Q.12 Define variations. Discuss their sources and types.
Answer:
Definition
Differences shown by the individual of same species are called as variations. Sources of variations
(a) Genetic Recombination
The genetic recombination produced through crossing over results in gametes with variations.
(b) Mutation
Mutation (changes in DNA) are important source of variations.
(c) Combination of Sperm with egg
During fertilization, one of the millions of sperms combines with a single egg. The chance involved in this combination also act as the source of variations.
(d) Gene Flow
Movement of genes from one population to another is also an important source of variations.
(e) Combination of Chromosomes
Variations are also caused by different combinations of chromosomes in gametes and then in zygote.
Types of Variations
(i) Continuous variations
In continuous variations, the phenotypes show a complete range of measurements from one extreme to the other.” Such variations are controlled by many genes and also affected by environmental factors.
Examples
Height in Man
Weight in Man
Feet size in Man
Intelligence in Man
Height in Man
In every human population, the individuals have range of heights (from very small to tall). No population can show only two or three distinct heights.
(ü) Discontinuous variations
Discontinuous variations show distinct phenotypes. The phenotypes of such variations cannot be measured. The individuals of population have distinct phenotypes which can easily be distinguished from each other, Discontinuous variations are controlled by the allele of a single gene pair. Environment has no effect on such variations.
Example
Blood groups in Man
In human population, an individual has one of the four distinct phenotypes (blood groups) and cannot have in between. ”
