B_T_2015_02_

• Important examples of trisomy in human beings with
2 n = 47 are :

(i) Down’s syndrome or Mongolian idiots (Mongolism)

• This is autosomal trisomic for chromosome 21,
which was reported by Langdon Down of England
in 1866.

• These are short-statured, having heavy eyelids and
heavy tongue, low mental ability and life span of 8–12
years.

(ii) Klinefelter’s syndrome (trisomy of XXY type)

• These are normal males, but having low sperm count,
so sterile.

(iii) Patau’s syndrome is trisomy of 13 th chromosome.

2. tetrasomy

• Organisms having genetic constitution as 2n + 2,

i.e., one homologous pair of extra chromosomes, or a

particular chromosome is present four times.

•    Both trisomics and tetrasomics are believed to have

given rise to new species through secondary polyploidy

e.g., apple, pear.

3. Pentasomy
   • XXXXX syndrome or pentasomy X is the presence
   of three additional X chromosomes.
   • Diagnosis is done by karyotyping. This condition was
   first described in 1963.
   • Approximately 25 females have been described in
   medical literature worldwide with this extremely rare
   condition.

mixed aneuploids

•    They are aneuploids with both hypoploidy and

hyperploidy, e.g., 2n + 1A – 1B.

Chromosome

nomenclature

Chromo-

some

formula

Clinical

syn-

drome

Estimated

frequency at

birth

Main phenotypic characteristics

47; + 21 2 n + 1 Down 1/700 Short broad hands with Simian-type palmar crease,

short stature, hyperflexibility of joints, mental

retardation, broad head with round face, open

mouth with large tongue, epicanthal fold.

47; + 13 2 n + 1 Patau 1/20,000 Mental deficiency and deafness, minor muscle

seizures, cleft lip and/or palate, polydactyly, cardiac

anomalies, posterior heel prominence.

47; + 18 2 n + 1 Edward 1/8000 Multiple congenital malformation of many organs;

low-set, malformed ears; receding mandible, small

mouth and nose with general elfin appearance;

mental deficiency; horseshoe or double kidney;

short sternum, 90% die in the first 6 months.

45; X 2 n – 1 Turner 1/2500 female births Female with retarded sexual development, usually

sterile, short stature, webbing of skin in neck region,

cardiovascular abnormalities, hearing impairment.

47; XXY

48; XXXY

48; XXYY

49; XXXXY

50; XXXXXY

2 n + 1

2 n + 2

2 n + 2

2 n + 3

2 n + 4

Klinefelter 1/500 male births Male, subfertile with small testes, developed

breasts, feminine pitched voice, long limbs, knock

knees, rambling talkativeness.

47; XXX 2 n + 1 Triple X 1/700 Female with usually normal genitalia and limited

fertility. Slight mental retardation.

Table : Aneuploidy resulting from nondisjunction in the human population.