Development of cytogenetic techniques to analyze chromosomes led to discovery and characterization of chromosome disorders (1950s)

Within years (from 1956), the cause of Down syndrome (47,XX/XY+21) Turner syndrome (45,X) and Klinefelter sysndrome (47,XXY) was established

Now more than 20,000 chromosome abnormalities have been reported but these conditions are very rare in the population

Chromosome abnormalities are a major cause for early pregnancy loses

1. Numerical abnormalities (Abnormal changes in the chromosome number in a

cell)

2. Structural abnormalities (abnormalities of the chromosome structure

3. Others

Normal Gametes Mono somic

gametes Nullisomic gametes

Occurs with advancing age; especially maternal age

Maternal oocytes have a long delay between different phase of the cell division

With advancing age, spindle formation become defective and leads to non dysjunction

A. Aneuploidy (loss or gain of one or more chromosomes)

1. Monosomies (loss of a single chromosome)1. 45,X - Turner syndrome (loss of one sex chromosome)2. Monosomy of autosomes are incompatible with life

2. Trisomies (Presence of an extra chromosome)1. Trisomy 21 (Down’s syndrome)2. Trisomi 18 (Edward’s syndrome)3. Trisomi 13 (Patau Syndrome)4. XXY (Klinefelter syndrome)5. XXX (XXX females)6. XYY (XYY males)

3. Tetrasomies (Presence of two extra chromosome)

B. Polyploidy

(Addition of one or more haploid set of chromosomes)

Triploidy (3n 46+23=69) Tetraploidy (4n 46+46=92)

Retension of polar bodies or fertilization with 2 or 3 sperms can lead to polyploidy

Ovum polar body

46

23

23

23

23

23

23

23

23

Tetraploidy (23+23+23+23=92)Triploidy (46+23=69) Triploidy (23+23+23=69)

Turner syndrome Karyotype is 45,X

Down SyndromeTrisomy 21 (47,XX/XY+21)

Edward syndrome Trisomi 18 (47, XX/XY, +18)

Patau syndrome Trisomi 13 (47, XX/XY, +13)

Klinefelter syndrome (47, XXY) Slightly femine figure Has low IQ andLearning difficulty Infertile

XXX females (47,XXX) Looks normal in phenotype Slightly less IQ Fertile

XYY males Tall Aggressive Low IQ and learning difficulties Normal fertility

1. Translocations2. Deletions3. Insertions4. Inversions5. Rings chromosmoes6. Isochromosomes

is caused by an error in cell division very early in the development of the unborn baby due to chromosome abnormality

chimera or chimaera is a single organism composed of two or more genetically distinct cellspopulations from different zygotes