Changes in chromosomal number can occur as a result of the addition of all or part of a chromosome, the loss of an entire set of chromosomes (monoploidy), or the gain of one or more whole sets of chromosomes (aneuploidy) (euploidy). Each of these circumstances deviates from the usual diploid chromosomal count.

1. {
Variation in Chromosome Number
- Aneuploidy -

2.  Variation in the number of particular chromosomes
within a set
 Aneuploid variations are regarded as abnormal
conditions
 Failure of chromosome division extra or
absence of one chromosome
Aneuploidy

3.  When observed in single set of chromosome
Hypoploidy
 Aneuploidy
Hyperploidy
 Hypoploidy – Loss of one or more chromosome. It is
of two types;
 Monosomy (2n – 1): It is due to loss of a single chromosome. They
produce two types of gametes, one with n chromosomes and
another with (n – 1) chromosomes. Monosomy causes lethality in
diploids but has no effect in polyploids.
 Nullisomy (2n – 2): Loss of a pair of chromosome. It causes
lethality.
Aneuploidy

4.  Hyperploidy – Gain of one or more chromosomes.
 Trisomy (2n+1): Gain of a single chromosome. In humans trisomy causes three types
of syndromes;
 A. Down's syndrome: It is caused by trisomy of 21st chromosome. It occurs in 1/650
births. It is also called mongolism. The children are mentally retarded, large tongue,
small ears, short stature and enlarged liver and spleen.
 B. Edwards syndrome: It is caused by trisomy of 18th chromosome. It occurs in
0.3/1000 births. The symptoms are mental retardation, cardiac malformations; and
deformity in skull, face and feet, Harelip and cleft palate often occurs. Death occurs
around 3 to 4 months.
 C. Patau syndrome: Trisomy of 13th chromosome. It occurs in 0.2/1000W births. The
symptoms are mentally retardation, hands and feet are deformed, heart and kidney
defects, death occurs within hours or days of birth.
Aneuploidy

5.  2. Double trisomy (2n+1+1): Gain of two non
homologous chromosomes. It causes lethality.
 3. Tetrasomy (2n+2): Gain of a pair of a chromosome.
It causes lethality
 NUMERICAL ABERRATIONS
• When there is increase in set of homologous
chromosome > POLYPLOIDY
• Triploid- 3n
• Tetraploid- 4n
• Pentaploid- 5n
Aneuploidy

6. Aneuploidy
 Meiotic Nondisjunction and Down Syndrome (>90% of Down's cases)
 • Nondisjunction refers to the failure of chromosomes to
segregate properly during anaphase
 • Meiotic nondisjunction can produce haploid gametes
that have too many or too few chromosomes
– If such a gamete participates in fertilization the
resulting individual will have an abnormal chromosomal
composition in all of its cells
 Down syndrome is caused by the meiotic nondisjunction
of chromosome 21

7.  Nondisjunction in Meiosis I
Aneuploidy

8.  Nondisjunction in Meiosis II
Aneuploidy

9. Aneuploidy

10. {
Variation in Chromosome
Number
Euploidy

11. Euploidy
 Euploidy is the variation in the chromosome number that
occurs due to increase or decrease of full set of
chromosomes.
 Types of Euploidy:
 Monoploidy
 Diploidy
 Polyploidy

13. Euploidy
 A genome that contains three or more full copies of
the haploid chromosome number are polyploidy
 Polyploidy means there will be a change in the
number of chromosome in set so instead of 2n we
can have one extra chromosome per set.
 It is commonly noticed in plants and rare in
animals.

14. Euploidy
 Autopolyploid - an individual that has an
additional set of chromosomes that are identical to
parental species; an autotriploid would have the
chromosomal composition of AAA and
an autotetraploid would be AAAA; both of these
are in comparison to the diploid with the
chromosomal composition of AA
 Addition of one or more haploid set of its own
genome in an organism results in autopolyploidy.
Watermelon, grapes and banana are autotriploids,
whereas apple is an autotetraploid.

16. Euploidy
 Allopolyploid - an individual that has an
additional set of chromosomes derived from
another species; these typically occur after
chromosomal doubling and their chromosomal
composition would be AABB; if both species have
the same number of chromosomes then the derived
species would be an allotetraploid

18.  Significance of ploidy:
 1. Polyploidy plays an important role in plant breeding and
horticulture.
 2. Polyploidy has more vigorous effect than the diploids and
results in the production of large sized flowers and fruits.
Hence, it has economical significance.
 3. It plays significant role in the evolution of new species.
 4. Polyploidy results in the changes in the season of flowering
and fruiting.
 5. Polyploids are vigorous invaders of new habitats.
 6. It leads to the formation of new varieties which show high
resistance to disease and increase in yield.
 7. Tetraploid cabbages and tomatoes contain more ascorbic
acid whereas tetraploid corn contains more vitamin A.
 8. Both euploidy and aneuploidy in man cause congenital
diseases.
 9. Polyploidy varieties like apple, pear, grape and
watermelons are cultivated because of their large size.

19. Euploidy
 An autotriploid could occur if a normal gamete (n) unites
with a gamete that has not undergone a reduction and is thus
2n. The zygote would be 3n. Triploids could also be produced
by mating a diploid (gametes = n) with a tetraploid (gametes
= 2n) to produce an individual that is 3n. The difficulty arises
when autotriploids try to mate because unbalanced gametes
are produced because of pairing problems with the additional
chromosome set. Thus, these are invariably sterile.
 Autotetraploids occur from a doubling of the chromosomal
composition. This can occur naturally by doubling sometime
during the life cycle or artificially through the application of
heat, cold or the chemical colchicine. Because an additional
set of chromosomes exists, autotetraploids can (but not
necessarily in all cases) undergo normal meiosis.