Inheritance of One Gene & Inheritance of Two Genes | Additional Study Material for NEET PDF Download

MONOHYBRID CROSS

When we consider the inheritance of one character at a time in a cross this is called monohybrid cross. First of all, Mendel selected tall and dwarf plants.

Parent

Checker Board Method :

First time, it was used by Reginald. C. Punnett (1875 - 1967)

The representation of generations to analyse in the form of symbols of squares. Male gamets lie horizontally and female gametes lie vertically.

T T = Tall (dominant homozygous),

T t = Tall (dominant heterozygous),

t t = Dwarf (recessive homozygous).

The ratio of characters (traits) appear/ visible morphologically is phenotypic ratio. It is 3: 1. Genetic constitution is called Genotype [using symbols for genes] it is 1 : 2 :1

Conclusions (results) of Monohybrid Cross

I^st Conclusion (Postulate of paired factors) :

According to Mendel each genetic character is controlled by a pair of unit factor. It is known as conclusion of paired factor or unit factor.

II^nd Conclusion (Postulate of Dominance):
This conclusion is based on F₁ - generation. When two different unit factors are present in single individual, only one unit factor is able to express itself and known as dominant unit factor. Another unit factor fails to express is the recessive factor. In the presence of dominant unit factor recessive unit factor can not express and it is known as conclusion of dominance.

•  There are two exceptions of law of dominance. [A] Incomplete dominance, [B] Co-dominance,

III^rd Conclusion (Law of segregation):

During gamete formation ; the unit factors of a pair segregate randomly and transfer inside different gamete.

Each gamete receives only one factor of a pair; so gametes are pure for a particular trait. It is known  as conclusion of purity of gametes or segregation.

• There is no exception of Law of segregation. The segregation is essential during the meiotic division in all sexually reproducing organisms. (Non-disjunction may be exception of this law).

DIHYBRID CROSS

A cross in which study of inheritance of two pairs of contrasting traits.

Mendel wanted to observe the effect of one pair of heterozygous on other pair.

Mendel selected traits for dihybrid cross for his experiment as follows :-

[1] Colour of cotyledons→ Yellow (Y)  & Green (y)

[2] Seed form → Round (R) and Wrinkled (r) yellow and round characters are dominant and green and wrinkled are recessive characters.

Mendel crossed, yellow and round seeded plants with green and wrinkled seeded plants.

All the plants in F₁–generation had yellow and round seeds.

When F₁ plants were self pollinated to produce four kinds of plants in F₂ generation such as yellow round, yellow–wrinkled, green round and green wrinkled,  there were in the ratio of 9 : 3 : 3 : 1. This ratio is known as dihybrid ratio.

Expression of yellow round (9) and green wrinkled (1)  traits shows as their parental combination.

Green Round and yellow wrinkled type of plants are produced by the results of new combination.

Demonstration by checker board method :-

F₂ - Generation 

Phenotype :-

Yellow Round = 9/16

Yellow Wrinkled = 3/16

Green Round = 3/16

Green Wrinkled = 1/16

Thus, Phenotypic Ratio = 9 : 3 : 3 : 1

Genotype:-

Homozygous yellow & Homozygous Round – YY RR = 1

Homozygous yellow & Heterozygous Round – YY Rr = 2

Heterozygous yellow & Homozygous Round – Yy RR = 2

Heterozygous yellow & Heterozygous Round – Yy Rr = 4

Homozygous yellow & Homozygous wrinkled – YY rr = 1

Heterozygous yellow & Homozygous wrinkled – Yy rr = 2

Homozygous green & Homozygous Round – yy RR = 1

Homozygous green & Heterozygous Round – yy Rr = 2

Homozygous green & Homozygous wrinkled – yy rr = 1

Thus, Genotypic Ratio = 1:2:2:4:1:2:1:2:1

Fork line method -To find out the composition of factors inside the gamete, we use fork line method.

AaBb = 4 types of gamete

Type of gamete / phenotypic category = 2ⁿ

 n = No of hybrid character or heterozygous pair.

Type of genotype = 3ⁿ

eg in dihybrid cross = 32 = 9 genotype

No. of zygote produced by selfing of a genotype = 4ⁿ

Conclusion (Law of Independent Assortment): The F₂ generation plant produce two new phenotypes, so inheritance of seed colour is independent from the inheritance of shape of seed. Otherwise it can not possible to obtain yellow wrinkled and green round type of seeds.

This observation leads to the Mendel's conclusion that different type of characters present in plants assorted independently during inheritance.

This is known as Conclusion of Independent Assortment. It is based on F₂ - generation of dihybrid cross.

The non-homologous chromosome show random distribution during anaphase-I of meiosis.

Explanation :-

A pure yellow and round seeded plant crossed with green and wrinkled seeded plant which are having genotype YYRR and yyrr to produced F₁ generation having YyRr genotype.

Both the characters recombine independently from  each other during gamete formation in F₁ generation .

Factor (R) of pair factor (Rr) is having equal chance to (Y) factor or (y) factor of gametes during recombination to form two type of gametes (YR) and (yr).

Similarly (r) factor also having equal chance with (Y) factor or (y) factor of gametes to form a two type gametes - (Yr) and (yr).

Thus, total four types of gametes - (YR), (yR), (Yr), and (yr) are formed.

Therefore, during the gametes formation in F₁ generation , independent recombination is possible.

– The law of independent assortment is most criticised. Linkage is the exception of this.

BACK CROSS

A back cross is a cross in which F₁ individuals are crossed with any of their parents.

(1) Out Cross : When F₁ individual is crossed with dominant parent then it is termed out cross. The generations obtained from this cross, all possess dominant character. so the any analysis can not possible in F1 generation.

[2] Test Cross : When F₁ progeny is crossed with recessive parent then it is called test cross. The total generations obtained from this cross, 50% having dominant character and 50% having recessive character. [Monohybrid test cross]. Test cross helps to find out the  genotype of dominant individual.

[a] Monohybrid Test Cross :- The progeny obtained from the monohybrid test cross are in equal proportion , means 50%  is dominant phenotypes and 50% is recessive phenotypes.

It can be represented in symbolic forms as follows.

[b] Dihybrid Test Cross:- The progeny is obtained from dihybrid test cross are four types and each of them is 25%.

The ratio of Dihybrid test cross = 1:1:1:1

Conclusion:-  In test cross phenotypes and genotypes ratio are same.  

RECIPROCAL CROSS

When two parents are used in two experiments in such a way that in 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. such type of a set of two experiments is called Reciprocal cross.

Characters which are controlled by karyogene are not affected by Reciprocal cross. In case of cytoplasmic inheritance result change by Reciprocal cross.

(a) 

 (b)