Heredity And Variation

Genetics

The science concerned with the precise understanding of biological properties which are transmitted from parents to offspring is called genetics.

OR

The branch of biology that deals with the study of heredity and variation is called genetics.

Contributors in the field of Genetics :

The few yet important contributors in the field of genetics are as follows :

Gregor Johann Mendel → Father of genetics

William Bateson → Father of modern genetics

Thomas Hunt Morgan → Father ofDrosophilagenetics.

The term or word 'genetics' was coined by W. Bateson in 1905 A.D.

[ The physical basis of heredity is gene while the chemical basis of heredity is D.N.A.]

Heredity-

The process of transmission of characters from one generation to another is known as inheritance or heredity.

Character-

The character is defined as the morphological or physiological feature of the individual. For example - Height of stem.

Trait-

It is defined as the distinguishable feature of a character and its detectable variant. For example - tallness, dwarf-ness.

Variation-

The differences that are shown by the individuals of a species and also by the offsprings of same parents are referred as variations. Variations may be harmful, useless and useful. Harmful or useless variations make individuals unfit for the struggle for existence, whereas the useful variation is quite significant and make organisms fit in nature.

Types of variation:

The types of variation are as follows-

A) Based on impact on cells

⇒Somatic or acquired variations

⇒Germinal variations

B) Based on degree of differences

⇒Continuous variation

⇒Discontinuous variation

A) As per nature of the cells it affects, variations are of two types :

⇒Somatic variations -

The variation acquired by an organism during its own lifetime and are lost with death are called somatic variations, Such variations are not inherited from parents to the offsprings. This variation is also known as acquired variation. Somatic variation may be caused by any one of the following factors :

I) Environmental factors - The factors like light, temperature, humidity, quality of nutrition e.t.c directly influence the form and functions of animals, plants, and micro-organisms. This may lead to creating somatic variations.

II) Use or disuse of organs- Constant and continuous use of an organ makes it better developed (According to Lamarck). Example - A player who uses his muscles acquires better and developed muscles.

III) Conscious efforts- Education, learning, training, nutrition, and habit formation are examples of somatic variations due to conscious efforts. Mutilations, castration are some somatic variations caused by human beings by the conscious effect.

⇒Germinal variations -

The type of variation caused due to the germ or reproductive cells is called germinal variation. For example-baldness, colourblindness e.t.c. Such variations are inheritable and caused by :

I)Recombination- Due to the reshuffling of parental genes, organisms with new genetic combinations are achieved.

II)Mutations- Due to mutation, structural changes in DNA or chromosomes occur resulting in the change of phenotype.

B) Based on degree of differences, variations are of two types:

⇒Continuous variations-

Such variations are small and indistinct when compared to average. It is also called fluctuating variations. Such variations are unstable and non-inheritable. These variations produce a bell-shaped graph. This variation is of 2- types -

I) Substantive - Such variations bring change in weight, shape, size, colour e.t.c. For example- skin colour, height in men e.t.c These variations affect the morphology of living beings.

II) Meristic- Such variations bring change in number if certain body parts. For example- Number of segments gets changed in the earthworm, a number of sepals and petals gets changed, milk yield in dairy cattle e.t.c.

⇒Discontinuous variations-

Such variations are large and conspicuous and show a clear difference of progeny from parents. Such variations are also called mutations or sports or saltations. Mutation is the sudden change in genetic sequence and is stable and inheritable. There is no intermediate stage between parents and individuals and thus no average or mean is seen. It is of two types-

I) Substantive- Such variations bring change in size, weight, shape, colour e.t.c. For example- Abnormally short fingers, hornless cattle e.t.c.

II) Meristic- Such variations bring change in a number of certain body parts. For example- Polydactyly(additional number in fingers.)

Difference between continuous and discontinuous variations:

Heredity and variations in asexually reproducing organisms-

The new individuals obtained through asexual reproduction are the carbon copy of their parents. Such carbon copied are known as 'clone'. The individual of a clone is called 'remete'. They are mono-parental and the variations are unstable and non-inheritable. Thus, they don't have any role in evolution.

