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MONOHYBRID CROSS

Inheritance of One Gene & Inheritance of Two Genes | Additional Study Material for NEETWhen 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


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


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.

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

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

Ist 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.

IInd 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.

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

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

IIIrd 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.

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

  • 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.

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

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 :-

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

F2 - Generation 

 

Y R

Yr

yR

yr

Y R

YYRR

YYRr

YyRR

YyRr

Yr

YYRr

YYrr

YyRr

Yyrr

yR

YyRR

YyRr

yyRR

yyRr

yr

YyRr

Yyrr

yyRr

yyrr

 

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

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

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

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.

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

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

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

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

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

 (b) Inheritance of One Gene & Inheritance of Two Genes | Additional Study Material for NEET

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FAQs on Inheritance of One Gene & Inheritance of Two Genes - Additional Study Material for NEET

1. What is inheritance of one gene?
Ans. Inheritance of one gene refers to the transmission of a specific trait or characteristic from one generation to another through the passing of a single gene. This type of inheritance is commonly observed in simple Mendelian traits, where a single gene controls the expression of a particular phenotype.
2. What is inheritance of two genes?
Ans. Inheritance of two genes, also known as dihybrid inheritance, involves the transmission of two different genes from parents to offspring. This type of inheritance considers the simultaneous inheritance of two traits controlled by different genes located on separate chromosomes. It takes into account the assortment of alleles during meiosis and the potential for genetic recombination.
3. How does inheritance of one gene differ from inheritance of two genes?
Ans. Inheritance of one gene involves the transmission of a single gene controlling a specific trait, while inheritance of two genes considers the simultaneous inheritance of two different genes controlling two different traits. In one gene inheritance, the inheritance pattern follows Mendelian genetics, whereas in two gene inheritance, it involves more complex patterns due to the assortment and recombination of alleles.
4. Can inheritance of one gene influence inheritance of two genes?
Ans. Yes, inheritance of one gene can influence the inheritance of two genes in certain cases. This can occur when the expression of one gene affects the expression of another gene, creating a genetic interaction. For example, one gene may act as a modifier for the expression of another gene, leading to variations in the phenotypic outcome. These interactions can further complicate the inheritance patterns of multiple genes.
5. What are some examples of traits inherited by one gene and two genes?
Ans. Examples of traits inherited by one gene include Mendelian traits such as tongue rolling, blood type, and attached earlobes. These traits are controlled by a single gene and exhibit a clear dominant-recessive pattern of inheritance. On the other hand, examples of traits inherited by two genes include eye color, height, and skin pigmentation. These traits are influenced by the interaction of multiple genes and often exhibit more complex inheritance patterns.
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