Mendel’s laws of Inheritance/Mendelism

      Mendelism or Mendelian principles are rules of inheritance first discovered by Mendel.

      All the principles proposed were based on Monohybrid and dihybrid crosses.

      Mendel proposed four principles or laws ( FRAMED TO THREE LAWS)

One gene inheritance:

1)   Every character is controlled by a gene that has at least two alleles (monogenic inheritance).

2)   Study of inheritance of a single pair of alleles or factors for a characters of trait at a time is known as one gene inheritance.

3)   On this basis Mendel proposed a set of postulates which are known as three law of inheritance.

 

Principle of Paired Factors:

1)   A characters is represented in an organisms generally diploid by at least two factors which are present on the two homologous chromosomes occupying the same locus.

2)   These two factors may be same i.e. homozygous or different i.e. heterozygous.

3)   Factors representing the alternate or same form of character are called as alleles or allelomorph.

      E.g. Tallness in pea plant exists in two different forms as ‘TT’ i.e. homozygous or ‘Tt’ which is heterozygous.

I) Law or Principle of Dominance:

1)   In a heterozygote condition or hybrid each character is represented by a pair of contrasting factor called alleles.

2)   Of these two alleles, only one is able to express itself in the organism and is called as dominant allele or dominant factor.

3)   The other factor which fails to express itself in the heterozygote condition is called as recessive allele or recessive factor.

4)   Mendel used upper case letter for dominant allele and small letter for recessive allele as a symbol.

5)   For a trait height of the pea plant, tallness is dominant over dwarfness which is recessive.

6)   Significance:

a)   It helps to explain why F1 generation express characters of only one parent.

b)   It is satisfactorily explains the occurrence of 3:1 ratio in F2 generation in a monohybrid cross.

c)   It also explain why mixed population is superior as it suppresses many of the defective recessive alleles.

II) Principle or Law of Segregation:

1)   The two factors of a characters present in an individuals keep their identity distinct, separate at the time of gametogenesis or sporogenesis,get randomly distributed  to different gametes and then get paired again in different offsprings as per the principle of probability.

2)   The principle of segregation can be deduced from a reciprocal monohybrid cross.

3)   The principle of segregation is the most fundamental principle of heredity that has universal application with no exception.

4)   Bateson called this law as law of purity of gametes because segregation of the two Mendelian factors of a trait results in gametes receiving one factor out of a pair.

      As a result of which gametes are always pure for a character.

5)   It is also known as law of non-mixing of alleles.

 

Inheritance of two genes:

1)   To verify his results of monohybrid crosses, Mendel also crossed pea plants differing in two characters which helped him to understand inheritance of two genes at a time.

2)   It was found that inheritance of one pair is independent of inheritance of the other.

3)   On this basis, Mendel proposed a second set of generalization which is now called as law of independent assortment.

III) Principle or Law of Independent Assortment:

1)   Correns called this law as second law of Mendelism.

2)   The law states that, the two factors of each character assort or separate independent of the factor of other characters at a time of gamete formation and get randomly rearranged in the offsprings producing both parental and new combinations of traits.

3)   The law can be explained by studying a dihybrid cross, between two homozygous plants.

4)   Mendel obtained following data in the above said cross:

      Yellow and Round = 9/16

      Yellow and Wrinkled = 3/16

      Green and Round - 3/16

      Green and Wrinkled = 1/16

      Thus, the phenotypic ratio obtained was 9: 3: 3:1.

4)   The occurrence of four type of plants in F2 generalization of dihybrid cross shows that the factors of each of the two characters assort independent of the others as if the other pair is absent.

5)   This can be proved by following results:

      Seed colour = Yellow (9+3) = 12: Green (3+1) = 4 i.e. 3:1

      Seed texture = Round (9+3) =12, Wrinkled (3+1) = 4 i.e. 3:1.

      The result for each characters is exactly identical to monohybrid cross.

6) Objections:

a)   The law is applicable to only those factors or genes which are either distantly located on the same chromosome or occur on different chromosomes.

b)   All the genes present over the chromosome are inherited together except when crossing over takes place.

c)   The phenomenon of inheritance of number of genes or factors due to their occurrence together on the same chromosomes is known as linkage.

 

BACK CROSS AND TEST CROSS

1] BACK CROSS: - A cross-made between F1 hybrid and any of its parents either homozygous dominant or homozygous recessive is called as back cross.

