hypoploidy
• Hypoploidy is further of two types : monosomy and
nullisomy.
- monosomy
• The organisms having genetic constitution as 2 n – 1
i.e., lacking one complete chromosome, are called
monosomics, and this phenomenon is known as
monosomy.
• Number of monosomics possible in an organism is
equal to its haploid number of chromosomes, e.g., 21
monosomics were produced in wheat by E.R. Sears
(as here n = 21).
• 2 n – 1– 1 (not 2n – 2) are called double
monosomics (i.e., the two chromosomes removed
are not homologous).
• Important example of monosomy in human beings is
‘Turner’s syndrome’, which is monosomic of XO
type. These are sterile females having webbed
neck.
- nullisomy
• Organisms having genetic constitution as 2 n – 2,
i.e., lacking a pair of homologous chromosomes,
are called nullisomics, and this condition is called
nullisomy.
• Number of nullisomics possible in a plant is again
equal to its haploid number of chromosomes.
• Nullisomics do not survive, except among polyploids.hyperploidy
• Hyperploidy is of the following three types : trisomy,
tetrasomy and pentasomy.- trisomy
• Organisms having genetic constitution as 2 n + 1, i.e.,
having one extra chromosome, are called trisomics,
and this condition is called trisomy.
• Double trisomic has two different chromosomes in
triplicate (2n + 1 + 1).
• Number of possible trisomics in an organism is equal
to its haploid number of chromosomes.
• Trisomics were first of all obtained by Blakeslee
et al. in Jimson weed (Datura stramonium).
nondisjunction
Nondisjunction is the failure of chromosomes or chromatids to disjoin and move to opposite poles during division. When
this occurs in meiosis, the normal distribution of chromosomes into gametes is disrupted.
The results of nondisjunction during meiosis I and meiosis II from a single chromosome of a diploid organism are shown
below: