Genetic Basis Of Inheritance of Class 12
Variations are (morphological, physiological, cytological and behaviouristic) differences amongst the individuals of the same species and the offspring of the same parents. They are found in all the characters and in every conceivable direction therefore, no two individuals are similar.
TYPES OF VARIATIONS
Variations are of two types regarding the nature of cells affected, somatic and germinal (blastogenic).
- Somatic variations affects the somatic cells of an organism. These are neither inherited from the parents nor transmitted to the next generation. These are acquired by an individual during its own life and is lost with its death (i.e., called the acquired variations or Modifications). These are produced by three types of factors: environment, use and disuse of organs, and conscious efforts.
- Germinal variations or Blastogenic variations affects the germ cells of an organism and is, consequently, inheritable. These are received by the individual from the parents and are transmitted to the next generation.
The germinal variations further are of two types regarding the degree of difference among the individuals :
- Discontinuous variations
- Continuous variations
These are also called as sports, saltations or mutations.There are certain characteristics within a population that exhibit a limited form of variation. Variation in this case produces individuals showing clearcut differences with no intermediates between them e.g., blood groups in humans, wing lengths in Drosophila, melanic and light forms in Biston betularia, style length in Primula and sex in animals and plants.
Characteristics showing discontinuous variations are usually controlled by one or more major genes that may have two or more allellic forms and their phenotypic expression is relatively unaffected by environmental conditions.
These are also called as Fluctuating variations i.e., they fluctuate on either side (both plus and minus) of a mean or average for the species.
Many characteristics in a population show a complete gradation from one extreme to the other without any break and is referred to as continuous variations. The continuous variation can be explained by certain characteristics e.g., mass, linear dimension, shape and colour of organs and organisms. The frequency distribution for characteristics exhibiting continuous variation is a normal distribution curve. Most of the organisms in the population fall in the middle of the range with approximately equal numbers showing the two extreme forms of the characteristics.
Characteristics exhibiting continuous variation are produced by the combined effects of many genes i.e., polygenes, and environment factors.
INFLUENCE OF THE ENVIRONMENT ON VARIATIONS
The ultimate factor determining a phenotypic characteristic is the genotype. At the moment of fertilization the genotype of the organisms is determined but the subsequent degree of expression allowed to this genetic potential is influenced greatly by the action of the environmental factors during the development of the organisms.
SOURCES OF VARIATIONS
The sources of genetic variations are as follows:
Crossing over : The reciprocal crossing over of genes between chromatids of homologous chromosomes may take place during prophase I (pachytene sub stage) of meiosis. This produces new linkage groups and so provides a major source of genetic recombination of alleles.
Independent assortment : The orientation of the chromatids of homologous chromosomes (bivalents) on the equatorial spindle during metaphase I of meiosis determines the direction in which the pairs of chromatids move during the anaphase I.
This orientation of the chromatids is random. During metaphase the orientation of pairs of chromatids once more is random and determines which chromosomes migrate to opposite poles of the cell during anaphase II. These random orientations and the subsequent independent assortment i.e., segregation of the chromosomes gives rise to a large calculable number of different chromosomal combinations in the gametes.
Random fusion of gametes : In sexual reproduction the fusion of gametes (both male and female gametes) is completely random. So any male gamete is potentially capable of fusing with any female gamete. The above-mentioned sources of genetic variations, account for the routine gene reshuffling that is the basis of continuous variation.
The environment acts on the range of phenotypes produced and only those best suited to it thrive. This leads to changes in allele and genotypic frequencies. Sources of variations do not generate the major changes in genotype that are necessary in order to give rise to new species. These changes are produced by mutations.