Explain monohybrid cross with suitable example.

Ans.: 1] Monohybrid cross: - The cross made between two homozygous plants for a single pair of contrasting character is called as monohybrid cross.

2] Monohybrid ratio: - The phenotypic and genotypic ratios obtained in monohybrid cross are called as monohybrid ratio.

3] The cross can be explained with the help of following example,

A] Tallness is the dominant character and dwarfness is the recessive character in sweet pea plant.

B] A homozygous dominant pea plant (TT) is crossed with a homozygous recessive pea plant (tt).

C] Actual crossing between these two plants can be explained as under,

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 F2 Generation:-

Parents: - Heterozygous Tall    X    Heterozygous Tall

Genotypes: -              Tt                 X              Tt

 

Gametes:-       T        t              T           t

Punnet’s square/ checker board:-

                         

Results:- a] Phenotypic ratio: -     Tall : Dwarf

                                      3   :    1

      b] Genotypic ratio:-      

Pure tall : Hybrid tall : Pure dwarf

 1    :          2           :     1

D] Significance: - Monohybrid ratio obtained by Mendel explains the Law of Dominance and Law of Segregation.

 

Dihybrid cross and its explanation:

1) When two allelic pairs are used for crossing, it is called as dihybrid cross.

2) Mendel conducted experiments to study the segregation and transmission of two pairs of contrasting traits at a time.

3) Mendel observed that a cross between round yellow and green wrinkled seeds (P1) produced only round and yellow seeds in F1 generation.

4) But in F2 generation four different types of combinations were produced.

    Round yellow – 9 parental combinations

    Round green – 3 non parental combinations

    Wrinkled yellow – 3 non-parental combinations

    Wrinkled green – 1parental combination.

5) Thus, the offspring of F2 generation were produced in the ratio of 9 : 3 : 3 :1 phenotypically and 1 :2 :1:2:4:2:1:2:1 genotypically.  The ratio is called dihybrid ratio.

6) Mendel’s explanation :

a) Mendel explained the results by assuming that the round and yellow characters are dominant over wrinkled and green so that all F1 offsprings are round and yellow.

b) In F2 generation since all the four characters were assorted out independently he said that a pair of alternating or contrasting characters behave independently of the other pair i.e. seed colour is independent of seed coat.

c) Therefore, at the time of gamete formation genes for round or wrinkled character of seed coat assorted out independently of yellow or green colour of the seed.

d) As a result of this four types of gametes with two old and two new combinations i.e. YR, Yr, yR, yr are produced from F1 hybrid.

e) These four types of gametes on random mating produce four types of offsprings in the ratio 9:3:3:1 in F2 generation (Law of independent assortment).

    Dihybrid test cross ratio is 1:1:1:1