Question

Genetic variation is essential for plant breeding. What are the main causes of genetic natural variation in plants? Describe at least three possible mechanisms for genetic natural variation to arise naturally, and how they occur.

Answer 1

Genetic variation means difference in genes among population of a species. Genetic variations gives rise to genetic diversity. In other words the number of different alleles of a gene in a population is a measure of genetic diversity.

The main causes of genetic natural variation in plants are :

1 Mutation : deletions , inversion, translocation and insertions are different types of mutations which can cause change in gene sequences, ultimately giving rise to genetic variation.

2 Outbreeding : It is process where individuals of a particular gentoype mate with individuals of different genotype. outbreeding increases genetic variation.

3 Sexual reproduction : Independent assortment and recombination between alleles give rise to variation among offsprings. In Independent assortment movement of an allele of a particular chromosome is independent of another allele. example : AaBb genotype cell can  produce different types of gametes like AB,Ab, aB,ab i.e there is no compulsion that A will move with B or a will move with b, into the gametes.

Recombination between sister chromatids during meiosis can also give rise to variation. Genes ABC present on a chromatid can crossover (recombine) with genes abc on another sister chromatid to produce gametes like Abc or aBC. In case of double cross over gametes like AbC and aBc are produced. Such events produce novel genetoypes leading to genetic variations among individuals.

4 Non disjunction during meosis or mitosis : This phenomenon gives rise to polyploidy i.e chromosome set is more than the normal set . In this process sister chromatids are not separated during meiosis or 2 homologous chromosomes are not separated during mitosis. if only some chromosomes undergo non disjunction then the offspring is usually aborted. However if all of the chromosomes under non disjunction then an unreduced gamete is produced. When this unreduced gamete fuses with a normal one the resultant progeny will contain 3n set of chromosomes. Polploidy is an extremely useful tool in producing better crops.

polyploidy can be of two types : autopolyploidy and allopolyploidy. If a self fertilizing plant produces unreduced gamtes then fusion will result in progeny having increase number of the same genome. Example AA x AA = AAAA

allopolyploidy occurs when unreduced gametes come from different species. This happen when a plant will outbreeds produces unreduced gamtes. AA x BB = AABB