- 1 Principles of Inheritance and Variation – Variation Exists Among Individuals of One Species
- 1.1 Variation
- 1.2 Classification of Variation
- 1.3 Type of Variation According to Site of Origin
- 1.4 Variation According to Its Impact
- 1.5 Variation According to its Intensity and Quality
- 1.6 Importance of Variation
- 1.7 Heredity
- 1.8 Concepts of Heredity During the Pre-Mendelian Period
- 1.9 Moist Vapour Theory
One of the interesting Biology Topics is the study of animal behavior and how it is influenced by genetics and the environment.
Principles of Inheritance and Variation – Variation Exists Among Individuals of One Species
The living world is a valuable property of nature and it is full of diversity. Any depletion in this diversity is a matter of concern to us. How does such diversity appear? If we analyze the life phenomena of an individual it appears that everyone amongst us took birth from a single-celled body known as a zygote. The zygote is formed as an oval body by the union of egg or ovum and sperm coming from the mother and father respectively. With this, the genes from the father and mother unite in the zygote. Under the influence of these genes, one human being takes its existence in this world in the long run.
The diversity among individual organisms or differences between one organism to the other is based on their genetic constitution. The genes from the father and mother constitute the genetic makeup of the progeny. Therefore they show a resemblance to their parents. The flow of genes from the parents to the progeny is known as heredity. The subject that deals with the structure of genes their mode of action, their behaviour, and their flow from one generation to the other is known as Genetics. However, the term ‘genetics’ was coined by Bateson in 1905.
The progeny inherit the characters as present in the parents. This is known as heredity or inheritance of characters. The genes remain at the root of all the characters present in an organism and transmission of the genes through the gametes in the sexually reproducing organisms gathers the genes for characters in the offspring of the parents. Therefore, a precise definition of heredity may be stated as “heredity is the mechanism by which the characters present in the parents are transmitted to the progeny”.
All the progenies resemble their parents with respect to many characteristics and bear similarities with each; other. Because of this resemblance, the progenies are called siblings. Such uniparental organisms are usually called ramets or clones. The organisms belonging to a remete sometimes vary and these variations develop either due to environmental pressure or due to mutation. The gradual accumulation of variations in the progeny population in time and space causes the origin of diversity. Therefore, variations are sometimes regarded as raw materials for evolution.
The members of a species also differ due to the accumulation of variations, but they are identical in consideration of certain aspects. When the members of a species accumulate such variations that the groups become reproductively isolated from other such groups of the species, they attain the identity of a different species. Therefore, the definition of a species is given as “species represent a group of organisms that is reproductively isolated from another such group”. In another way, the definition of a species may be given as species is such, a group of organisms that shares a common gene pool. The species in nature is maintained through reproduction which promotes the continuation of the existence of the species and in this respect also heredity or inheritance of characters becomes of prime importance in the living world.
Except for the monozygotic twins, no two individuals are exactly identical. Therefore, the siblings or offsprings born of the same parents are alike. These differences in the offspring usually develop due to recombination. But the differences among the ramets mostly develop due to mutation. Variations are of great concern in the discussion on genetics. The development of variations and their transmission to the progeny population remains at a pivotal position in the discussion of all genetics.
In this world except for the progeny from one organism or monozygotic twins that have developed from a single zygote, no two individuals are alike. This occurs due to variation. Because of this variation, the descendants from the same species or progeny from the same parents are dissimilar with respect to their many characteristics. How many variations appear in the organisms has been explored to some extent. However, among many reasons, some prominent events are
- a random assemblage of the chromosomes during gamete formation.
- crossing over and recombination between the genes on the chromosomes.
- mutation of the genes.
- influence of the environment.
Whatever the method, variation is achieved through the alteration of the genetic constitution. The variation flows through generations and heredity gets important in the arena of evolution. Because of this in the study of genetics, a vivid concept of variation is important to know.
Classification of Variation
Classification of variation is done from various angles according to the constitution, its impact, characteristic nature, or on the basis of intensity and according to the site of origin.
Type of Variation According to Site of Origin
1. Somatic Variation:
When variation appears in the somatic cells of an organism, it is called somatic variation. Such variation is not inherited in the subsequent generations. Such variation is acquired during the lifetime of the individual and such variation has no influence in evolution. This type of variation may appear due to environmental influence as found in the case of insufficient nutrition in babies. It is found in man due to use and disease or by continuous effort.
2. Germinal or Blastogenic Variation:
If the variation appears in the germ cells of an organism, it is called germinal variation. The germinal variation may be accumulated in the gametes. Therefore, germinal variation may be inherited in the progeny. Hence, such variation is important in evolution.
Variation According to Its Impact
1. Useful Variation:
The variation that is useful to an organism for its survival is known as useful variation.
2. Unuseful Variation:
The variation that is not at all beneficial to an organism is known as unuseful variation. The gigantic size of the dinosaurs was detrimental to them and because of that, they could not last long on the earth.
On the basis of impact on the organism, variation may again be classified as morphological variation, physiological variation, and psychological variation. When a variation affects the phenotype related to the morphology of the organism it is designated as morphological variation. On the other hand, if the variation relates to the physiology of the organism, it is called physiological variation. On the other hand, a variation changing the mental status of the organism is designated as a psychological variation.
