- It is caused due to absence or excess or abnormal arrangement of one or more chromosomes.
- Aneuploidy – failure of segregation of chromatids during cell division cycle results in the gain or loss of chromosome.
- Polyploidy – Failure of cytokinesis after telophase stage of cell division results in an increase in a whole set of chromosomes in an organism.
- Down’s syndrome – gain of extra copy of chromosome 21 (trisomy 21)
- It was first described by Langdon Down (1866).
- Affected individual is short with small round head, furrowed tongue and partially open mouth.
- Broad palm with characteristic palm crease.
- Physical, psychomotor and mental development is retarded.
Pic shows baby suffering from Down’s syndrome.
- Turner’s syndrome – loss of an X-chromosome in human females i.e. 45 with XO
- Such females are sterile as ovaries are rudimentary
- Klinefelter’s syndrome – presence of an additional copy of X-chromosome resulting into karyotype, 47, XXY.
- Sterile individuals