Topic-3. Inheritance and variation

Chromosomes and Mechanism of inheritance.

The transmission of genetic information from one generation to other generation is known as heredity or inheritance.

1.    The mechanism of inheritance was successfully investigated before the study of chromosomes or genes

2.    Gregor Mendel, son of the peasant farmer, was born in Moravia in 1822.

3.     Gregor Mendel first gave the accurate explanation for the mechanism of inheritance by using hybridization technique.

4.    Mendel studied seven traits in garden pea plant individually one at a time or in combination of two or three character at a time.

5.    These characters are -

Sr. No.	Character	Dominant	Recessive
1	Height of stem	Tall (TT)	Dwarf (tt)
2	Colour of flower	Coloured (CC)	White (cc)
3	Position of flower	Aerial (AA)	Terminal (aa)
4	Colour of pod	Green (GG)	Yellow ( gg)
5	Shape of pod	Inflated (II)	Constricted (ii)
6	Shape of seed	Round (RR)	Wrinkled (rr)
7	Seed colored	Yellow (Y)	Green (y)

6.     He processed the data mathematically and statistically.

7.     Mendel postulated the principles of heredity which are known as fundamental laws of heredity, as proposed by Correns (1900).

8.    According to Mendel, transmission of characters due to ‘something’ present inside the gametic cell.

9.    To this ‘something’, he coined term ‘factors’ that are responsible for expression of a particular trait/ character.

10.  He proposed that factors are particulate in nature.

11. The term the factor is now known as gene which is given by Johannsen.

12. These factors occur in pairs in the parents and segregate from each other during gamete formation without blending/ mixing.

Reasons for Mendel’s Success:

1)    Mendel chose garden pea plant for his experiment which was an annual, naturally self-pollinating plant with several pairs of contrasting character.

2)    experiments were carefully planned and involved large sample.

3)    Mendel used pure breeding varieties which are verified personally.

4)    He considered contrasting characters for his experiment.

5)    Mendel considered only one character at a time

6)    Each character in pea plant was controlled by a single factor.

7)    These factors are located on separate chromosomes and these factors are transmitted from generation to generation

8)    He introduced the concepts of dominance and recessive.

9)    He kept accurate records.

10) He used statistical method for analyzing of the results.

11) The characters selected by Mendel where present on different chromosomes.

Genetic Terminology:

1.    Character: It is a specific feature of an organism e.g. height of stem.

2.    Trait:  An inherited character and its detectable variant e.g. Tall or dwarf.

3.    Factor: According to Mendel, it is a unit of heredity, a particle present in the organism which is responsible for the inheritance and expression of a character.  (factor is passed from one generation to the next through gametes). Factor determines a genetical (biological) character of an organism.

4.    Gene: It is a particular segment of DNA which is responsible for the inheritance and expression of that character.

5.    Alleles or Allelomorphs: The two or more alternative forms of a given gene (factor) present on identical loci (positions) of homologous chromosomes is known as allele.

Allele is a short form of Allelomorph.

6.    Dominant: It is an allele that expresses its trait even in the presence of an alternative allele i.e. in heterozygous condition only.

The allele that expresses in F1 is called dominant. (It is an allele of a pair that hides the expression of other allele in F1 generation.)

7.    Recessive: This allele is not expressed in the presence of an alternative allele (in heterozygous condition).

                                       It expresses only in the presence of another identical allele.

                                       It is an allele that does not express in F1 hybrid.

8.    Phenotype: The external apperance of an individual for any trait is called phenotype for that trait.

It is observable and is determined by different combinations of alleles. e.g. In pea, for the height of stem (plant) tall and dwarf are the two phenotypes (Tall is determined by TT or Tt and dwarf by tt).

9.    Genotype: Genetic constitution or genetical make up of an organism with respect to a particular trait.

It is representation of the genetic constitution of an individual with respect to a single character or a set of characters. e.g. pea tall plants can have genotype TT or Tt and dwarf has tt.   

10. Homozygous (pure): An individual possessing identical alleles for a particular trait, is called homozygous or pure for that trait.

                       Homozygous breeds true to the trait and produces only one type of gametes e.g., tall with TT and dwarf with tt.

11. Heterozygous: An individual possessing contrasting allele for a particular trait, is called heterozygous.  

         Heterozygous does not breed true for that trait and produces two types of gametes e.g. F1 generation hybrids (Tt).

Heterozygous individual is also called hybrid.

12. Pure line: An individual or a group of individuals (population) which is homozygous or true breeding for one or more traits, constitutes pure line i.e. plant which breeds true for a particular character. It is a descendent of a single homozygous parent produced after self-fertilization.

13. Monohybrid: It is heterozygous for one trait and is produced from a cross between two pure parents differing in single pair of contrasting characters

e.g. Hybrid tall produced in a cross between pure tall and pure dwarf parents. It is a heterozygote for a single pair of alleles.

14. F1 generation: It refers to the first filial generation. It consists of all off-springs produced from a parental cross.

15. F2 generation: The second generation (progeny) produced by selfing (inbreeding) of F₁ generation offsprings is called second filial generation.

            e.g. Progeny produced from a cross between two F₁ individuals (e.g. Tt × Tt).

16. Punnett square/checker board : It is a probability table representing different permutations and combination of fertilization between gametes of the opposite mating types.

In short, it is a diagrammatic representation of a particular cross to predict the progeny of a cross.

17. Homologous Chromosomes: The morphologically, genetically and structurally essentially identical chromosomes present in a diploid cell, are called homologous chromosomes.

               Such chromosomes synapse during meiosis.

18. Back cross: It is a cross of F₁ progeny with any of the parents (e.g. F₁ tall, pure tall× F₁ tall, pure dwarf (Tt,TT×tt).

19. Test cross: It is a cross of F₁ progeny with homozygous recessive parent (e.g. F₁ tall ×pure dwarf (Tt × tt ).

                    It is used to test the homozygous/ heterozygous nature of hybrid. It is a kind of back cross.

20. Phenotypic ratio: It is the ratio of the offsprings produced in F2 and subsequent generation with respect to their physical appearance

      e.g. 3Tall: 1 dwarf, is F2 ‘Phenotypic ratio’ in monohybrid cross.

21. Genotypic ratio: It is the ratio of the offsprings produced in the F2 and subsequent generation with respect to their genetical makeup

                  e.g. 1 TT: 2 Tt: 1 tt, is F2 genotypic ratio in monohybrid cross.