Variation According to its Intensity and Quality
On the basis of intensity and quality, variation may be categorized as:
- Continuous and discontinuous variation.
- Determinate and indeterminate variation.
- Meristic and substantive variation.
1. Continuous and Discontinuous Variation:
Variations in a character are continuous in their distribution and if they are placed on graph paper, with their variable properties along the abcissa and their numbers along the ordinate the graph will have a bell-shaped appearance. The graph also shows that the maximum number represents an average quality. Such a variation may be observed in the height and weight of persons. On the other hand when the variables of a character are discrete and either large or small the values of the distribution cannot be graphically placed as above. Such variations may be the result of a mutation called saltation. Hugo de Vries opined that such changes are important in evolution.
2. Determinate and Indeterminate Variations:
When the variables for a character appear as indicative of a fact or purpose, it is called determinate variation. The height of one plant in a jungle or the weight of the fish in a pond come under this category. For instance, the weight of a Katla fish will be more with its age, while the weight of the same type of fish in the pond will be less if it is younger. Therefore, the variables are indicative of the age and growth of the fish. On the contrary, the variables of a character are haphazard and not indicative of any purpose. For example, the IQ of the persons in one human population varies haphazardly. The similarities will be among the persons in a family. Determinate variations are also called directional variations or orthogenic variations.
3. Meristic and Substantive Variation:
When the variables of a character are represented by a number, it is known as meristic variation. The character in this context is known as a meristic character. The number of bananas and a number of paddy seeds in an ear are meristic characters. So their numbers are meristic variables. On the other hand when the variables of a character are judged on the basis of quality the variables are known as substantive character. The colour of hair and eyes, and the shape of the nose and ear of persons come under this category.
Discussion: Whatever may be the variation type and form each of the variations has a genetic basis. Therefore, any variation is determined by genetic makeup. Hence, variations are hereditary and they are representative of some genetic constitutions. As the variations are heritable, they follow the role of principles of inheritance. On the background of this discussion, a treatise on the mechanism of inheritance may be done.
In this context, it should be mentioned that there are some variations concerning some characteristics of living organisms. Such variations are determined by environmental factors and have no genetic basis. These variations are called phenocopy. The phenocopies are non-hereditary. For example, leaves of water lilies float on the surface of the water as continuous broad round, and flat leaves. In rainy season when some plant leaves submerge under water the submerged leaves are torn into pieces.
A detailed discussion on variation has been given in the chapter evolution and to take an idea of variation the reader may consult chapter evolution. In most cases, variation in the living organism originates due to mutation which means a sudden heritable change in the living organisms as an effect of environmental pressure. However, in sexually reproducing organisms, recombination during meiotic cell division may also be the way to the origin of variation through inheritance. The variations developed by way of mutation may be transmitted from one generation to the other and variations developed in nature get stabilized.
DNA is the genetic material and within the genetic material, a gene resides. A segment of a DNA molecule that contains the information required for the synthesis of a functional biological product, whether protein or RNA, is referred to as a gene part of the DNA polynucleotide chain that varies in dimension according to its nature and property. Any change in these polynucleotides may cause a change in gene structure leading to mutation resulting in variation. This changed gene structure is an alternative form of the wild-type gene. These alternative structures of a gene are known as alleles. There are multiple ways by which such change in gene structure may be achieved.
In most of the organisms, DNA is located in the chromosomes. Usually, these chromosomes are constant in number in a species. The structure of the individual chromosomes is also static. Any change in the number of chromosomes or in the structural organization of the chromosome results in a change of characters. This type of change is known as chromosomal aberration. Numerical as well as structural aberrations in chromosomes in most cases are non-hereditary. Sexually reproducing organisms transmit the change in character achieved through their gametes. Therefore, for transmission of variation from one generation to the other and genetic change must be incorporated into the gametes. In sexually reproducing organisms gametes may be formed by special types of cells in the body. These cells are called germ line cells.
In the constitution of the body, there are two types of cells in the body. These two types of cells are somatic cells and germ cells. Whether it is a somatic cell or germ cell, by the genetic constitution they are exactly identical containing the same amount and identical genetic material. Sometimes a change in genetic material may occur in the somatic cells and some variation in the expression of a character may be visible. But such variations cannot be inherited since somatic cells of the body do not participate in the formation of gametes.
A variation that is resulted from a change in gene structure should be stable enough and should be capable of transmission from generation to generation. Therefore, a variation should get a natural selection to get stability. Usually, stable variations are of adaptive value. Chromosomal changes though develop variation, they have little adaptive value in most of the cases. Hence, they are non-heritable. A comprehensive account of the mechanisms leading to the origin of variation may be presented in the following table.
Major Events Creating Variation in Organisms:
Sometimes alteration of character in organisms appears to be transitory and is found to be affected due to certain conditions in the environment. No genetic alteration is involved in such variation in character. Such a change of character is known as phenocopy. Phenocopy is nonhereditary and has no importance in evolution. Variations may be classified into different types.
