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• This phenomenon has occurred in the phylogeny of
many species, and has led to a reduction in the number
of chromosomes. This mechanism has played an
important part in the evolution of many species.

• Centric fusion has also occurred during the phylogeny
of Drosophila, grasshoppers, reptiles, birds, mammals,
and other groups.

• In human beings also, chromosome number 2 has
been formed by centric fusion of two chromosomes
of primate ancestors.

• Myelogenous leukemia is caused by translocation of
a part of long arm of chromosome 22 (Philadelphia
chromosome) to chromosome 9.

• Translocation was first of all observed in evening
primrose (Oenothera) and it was considered as
mutation by Hugo de Vries initially.

• Translocations in heterozygous condition are reported
in Oenothera, Rhoeo and Tradescantia.

•    Translocations and inversions are both associated with

poor seed setting and considerable semisterility.

Changes in number of Chromosomes

•    Each organism has a definite number of chromo-

somes.

•    Sometimes, however the number of chromosomes

is changed, causing numerical changes in

chromosomes, which can be diagnosed by prenatal

tests as amniocentesis and chorionic villus sampling.

•    There are two main kind of changes in the number

of chromosomes. These result in euploids and

aneuploids.

•    In euploids, the change is in the haploid number, but

the entire set of chromosomes is kept balanced.

•    In aneuploids, there is a loss or gain of one or

more chromosomes, causing the set to become

unbalanced.

aneuploidy

• If the number of chromosomes in an organism is not

a whole number multiple of the basic chromosome

number, it is called aneuploidy and such organisms

are called aneuploids.

• Hence, if basic chromosome no. x = 7 and 2n or

diploid no. = 14, then 2n = 15, 2n = 16, 2n = 13,

2 n = 12 are all aneuploids.

• Aneuploidy is produced by a failure in the separation of

chromosomes during meiosis, called nondisjunction.

•    Aneuploidy may also occur in somatic cells, in which

the nondisjunction occurs at mitosis.

•    This process may produce individual cells in various

tissues or parts of the body that have different

chromosome numbers, e.g., in mosaic individuals,

variegations, and gynandromorphs.

•    Aneuploidy is of two types – hypoploidy or loss

of chromosomes, and hyperploidy or addition of

chromosomes.

amniocentesis

•    A genetic counselor may recommend a prenatal

diagnostic technique, where fetal cells are isolated and

cultured.

•    Amniocentesis and chorionic villus sampling (CVS) are

the two most familiar approaches, whereby fetal cells

are obtained from the amniotic fluid or the chorion of

the placenta, respectively.

•    Once fetal cells are obtained, the karyotype can then

be determined by cytogenetic analysis.

•    If the fetus is diagnosed as having Down’s syndrome, a therapeutic abortion is one option currently available

to parents.

•    Obviously, this is a difficult decision involving a number of religious and ethical issues.

Biochemical

analysis

Chromosomal

analysis

Biochemical

Amniotic cavity analysis

Fetal cells

Amniocentesis

Placenta

Uterine

wall

Fetal cells Fetal cell culture

Fig.:Amniocentesis and procedure for prenatal diagnosis of

biochemical and chromosomal disorders.

Centrifuge