Introduction
The phenomenon of transmission of characters from parents to the offspring is called heredity. The character s that is transmitted from parents to offspring is called hereditary characters. For example, colors of the eye, rolling of the tongue, folding of tongue etc. The characters which are not inherited and learnt by the organism during the life time are called acquired characters. For example, ability to swim, riding bicycle, operating machines etc.
Genetics
The branch of biology which deals with the study of heredity and variation is called genetics.
Terms related to heredity
- Genes: Genes are tiny units of heredity that transmit hereditary characters from parents to the offspring. They control the development of a set of hereditary character. They are located in the chromosomes and made up of DNA.
- Allele or allelomorphs: the factors of genes for the two contrasting characters are called allele. For example the pair of gene ‘Tt’, ‘T’ is the allele of ‘t’.
- Dominant characters: The character that appears in successive generation is called dominant characters.
- Recessive Characters: The hidden or suppress characters that do not express themselves in successive generation are called recessive characters.
- Genotype and phenotype: The internal genetic composition present in the organism is called genotype. It is express in letters. For example TT (pure tall), tt (pure dwarf) etc.
The external physical appearance of an organism is called is called phenotype. Tallness and dwarfness are phenotype of a pea plant.
- Hybrid: The organism produced by crossing two genetically different individual is called hybrid.
The process of obtaining hybrids after crossing two genetically dissimilar individual is called hybridization.
- Monohybrid Cross: The cross between two organisms differing in single air of contrasting character is called monohybrid cross. For example cross between tall and dwarf pea plant.
- Dihybrid Cross: The cross between two organisms differing in two pairs of contrasting characters is called Dihybrid cross. For example cross between tall pea plant with red flower and dwarf pea plant with white flower.
- Homozygous and heterozygous: An organism having similar gene is called homozygous. Pure tall ‘TT’ pea plant is homozygous for tallness.
An organism having two members of pair of gene or dissimilar genes is called heterozygous. Tallness ‘Tt’ is a hybrid tall which is formed by different genes.
Mendel’s Experiment on Pea plant
Gregor Mendel (1822-1884) who was also known as father of genetics experimented on pea plant to know how hereditary characters are transmitted in the next generation. He developed the theory which explains how the hereditary characters are transmitted with the help of genes. For his experiment e selected pea plant. He took seven characters for his study such characters having two alternatives. They are as follows
Traits | Dominants | Recessive |
Height | Tall(T) | Dwarfs(t) |
Colors of flower | Red(R) | White(r) |
color of pod | Green(G) | Yellow(g) |
Shape of pod | Smooth(I) | Constricted(i) |
Position of flower | Axial(A) | Terminal(a) |
Color of seed | Yellow(Y) | Green(y) |
Color of seed coat | Brown(B) | White(b) |
Mendel cross pollinated between tall pea plant and dwarf pea plant. The seed produced were allowed to germinate which were called first filial generation. The plants produced were allowed to self pollinated to obtain second filial generation. He noted that the plants of F1 generation were not intermediate between tall and dwarf. However, inF1 generation only one character, tallness was expresses which was named as dominant character and other remained hidden which was named as recessive character. In F2 generation both tall and dwarf character appears in the ratio3:1. Similarly, he did experiment on the other pairs of contrasting characters and found the same result. Based on his result he developed the laws called laws of heredity or Mendelism.
Phenotypic ratio= 3:1 (tall: dwarf)
Genotypic ratio= 1:2:1 (pure tall: hybrid tall: pure dwarf)
Reasons for selecting pea plant by Mendel’s
- There are varieties of pea plant having many contrasting characters.
- They are self pollinated in nature but cross pollination can be done.
- The hybrids produced through cross pollination are also fertile.
- A single plant can produce large numbers of seed.
- The produce new generation in short period of time.
- They can be cultivated easily in the garden.
Mendelism of Law of Heredity
Law of Dominance
- When a cross is made between plants having a pair of contrasting characters only one of the character appear in F1 generation and the other remain hidden. This law is called law of dominance.
For example, when a cross is made between with pea plant of red flower (RR) with white flower (rr) all the plants produce in F1 generation are red flower. The white character does not express in F1 generation. Here red is dominant character and white is recessive character.
Law of purity of gametes (Law of segregation)
- When a pair of allele is brought together in a hybrid they do not mix with each other but simply remain together and separate from each other during gamete formation. This law is called law of purity of gametes.
For example, if a cross is made between pure tall (TT) and pure dwarf (tt) pea plant, all the plants in F1 generation are hybrid tall (Tt). When these plants are allowed for self-pollination dwarf characters appears in the plants of F2 generation. The plants in F1 generation carry both tall and dwarf factor and these factors do not mix with each other but simply remain together without affecting each other. When the plants of F1 generation for gametes the tall and dwarf separate from each other as pure gamete and in F2 generation the 3:1 phenotypic ratio and 1:2:1 genotypic ratio are obtained.
Phenotypic ratio= 3:1 (tall: dwarf)
Genotypic ratio= 1:2:1 (pure tall: hybrid tall: pure dwarf)
Law of Independent Assortment
When two pair of allele is brought together in hybrid they show independent dominant effect. In gamete formation, separation takes place but the factors assort independently at random and freely.
Significance of Mendelism
- Disease resistant and high yielding varieties of plants and animals can be produced.
- Hybrids with desired characteristics can be produced.
- The phenotype and genotype likely to be produced in successive generation can be predicated.
- Dominant and recessive characteristics of the organism can be known.
- Some common hereditary characters found in human are
- Dimpled cheeks (dominant) and non-dimpled cheeks(recessive)
- Free ear lobe(dominant)and attached ear lobe (recessive)
- Curly hair(dominant) and straight hair(recessive)
- Rolling of tongue(dominant)and non-rolling of tongue (recessive)
- Thick lips(dominant) and thin lips(recessive)
- No hitcher’s thumb(dominant) and hitcher’s thumb(recessive)
- Certain diseases are found only in male and others are found only in females. Such diseases are called sex-linked diseases.
For example, baldness and color blindness are found only in males whereas breast cancer and uterine cancer are found only in females.
Variation
The physiological and morphological change in the organism due to change in genetic composition is called variation. Thus, variation is the cause of difference between the species of same parents. The difference may be in color, structure of body, height etc. There are two types of variation. They are,
- Continuous Variation: The variation that occurs due to gradual change in genetic composition of ancestors to offspring is called continuous variation. In this variation, gradual change in hereditary characters takes place from one generation to another generation.
- Discontinuous Variation or Mutation: The sudden heritable change in a genetic material of an organism due to sudden defect in genes is called discontinuous variation. For example, missing limbs, extra toe in limbs etc.
Causes of variation
- Change in environmental condition.
- Sudden change in genetic composition.
- Meiosis cell division in which the exchange of genetic material occurs.
Significance of variation
- It helps in organic evolution.
- It increases the chance of survival of organism in changing environment.
- It provides individuality to an organism of a species from member of other species.