2] TEST CROSS: - A cross made between F1 hybrid and its homozygous recessive parent only is called as test cross.

“Every test cross is a back cross but every back cross is not a test cross.”

 

Monohybrid back cross

1] A cross made between F1 hybrid and any of its parent either homozygous dominant or homozygous recessive by considering single character is called as monohybrid back cross.

2] It can be explained with the help of following example,

Character selected: - Height of plant

                        1] Dominant—Tall

                        2] Recessive—Dwarf

Parents: -   Homozygous dominant X   Homozygous recessive

Phenotype: -      Pure tall             X     Pure dwarf

                                                   

Genotype: -     TT           X             tt

                                               

Gametes: -    T               t

 

F1 generation: -                        Tt         Heterozygous tall (Hybrid)

 

For monohybrid back cross:-

Parent: -      F1 hybrid         X  Dominant Parent

 

Phenotype: - Hybrid tall           Pure tall

                                         

Genotype: -       Tt             X      TT

                                         

Gametes: -       T   t        X         T

 

Punnett’s Square :- TRY THIS

       

Genotypic ratio:-

                A] Pure tall   :   Hybrid tall

                  1       :       1

                B] Phenotypically all are same i.e. tall.

Monohybrid test cross

1] A cross made between F1 hybrid and its homozygous recessive parent by considering single character is called as monohybrid test cross.

2] It can be explained with the help of following example,

Character selected: - Height of plant

                        1] Dominant—Tall

                        2] Recessive—Dwarf

For monohybrid test cross:-

Parent: -    F1 hybrid   X      Recessive parent

 

Phenotype: -   Hybrid tall   X          Dwarf              

 

Genotype: -        Tt             X         tt

       

Gametes: -        T    t            X         t

 

Punnett’s Square :- TRY THIS

Genotypic ratio:-

                Hybrid tall: Dwarf

                        1       :     1

Phenotypic ratio: -   Tall:  Dwarf

                                 1    :     1

Dihybrid back cross

1] 1] A cross made between F1 hybrid and any of its parent either
homozygous dominant or homozygous recessive by considering two character is called as dihybrid back cross.

2] It can be explained with the help of following example,

Character selected: - Height of plant and colour of flower.

        1] Dominant—Tall, red

        2] Recessive—Dwarf .white

Parents: -  Homozygous dominant   X   Homozygous recessive

Phenotype: -    Pure tall red              X      Pure dwarf white

 

Genotype: -     TTRR                X              tr

                                                                       

Gametes: -    TR                   tr

 

F1 generation: -      TtRr (Heterozygous tail) (hybrid)

 

For dihybrid back cross:

Parent: -       F1 hybrid                   X      Dominant Parent

Phenotype: - Hybrid tall red                    Pure tall red

Genotype: -     TtRr                  X          TTRR

                                        

Gametes: -     TR, Tr , tR, tr           TR

Punnett’s Square :- TRY THIS

               

Genotypic ratio.-

      A]   1   : 1  :  1  :   1

        B]     Phenotypically all are same i.e. Tall red .

Dihybrid test cross

1] A cross made between F1 hybrid and any of its homozygous recessive parent by considering two characters is called as dihybrid back cross.

2] It can be explained with the help of following example,

Character selected: Height of plant and colour of flower.

                1] Dominant—Tall, red

                2] Recessive—Dwarf, white

For dihybrid back cross:

Parent: -  F1 hybrid                  X     recessive Parent

Phenotype: -    Hybrid tall red          dwarf white

Genotype: -    TtRr         X     ttrr

 Gametes: -      TR, Tr, tR, tr             tr

Punnett’s Square - TRY THIS

 Phenotypic ratio:

      A] TallRed   : Tall white, Dwarf red: Dwarf white

            1     :         1    :   1       : 1

        B] Genotypic ratio is,

            1     :         1   :     1       : 1

Significance of test or back cross.

1] Pure line varieties: - It helps in development of homozygous true breeding variety.

2] It explains the law of dominance and law of segregation.

3] It explains homozygous and heterozygous conditions of the individual’s i.e. determine the genotypes of the unknown plants.

4] It is easy and quick method of obtaining exotic variety.

5] Back cross is usually used in hybridization programme due to its simple ratios.