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Heredity and Variation

Variation 1

In a family, the children are generally similar to  their parents. The  children resemble the parents in skin color, pattern of hair, color of eyes, etc. It is because the parental characters transfer into the offspring. The process through which the parental characters pass from generation  to generation is called heredity or inheritance. Though similarity  is found among the members  of a family, they are not exactly alike i.e. there are some dissimilarities among them which is called variation. The branch of biology which deals with the mechanism of heredity and variation is called genetics.

VARIATION

The structural/ morphological / physiological differences that provide individuality to each member of a species is called variation. OR,

The difference /dissimilarity from parents to offspring or from the member of the species is called variation.

Types of Variation

A. Based on the type of cells

1) Somatic Variation

They are the variation that affect the somatic cells only. They are not inheritable. They are acquired by the organisms during their own life time and are lost with their death. These variations are also called as Acquired Variations. They are not important for the evolution point of view.

The possible causes of somatic variations are:

i) Environmental Factors

ii) Use and Disuse of organs

iii) Conscious efforts

2. Germinal Variation

They are the variation that affects the germinal or reproductive cells only. They are inheritable and are important foe evolution.

The possible causes of germinal variation are:

i) Recombination

Offspring with new genetic combination are born due to reshuffling of parental genes.

ii) Mutation

Structural changes in DNA or chromosomes are caused due to mutation which results in change in phenotype of the organism.

B. Based on the degree of differences produced

1. Continuous Variation

Continuous variation are small and indistinct. They fluctuate on the either side on the average condition. These are also described as fluctuating or plus-minus variation. They don't have any role in evolution as they are unstable and non-heritable. They affect the size, weight, shape or color of an organism or its parts.

2. Discontinuous Variation

Discontinuous variation are large and sudden. These deviate to a great extent from the normal and appear as totally new. They are also described as Mutations. They play an important role in origin and evolution of species as they are stable and inheritable.

Importance of Variation

1. Variation forms the basis of heredity.

2.They are the source of evolution.

3. They help with the adaptation of the organisms to the changing environment.

4. Due to variation, useful variants of organisms can be created.

5. They provide individuality to each organism.

Differences between Continuous and Discontinuous Variation


Continuous Variation
Discontinuous Variation
1. Continuous variation is small and indistinct.
1. Discontinuous variation is large and distinct.
2. These are unstable.
2. These are stable.
3. They are inheritable.
3. They are not inheritable.
4. They have no role in evolution.
4. They have significant role in evolution.
5. They are very common and occur in all organisms.
5. They are not common and appear suddenly.


Heredity and Variation in Asexual Reproduction

Many eukaryotic organisms like protists, fungi, algae, and lower animals reproduce by asexual methods also. Organisms produced by asexual reproduction are the carbon copies of their parents. They are genetically identical. Such a genetically uniform population produced from single organism by asexual method is called clone. Clones due to their identical hereditary character exhibit no or little variations. Variations in them may be due to environmental factors.

Similarly, identical twins arising from a single fertilized egg are clone to each other, though they are the offspring of their parents. Hence, twins are both genetically and phenotypically identical.

Difference between Clone and Offspring


Clone
Offspring
1.They are produced from asexual reproduction.
1. They are produced from sexual reproduction.
2. They are carbon copy of parents.
2. They differ from parents.
3. They are derived from single parent.
3. They are derived from two parents.
4. Only mitosis takes place.
4. Meiosis takes place to form gametes.
5. No recombination of genetic materials.
5. Genetic recombination causes variation.