow mental growth. The life expectancy of these idiots is about 8 to 12 years. Later, it was shown that trisomy 21 had the same phenotypic effect which were described for mongolian idiots or Down's syndromes.
The trisomy for chromosome 17 or for 13-15 (D group) has more serious effect and is, therefore, rarer. Trisomy 17 and trisomy 18 had effects like small mouth, low-set-ears, heart defect, flexion anomaly of fingers and toes and sometimes also syndactyly, malformed chest and webbed neck. The trisomic for D group (13-15) has heart defect, polydactyly, mental retardation, harelip, cleft palate and severely defective eyes.
Structural changes. In certain cases of chromosomal aberrations, the chromosome number does not change and remains 2n = 46, but there may be structural changes involved. One of these structural changes may lead to Down's syndrome. In some such cases, it was discovered that extra chromosome 21 (in trisomy 21) was attached to another perhaps acrocentric chromosome of the D group or the G group, so that although it is a trisomic in chromatin content but has a chromosome number 2n = 46. Another translocation 13-22 gives rise to 2n = 45 and leads to delayed development of speech and to a low I.Q.
A large number of other structural changes in the chromosomes leading to a variety of phenotypic effects are known in human beings.
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