Changes in Chromosome Number Origin Numerical changes in
Changes in Chromosome Number
Origin Numerical changes in the chromosome may be arises due to- • Meiotic irregularities, Nondisjunction • Translocation heterozygote • Through delayed pollination e. g. haploid production • Occasional chromosome doubling in somatic cells • Colchicine treatment to seeds, seedlings or shoottips
Genome: A basic or monoploid set of chromosomes of an individual is called genome. In a genome, each type of chromosome is represented only once. Basic chromosome number: The number of different chromosomes that make up a single complete set.
Heteroploidy: Any change in the chromosome number from the diploid state is referred to as heteroploidy
Types of heteroploidy i. Euploidy ii. Aneuploidy
Euploidy The change in chromosome number which involves entire set is called euploidy. The somatic chromosome number of an euploid individual is exact multiple of basic number of that species.
Types of euploidy
Monoploids contain a single chromosome set and are characteristically sterile. Haploids which carry chromosome number (n). half or gametic
Types of haploids 1. Euhaploids (a) Monohaploids (b) Polyhaploids (i) Allohaploids (ii) Autohaploids (iii) Dihaploids
2. Aneuhaploids (a) Disomic haploids (b) Nullisomic haploids (c) Substitution haploids (d) Misdivision haploid
Diploids Normal diploids are known as disomics. They have regular bivalent pairing during meiosis.
POLYPLOIDY An organism or individual having more than two basic or monoploid sets of chromosomes is called ployploid and such condition is known as ployploidy.
Types of Polyploidy (1) Autopolyploidy (2) Allopolyploidy
AUTOPOLYPLOIDY Autopolyploidy refers to the situation in which additional sets of chromosomes arise from the same species.
Autotriploids They have three sets of chromosomes of the same species. Triploids are generally highly sterile due to defective gamete formation.
Autotetraploids • They have four copies of the genome of same species. • Tetraploids are usually very stable and fertile because pairing partners are available during meiosis. • Examples: Rye, grapes, groundnut, potato etc.
Allopolyploidy An allopolyploid which arises by combining genomes of two diploid species is termed as allotetraploid or amphidiploid.
Natural Alloploids
U’s triangle The Triangle of U demonstrates the evolution and relationships between members of the plant genus Brassica. It was named after Nagaharu U, a Korean botanist who published theory in 1935 as part of his studies on Brassica species.
Figure: The Triangle of U demonstrates the evolution and relationships between members of the plant genus Brassica.
Origin of new world cotton from old world cotton and American cotton
Some examples of artificial alloploids • Raphanobrassica • Triticale
LIMITATIONS OF POLYPLOIDY 1. Limited Use 2. Difficulty in Maintenance 3. Undesirable Characters 4. Some other Defects 5. Chances of developing new species through allopolyploidy are extremely low.
Aneuploidy The change in chromosome number which involves one or few chromosomes of the genome is called aneuploidy and individuals are known as aneuploids such
Types of Aneuploid 1. Hypoploidy (a) Monosomics (b) Nullisomics 2. Hyperploidy (a) Trisomics (b) Tetrasomics
Uses of aneuploids 1. Locating Genes: 2. Interspecific Gene Transfer: 3. Aneuploids are used for developing alien addition and alien substitution lines in various crops. 4. Primary trisomics are useful in identification of chromosomes involved in translocations.
Limitations of Aneuploids 1. In diploid species, monosomics are inviable. 2. Nullisomics are inviable in diploid species. 3. Trisomics can survive in diploid species. 4. Maintenance of nullisomics and monosomics is difficult.
Common name Ploidy Name Maize 2 x=20 Diploid Wheat 6 x=42 Hexaploid Rice 2 x=24 Diploid Potatoes 4 x=48 Tetraploid Soybeans 2 x=40 Diploid Barley 2 x=14 Diploid Cassava 2 x=36 Diploid
Common name Ploidy Name Tomatoes 2 x=24 Diploid Bananas 3 x=33 Triploid Watermelon 2 x=22 Diploid Sugarcane 8 x=80 Octaploid Sugar beet 2 x=18 Diploid
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