Describe Aneuploidy and Euploidy in relation to changes in the chromosome number
Change in
chromosome number occur occasionally which produces a variety of effects.
Aneuploidy in change in number of chromosomes which can be either due to loss
of or more chromosomes or due to addition or deletion of one or more
chromosomes. It leads to variation in chromosome number and do not involve the
whole karyotype. The nuclei of Aneuploids contain chromosomes whose number is
not true multiple of the basic number (n).
The Aneuploidy
arises due to non disjunction. The loss of one chromosome produces a Monosomic
(2n-1) and the condition is termed as Monosomy. The gain of one chromosome
produces a Trisomic (2n+1) and the condition is known as Trisomy. In the same
way the addition of two or more chromosomes is respectively known as Tetrasomy
and Pentaasomy, the individuals are known as Tetrasomic and Pentasomic. In some
cases a pair of homologous chromosomes is lost (2n-2); such individuals are
termed as nullisomic and the condition is called Nullisomy.
Effects of
Aneuploidy are
A. MONOSOMY
The monosmoic
lack a complete chromosome which creates genetic imbalance as the expression of
only one allele at each locus of the chromosomes is inadequate. The expression
of a genetic information during early development is very delicately regulated
so that a sensitive balance of gene product is required to ensure normal
development. Due to loss of one chromosome the equilibrium is not achieved
therefore monosomy is not tolerated in diploids.
In many plants
monosomy is observed such as maize, tobacco, the evening prim rose and
oenothera. Such monosomic plants are usually less viable than their diploid
derivatives. The monosomic can easily be produced in polyploids. A polyploid
has several chromosomes of same type, therefore the loss becomes viable.
The number of
possible monosomics in an organism will be equal to haploid chromosome number.
In common wheat since 21 pairs of chromosomes are present, 21 possible
monosomics are known. Monosomics were also isolated in cotton (2n=52) and in
Tobacco (2n=48). In tomato which is a diploid (2n=24), rarely monosomics could
be produced similarly monosomics have been produced in diploid maize.
Double
monosomics (2n-1-1) i.e. loss of two chromosomes, but differ from nullisomics
in respect that the chromosomes lost are non homologous. Also triple monosomics
(2n-=-1-1-1) could be produced in polyploids like wheat.
B. TRISOMY
The addition of
extra chromosome produces somewhat more viable individuals in both animals and
plants than does the loss of a chromosome.
As in monosomy
the sex chromosome variation of trisomic type has less drastic effect on the
phenotype that autosomal variation. Drosophilla females with three x
chromosomes and a normal complement of autosomes (3x: 2A) may be fertile but
less viable than normal (2x: 2A) females.
In some plants
trisomic individuals are viable but their phenotype may become changed e.g. the
diploid number of Jimson weed (Datura) in 24. Twelve different primary trisomic
chromosomes are recognised in Jimson weed. Each kind of trisomy alters the
phenotype. Secondarytrisomics, the chromosome is an isochromosome. Tertiary
trisomics are also possible but rarely. The extra chromosome in tertiary
trisomics is produced in the result of translocation.
Trisomy usually
originates spontaneously due to production of n+1 type of gametes due to non
disjunction of a bivalent. The trisomic are more often produced artifsicially
by selfing triploids or by crossing triploid females with diploid males (3X ×
2X). In plants and animals the trusomy may be delted during cytological
observation of meiosis. As there copies of a chromosome are present, pairing of
configurations are different. In some cases only two of these there homologues
may synapse. At regions different members of trio may be paired and are known
as Trivalents. In some cases one bivalent and one univalent may be present. The
trivalent is usually arranged on the spindle so that during avaphase one member
moves to one pole and two go to opposite pole. Trisomis are used to locate
genes on specific chromosomes. If a particular gene is located on the
chromosome involved in trigomy, segregation in the progemy of this trisomic
will not follow a Mendelian pattern but the ratio will deviate from normal 3:1
and 1:1 test cross ratios.
C. Tetrasomy:
They have
particular chromosome, there are at least tetrasomics available in wheat. E. R
sears was able to synthesize a complete set of compensating Nullisomic Tetra
somic (2n + 2 + 2) where addition of a pair or homologous chromosomes would
compensate for the loss of another pair of homologous. Sun non homologous
chromosomes are able to compensate for each other are called Homoeologous
chromosomes.
2) Euplody is a
condition where one or more full sets of chromosomes are present in an
organism. Euploids may be Monoploids, Diploids of Polyploids.
Monoploids can
be distinguished from haploids as they have a single basic set of chromosomes
as in Barly 2n = x = 7 or in corn 2n = x = 10 while the Haploids have half the
somatic number of chromosomes found in normal individual i.e. each chromosome
is represented once. In some cases as in the male insects the haploid are
produced due to parthenogenesis. In these insects the queen and drones are
diploid females. The haploids also originate due t development of egg
parthenogenetically in flowering plants such as Tomatoes and Cotton. The
haploids also originate from pollen tube rather than form egg, synergids or
antipodals of the embryo sac. Such haploids are known as Androgenic Haploids.
Haploids can be
produced artificially by any one of the following methods.
(1) X-ray Treatment (2) Delayed Pollination
(3) Temperature shock (4) Colchicine
treatment
(5) Distant hybridization (6) Anther or pollen culture
SIGNIFICANCE OF
EUPLOIDY:
Haploids are
characterized by reduction in size of all vegetative and floral parts than a
diploid. The haploids are used in production of homozygous diploid as haploids
can be doubled by colchicines treatment. These homozygous diploids are used for
cultivation e.g. rice, wheat and Tobacco.
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