Genome – A complete (monoploid or basic) set of chromosome of a diploid species. A deviation from diploid state represent NCA or heteroplo...
Genome – A complete (monoploid or basic) set of chromosome of a diploid species.
A deviation from diploid state represent NCA or heteroploidy.
1. Euploidy
2. Aneuploidy
1. Euploidy
Eu=true; ploidy=unit
Change in entire set of chromosome or genome.
Have one or more complete genome may be identical or distinct from each other.
I. Monoploidy and haploidy
Monoloids – single copy of single genome
Contain single chromosome set.
Monoploids are haploid of true diploids (two set of chromosome)
Have basic chromosome number (x) of a species.
Characteristically sterile.
Haploid – Individuals or tissue with gametic chromosome number (n).
It is a gametic chromosome number of species irrespective whether it is a diploid or polyploids.
Sr. no. | Monoploids | Haploids |
1 | Represents gametic chromosome no. of a diploid species. | Represent gametic chromosome no. of any species. |
2 | Denoted by ‘x’ | Denoted by ‘n’ |
3 | Monoploid are always haploid | Haploids can’t always be monoploids |
4 | Contain single set of genome. | May contain one or more copies of genome |
Eg:
Maize 2n=20 x=10 n=10
Wheat 2n=42 x=7 n=21
Origin/production –
Arise spontaneously at low frequency – tomato, cotton, coffee, wheat
These haploids develop from unfertilized egg cell (parthenocarpy).
Haploid can be produces through delayed pollination, incompatible pollen, and temp sock and by x-rays.
Shoot apical meristem of shorghm treated by colchicines induces small proportion of haploids.
Anther or pollen culture also use in some crops like – brassoca, rice, tobacco and triticale.
Interspecific cross like Hordeum bulbosum*H. vulgare used in haploid production in barley. Chromosome specially hordeum bulbosum eliminate from hybrid embryo production as H.bulbosum*T. aestivum.
Unfertilized embryo, egg cell also use in haploid production
Induced artificially first time by Jorgensen in 1928 by crossing Solanum nigrum*Solanum leteum
Guha and maheswari in 1964 obtained haploid plants of Datura innoxia from the pollen by culturing the anthers.
Diploids obtained through the chromosome doubling of haploids are known as dihaploids.
Haploid also be produced through parthenocarpy and delayed pollination etc.
Uses
Pure line
Haploid doubling to produce homozygous disomic plants in just two year. As compare to 6-7 generation of selfing.
Isolation of mutant because express in M1 generation.
Selection at haploid level are more efficient than diploid level, in potato where breeding is preferred at haploid level such approach called analytical breeding
Disease resistance
Inbreds
II. Diploidy
Presence of two genome in each cell of an organism.
Fertility, balanced growth, vigour, adoptability.
Regular bivalent formation during meiosis.
Have two allele at each locus.
III. Polyploidy
Have more than two genome.
Individual having more than two basic or monoploid set of chromosome.
About 1/3 population are polyploidy.
Types of polyploidy
A. Autopolyploidy/autoploids/simple polyploids
Multiplication of chromosome of single species.
Increase number of same set of chromosome of a genome.
a. Autotriploidy
Three set of chromosome or genome of same species (AAA).
Occurs naturally and can be produced artificially.
Triploids are highly sterile.
Useful in asexually propagated plants.
Ex. Banana, sugarcane, apple, sugarbeet, watermelon, tomato, Cynodon dactylon.
Banana- Seedless, larger size than diploid one.
Watermelon- Commercially grown in Japan.
Produced through crossing of tetraploid and diploid (4x used as female), (2x used as male).
Seedless.
Apples – some are tetraploid.
Sugarbeet- Larger roots and Higher sugar yields than diploid one
Resistant to moulds.
4x*2x are difficult.
Tea- released in commercial cultivation in India.
Higher leaf yield and leaf quality than diploids.
b. Autotetraploidy
Four copies of genome of same species (AAAA).
Occurs naturally and can be produced artificially.
Quadrivalent are formed,
Very stable and fertile.
Eg; rye, alfalfa, grapes, groundnut, potato, coffee, Oenothera lamarckiana
Four chromosome are homologous to each and each gene has four copies (allele).
Nulliplex – none dominant allele and all recessive alleles (aaaa)
Simplex - one dominant and three recessive alleles (Aaaa)
Duplex – two dominant and two recessive alleles (AAaa)
Triplex – three dominant and one recessive alleles (AAAa)
Quadruplex – four dominant and none recessive alleles (AAAA).
Rye- grown in Sweden and Germany.
Double Steel, Tetra Petkus
Larger seeds and higher protein.
Grapes- developed in California.
Larger fruits and fever seeds.
Alfalfa- better yield and higher recovery after grazing.
Barseem- Pusa Giant
B. Allopolyploids/alloploids/hybrid/bispecies/multi species polyploids
Having complete set of chromosome from two or more species.
Allopploids developed by interspecific crosses ,fertility restored by colchicines chromosome doubling.
More important than autoploids.