Differences between Clone and Offspring:
|Point of Difference||Clone||Offspring|
|Parental Generation||Single||Two (Father and Mother)|
|Mode of Origin||Asexual Reproduction||Sexual Reproduction|
|Cell Division Involved||Mitosis||Meiosis|
|Recombination||Not Found||Usually Occurs|
|Resemblance||Every individual is identical in character.||Dissimilar in appearance.|
|Genic Constitution||Genetic Constitution Similar.||Siblings Show Genetic Difference.|
Importance of Variation
Variation is important for various reasons and they may be pointed out below:
- Variations are associated with environmental variability and hence variation promotes the adaptability of organisms.
- Variations promote the evolution of living organisms.
- The individuality of organisms is expressed through variations.
- Variations may be used for identifying one organism from the other.
- Variation sometimes indicates the quantity of an organism.
- Accumulation of variations in the course of time gives the origin of new species.
- In the case of Natural Selection survival of the fittest is selected due to suitable variation.
Heredity is a process by which characters of the parents are transmitted to the progeny. The parental gametes help in this process of transmission. A variation is important when it is inherited through generations. Therefore, heritable variations are favored by nature and they promote the evolution of species. But how the variation is inherited was not known to man before the discovery made by Mendel in 1865.
Concepts of Heredity During the Pre-Mendelian Period
Some concepts regarding heredity during the pre-Mendelian period prevailed. From 580-500 BC, some philosophers tried to analyze the incidence of heredity by proposing several theories. Among these moist vapor theory of Greek philosophers, Pythagoras, the Fluid theory of Empedocles, the Reproductive blood theory of Aristotle, and the Preformation theory of Malpighi are noteworthy. These theories at least say that people of the pre-Mendelian period were having much interest in the inheritance of characters. However, a scientific study on the inheritance of characters follows the Mendelian discovery.
Moist Vapour Theory
Greek philosopher, Pythagoras proposed this theory during the 6th century BC. According to this theory from various organs of humans, the vapour is liberated and the vapour condenses to initiate life.
(a) Fluid Theory:
Empedocles (504-433 BC) was the proposer of this theory. Empedocles opined that some fluids are liberated from different organs. These fluids from the parental body mix to produce the embryonic body.
(b) Reproductive Blood Theory:
This theory was given by the famous Greek philosopher Aristotle (384-322 BC). Aristotle believed that for the purpose of reproduction, males, and females produce comparatively pure blood within which purified nutrients from different: organs are accumulated. Within the body of the female, the pure nutrients from the father and mother are condensed to form the body of the embryo.
(c) Preformation Theory:
In 1712 Malpighi proposed this theory. Malpighi would believe that in the male gamete, a miniature human resides which was called him humancule. The humancule grows within the female body and ultimately produces the baby. In 1888 Roux agreed with the concept of Malpighi. The mechanism which governs this flow of character through generation was first elucidated by the Austrian monk Gregor Johann Mendel in 1865. Following this discovery though a number of contributors have added valuable concepts to explain the mechanism of heredity, we still remember Mendel for his pioneering experiments leading to the formulation of basic principles of inheritance of characters and hence Mendel is honoured as the father of Genetics.
Besides the theories discussed above, two other theories deserve worth mentioning. There is the theory of pangenesis and germplasm theory.
1. Theory of Pangenesis:
Initially, Mopartus (1698-1757) proposed this theory. In the latter period, he made some changes of this theory and supported the concept. Darwin proposed the theory of pangenesis which advocates that some pangenes carry the characteristic features from parents to progeny through their gametes. The characters which are observed in a progeny may be called the blended expression of these pangenes coming from father and mother. During the pre-Mendelian period, this blending theory of inheritance was the only explanation for the inheritance of characters. In contrast to this concept, the Mendelian explanation of heredity advocates a particulate theory of inheritance. According to Mendelian proposition, a character in the progeny is primarily expressed by the influence of the nonblendable particulate material that may come from either of the parents through their gametes. Mendel termed the particulate material as a factor which later on was named a gene.
2. Germplasm Theory:
This theory was given by Agust Weissmann in 1892. The theory states that our body is formed of two entities namely germplasm and somatoplasm. From the germplasm gametes are produced and from somatoplasm major body parts are formed. The characteristic features of the male and female go to the progeny through the gametes. Hence, germplasm determines heredity.
Some Important Discoveries Related to Heredity:
|Year||Discovery||Scientists Related to Discovery|
|1902||Chromosome theory of inheritance||Sutton and Bovery|
|1908||Hardy Weinberg Principle||Hardy and Weinberg|
|1910||Sex Linked Inheritance||Morgan|
|1927||X-ray as mutagen||Muller|
|1941||One gene one enzyme theory||Beadle and Tatum|
|1952||DNA as genetic material||Harsey and Chase|
|1953||Structure of DNA||Watson and Crick|
|1956||Human Chromosome No. 46||Tjio and Levan|
|1961||Operon Model||Jacob and Monod|
|1966||Genetic Code||Nirenberg, Khorana and Holley|
|1976||Protooncogene and Oncogene||Bishop and Vermus|
|1997||Cloning and Birth of Dolly, the Sheep||Wilmut|