AGRIC/5: BASIC SCIENCE I: Variation

Variation among organisms

This refers to differences among organisms of the same species due to the differences in the genes they inherit and the environment they survive in.

Types of Variation:

There are two main types of variation namely, Environmental variation and Genetic variation.

Environmental variation:

This refers to differences amongst organisms of the same species due to the different factors of the environment they are exposed to.

Eg , exposure of organism to different temperatures, light, humidity, nutrients, loss of body parts via accidents, dehorning of cattle by man, lightening of the skin using cosmetics.

Such variations, because they are not genetically acquired but enviromentary acquired or influenced characters, can not be inherited from parents to off their springs.

Genetic variation:

This refers to differences amongst organisms of the same species due to the differences in the genes they inherit from their parents. Eg some individuals are tall and others are short. This is because they inherited different genes from their parents. Such variations can be inherited because they are genetically determined.

Each gene pool includes various allelles of genes. The variation occurs both within and among populations, supported by individual carriers of the variant genes.

Without genetic variation, some of the basic mechanisms of evolutionary change cannot operate.

There are three primary sources of genetic variation, which we will learn more about:

1. Mutations are changes in the DNA or a permanent change in the chemical structure of chromosomes. A single mutation can have a large effect, but in many cases, evolutionary change is based on the accumulation of many mutations.Mutations are likely to be rare and most mutations are neutral or deleterious, but in some instances, the new alleles can be favored by natural selection.Polypliody is an example of chromosomal mutation. Polyploidy is a condition wherein organisms have three or more sets of genetic variation (3n or more).
2. Gene flow is any movement of genes from one population to another and is an important source of genetic variation.
3. Sex can introduce new gene combinations into a population as it produces changes within alleles. This genetic shuffling is another important source of genetic variation.Crossing over (genetic recombination) and random segregation during meiosis can result in the production of new alleles or new combinations of alleles. Furthermore, random fertilization also contributes to variation.Variation and recombination can be facilitated by transposable genetic elements, endogenous retroviruses, LINEs, SINEs, etc.For a given genome of a multicellular organism, genetic variation may be acquired in somatic cells or inherited through the germline.

Types of genetic variation:

There are two type of genetic variation and these are,

1. Continuous variation
2. Discontinuous variation.

Continuous variation:

This is the type of variation of a given character/trait where by differences among organisms of the same species are slight and grade into each other.

These characters can be measured and mean, mode and median can be obtained. eg Height, weight, intelligence, waist line, length and width of structures, skin colour, yield of milk, fertility, number of grains on a maize cob.

When the above are measured for any group of organisms, the biggest percentage of the measurement are intermediates a few are of low grade and a few are of long grades

It give a normal distribution curve/bell shaped curve,/Gaussian curve when  we compare many organisms referring to one continuous character.

NB: No remarkable differences in value but continuous transmission from low to intermediate to high as shown by the normal distribution graph below.

Though genetically determined can be influenced by the environment. Such features can be used in the dichotomous key.

Discontinuous variation:

This the type of variation which shows clear-cut and sharp differences amongst organisms over a given trait.

In discontinuous variation differences do not merge into each other and there fore there are no intermediate grades. Features can not be measured but can be observed and there fore we can not obtain a normal distribution curve.

Features persist through out the life time of an organism. They show distinct differences. Eg sex, finger print, tongue rollers and non tongue roller, colour blindness, taster and non tasters to PTC (phenyl thio carbamide) blood groups, sickle cell anaemia, haemophilia, skin pigmentation (normal skin colour/albinism) eye colour etc. They are not affected by environmental condition.

These traits can be used when constructing a dichotomous key.

Causes of variation include;

1. Environmental factors such as a) diet, b) altitude, c) light intensity, d) temperature, e) pathogens and diseases, f) social function and g) age.

2. Genetic factors such as a) crossing over between homologous chromosomes during prophase I of meiosis, b) mutations (which bring about mainly discontinuous variation in a population)

Among individuals within a population

Genetic variation among individuals within a population can be identified at a variety of levels. It is possible to identify genetic variation from observations of phenotypic variation in either quantitative traits (traits that vary continuously and are coded for by many genes (e.g., leg length in dogs)) or discrete traits (traits that fall into discrete categories and are coded for by one or a few genes (e.g., white, pink, red petal color in certain flowers)).

Genetic variation can also be identified by examining variation at the level of enzymes using the process of protein electrophoresis. Polymorphic genes have more than one allele at each locus. Half of the genes that code for enzymes in insects and plants may be polymorphic, whereas polymorphisms are less common among vertebrates.

Ultimately, genetic variation is caused by variation in the order of bases in the nucleotides in genes. New technology now allows scientists to directly sequence DNA which has identified even more genetic variation than was previously detected by protein electrophoresis. Examination of DNA has shown genetic variation in both coding regions and in the non-coding intron region of genes.

Genetic variation will result in phenotypic variation if variation in the order of nucleotides in the DNA sequence results in a difference in the order of amino acids in proteins coded by that DNA sequence, and if the resultant differences in amino acid sequence influence the shape, and thus the function of the enzyme.

Between populations.

Geographic variation means genetic differences in populations from different locations. This is caused by natural selection or genetic drift.

Measurement.

Genetic variation within a population is commonly measured as the percentage of gene loci that are polymorphic or the percentage of gene loci in individuals that are heterozygous.

Forms

Genetic variation can be divided into different forms according to the size and type of genomic variation underpinning genetic change:

Small-scale sequence variation (< 1Kbp) includes base-pair substitution and indels.

Large-scale structural variation (>1Kbp) can be either copy number variation (loss or gain), or chromosomal rearrangement (translocation, inversion, or Segmental acquired uniparental disomy).

Genetic variation and recombination by transposable elements and endogenous retroviruses sometimes is supplemented by a variety of persistent viruses and their defectives which generate genetic novelty in host genomes. Numerical variation in whole chromosomes or genomes can be either polyploidy or aneuploidy.

Maintenance in populations

A variety of factors maintain genetic variation in populations. Potentially harmful recessive alleles can be hidden from selection in the heterozygous individuals in populations of diploid organisms (recessive alleles are only expressed in the less common homozygous individuals). Natural selection can also maintain genetic variation in balanced polymorphisms. Balanced polymorphisms may occur when heterozygotes are favored or when selection is frequency dependent.

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