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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