Amphidiploids- allopolyploid (allotetraploid) arises by combining genomes of two different species.
Natural alloplyploids-
Cotton-
Wheat-
Brassica-
Tobacco-
Artificial allopolyploids-
Triticale-
Raphanobrassoca-
C. Segmental allopolploids
In some allopolyploids, different genome present not quite different from one another.
Genome pair to some extent and multivalent are formed.
That are intermediate between autopolyploids and allopolyploids.
Common hexaploid wheat are segmental allopolyploids because three diploid genome A, B, D are related (homoelogous) to each other.
Effect of polyploidy
Genetic – sterility due to imbalanced gamete
Phenotypic effect –
Larger flower, pollen and seed
Leaves are thicker, larger darker in green colour
Roots are stronger and longer
Physiological effect –
Longer maturity duration
Slower growth rate
Higher ascorbic acid in cabbage and tomato
Corn flour tetraploid maize have 40% more vita-A
Limitation of polyploids –
Limited use like asexually propagated crops.
Difficulty in maintenance like monoploids and triploids difficult to recover in sexually propagated crops.
Undesirable characters like in raphanobrassica
Some other defects like low fertility, genetic instability, slow growth rate late maturity.
Chances of development of new species are very low.
2. Aneuploidy
Loss or gain of one or more chromosome as compare to somatic chromosome number of a species.
Changes in chromosome number never involve complete genome.
Origin-
Loss of chromosome in mitotic or meiotic cells due movement during anaphase
Irregularities of chromosome distribution during meiosis of polyploids with uneven number of basic genome like triploid and pentaploid
Occurrence of multipolar meiosis
Types of aneuploids
Hypoploids (monosomic and nullisomic)
Hyperploids (trisomic and tetrasomic)
I. Monosomy
One chromosome missing from somatic chromosome complement.
Genomic formula 2n-1.
Produce two types of gametes n an n-1
Monosomic found in polyploids because it can’t survive in diploid.
21 monosomics are produced by sears in 1954 in the Chinese spring variety of wheat.
A. Double monosomy
Loss of one chromosome each from two different pairs.
2n-1-1
Can be produced in polyploidy wheat.
B. Triple monosomy
Loss of one chromosome each from three different pairs.
2n-1-1-1
Monosomic originate –
Diploid – at the time of non disjunction n-1 gamete are produced when it fuse to normal n gamete then monosomics formed 2n-1.
Nullisomics – nullisomic 2n-2
Gametes n-1
Fuse with normal gamete n
Monosomic form 2n-1
Trisomics - non-disjunction of three homologous chromosome in trisomics during meiosis gives rise to n-1 gametes. It fuse with normal n gamete to form monosomic 2n-1,
Monosomics also produces two types of gamete one is n-1 and another one is n.
Most of the cases n-1 formed and n remain laggards and ultimately lost.
II. Nullisomy
Loss of one pair of chromosome (homologous chromosome pair) from diploid state 2n.
Genomic formula are 2n-2.
Nullisomic not found in natural population.
It is not important but use in genetic studies.
Exhibit reduced vigour, fertility, and survival.
A. Double nullisomic
Loss of two pair of homologous chromosome.
III. Trisomy
Gain of one chromosome
Genomic formula 2n+1.
Possible trisomics may be equal to haploid chromosome number of organism.
Barley 2n= 14 so haploid is n=7 trisomic may be equal to.
In plants first case in Jimson Weed=Datura stramonium by A. F. Blakeselee and J. Billing in 1924. It found 2n=25 in common 2n=24.
A. Simple trisomy
2n+1
B. Double trisomiy
Two extra chromosome each belonging to a different chromosome pair
2n+1+1
Depending on nature of extra chromosome it is of two types –
A. Primary trisomy
When addition chromosome are normal.
B. Secondary trisomics
When additional chromosome are isochromosome.
C. Tertiary trisomics
When additional chromosome are translocated.
Origin of trisomy –
2n+2*2n
n+1*n
=2n+1
IV. Tetrasomy
Addition of two chromosome at one or two pair of chromosome.
Genomic formula= 2n+2
21 possible tetrasomic in wheat are possible.
Application of aneuploids :
Determine the phenotypic effect of loos or gain of different chromosome.
Use to produce chromosome substitution line, provide information on the effect of different chromosome of variety in the same genetic background.
Use to produce alien addition and alien substitution line.
Monosomics use to transfer of chromosome with desirable genes fron one species to another.
Permits of location of genes as well as linkage group on to a specific chromosome, monosomics and nullisomics are used generally for this purpose.
Studies of nullisomics and tetrasomics combination made it possible to establish homoeology among the chromosome of A, B, and D of wheat.
Identifying the chromosome involve in the translocation.
Use in preparation of molecular map.
Use to obtain chromosome specific probes. (probe is a DNA sequence that use to detect presence of same DNA sequence in test DNA sample).
UPSC Agriculture Optional Previous Year Questions What do you understand by chromosomal aberration? Discuss in brief the major type of chromosomal aberration. 2018 |
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