Some early views on heredity (Pre-Mendelian Concepts) -

a) Moist vapour theory- This theory was given by Greek philosopher, Pythagoras in 580-500 B.C. HE proposed that some moist vapour is given out from the brain, nerves and other parts of the body of male during coitus that makes the offsprings similar to the male parent.

b) Fluid theory- This theory was proposed by Aristotle in 384-322 B.C. According to his view, males produced highly purified blood but females menstrual fluid was not as pure as male semen. The reproductive blood coagulates in the body of female and forms embryo.

c) Preformation theory- A.V Leeuwenhoek in 1632-1723 was the first to observe human sperms. His theory believed that the heredity traits must be transmitted through eggs or through sperms or both. It is suggested that development of an organism is a simple enlargement of a minute but pre-formed a miniature form called homunculus.

d) Epigenesis theory- K.F Wolff in 1738-1794 proposed that neither egg nor sperm had a structure like a homunculus but the gametes contained undifferentiated living substance are capable of forming the organised body after fertilisation. Such an idea was called the theory of epigenesis. This theory suggested that many new tissues and organs, which were originally absent, develop subsequently.

e) Pangenesis theory- This was proposed by Charles Darwin in 1809-1882 in which he suggested that every part of the body produces very small invisible body called pangenes or gemmules which are transported by the bloodstream to the sex organs and these assembled into gametes. During fertilisation, pangenes from both the parents are brought together for redistribution to different organs during development, thus determining the different characters.

Symbols and terminology used in inheritance-

1)Gene-It is a small D.N.A segment that determines a biological character of an organism. It is a functional unit of hereditary material.

2)Allele or allomorph-An allele is a particular form of the gene. It is the short form of allomorph. It indicates the alternative form of the same gene. Example- T and t are two alleles for the gene of plant height.

3)Genotype-A genotype is the genetic expression of an organism. It is not observation phenomenon.

4)Phenotype-It is the observable or physical expression of an organism. Example- Tall, dwarf e.t.c.

5)Homozygous-An individual having two identical alleles is known as homozygous. Example- TT or tt e.t.c.

6)Heterozygous-An individual having two different alleles is known as heterozygous or hybrid. Example- Tt e.t.c.

7)Dominant and recessive- The gene which expresses itself after the cross between two characters producing heterozygous in F1 is called dominant. The gene which remains hidden is called recessive.

8) Parental generation-The individuals used as parents in a cross are said to represent parental generation- P₁.

9)Filial generation-The progeny obtained as a result of the crossing between parents is a hybrid progeny and is called first filial generation, represented by F₁. The progeny obtained as a result of self-fertilization among F₁ plants represent the second filial generation (F₂.)

10)Back-cross-The F₁ individuals obtained in a cross are usually selfed to get F₂progeny. They can also be crossed with one of the two parents (P₁or P₂) from which they are derived. Such a cross of F₁hybrid individual with either of the two parents is called backcross.

11)Test-cross-In this cross, the individual of an unknown genotype is crossed with homozygous recessive parents.

12)Out-cross-In this cross, the individual of an unknown genotype is crossed with homozygous dominant parents.

13)Reciprocal cross-A set of two reciprocal crosses means that the same parents are used in two experiments in such a way that if one experiment 'A' is used as the female parent and 'B' is used as the male parent, in the other experiment, 'A' will be used as the male parent and 'B' as the female parent. The purpose of this is to find out whether both parents are making an equal contribution.

14)Gene locus-The point in the chromosome where the gene is located is gene locus.

15)Monohybrid cross-The cross performed by taking single determining biological character is called monohybrid cross.

16)Dihybrid cross-The cross performed by taking two determining biological characters as the reference is dihybrid cross.

17)Polyhybrid cross-The cross performed by taking multiple determining characters is poly hybrid cross.

18)Checkerboard (Punnett square) method-The possible phenotypes and genotypes resulting from random combinations of gametes during fertilisation in hybridization experiments are more conveniently determined by using checkerboard method devised by Punnett in 1906. This board is a geometrical device; a square with vertical and horizontal columns. Possible types of female gametes are shown in small spheres above the vertical columns. Similarly, the male is shown on the right of horizontal columns. Then the possible genotype and phenotype are then entered in the square boxes.

Reference

Keshari, Arvind K. and Kamal K. Adhikari. A Text Book of Higher Secondary Biology(Class XII). 1st. Kathmandu: Vidyarthi Pustak Bhandar, 2015.

Mehta, Krishna Ram. Principle of biology. 2nd edition. Kathmandu: Asmita, 2068,2069.

Jorden, S.L. principle of biology. 2nd edition . Kathmandu: Asmita book Publication, 2068.2069.