Class 12th Biology Chapter 4 Principles Of Inheritance And Variation

 NCERT Solutions for Class 12 Biology Chapter 4 : This chapter is about Principles of Inheritance And Variation. We are going to cover the topics like Mendel experiment,laws of Mendel's experiment, Inheritance of one gene, incomplete dominance, mutation,linkage & Recombination,etc. We have prepared this article very carefully and tried to include all those important topics and notes from the chapter which you can use in your preparation for 12th exam or any other upcoming competitive exams.

 

In this chapter we have covered all the topics related to Principles of Inheritance And Variation which are listed below:-

Mendel Experiment

Laws of Mendel's Experiment

Inheritance of one gene

Incomplete Dominance

Codominance or Multiple Allelism 

Linkage and Recombination

Sex determination

Mutation

 

 

             Principles of Inheritance And Variation

 

 

Heredity
It is the transmission of traits in which the traits from one generation transmitted to the following generation.
Many inheritance & types of genes are passed on because the offsprings are produced are better adapted  to new changing conditions.

Genetics
It is the branch of biology which deals with inherited variations and laws of heredity  and is known as genetics.
‘Gregor Johann Mendel’ is the father of genetics. There are three laws which are Mendel’ s laws proposed by him.

Inheritance 
It is the process by which the characters are passed from one parent to the progeny.

Variation It is a process in which the closely related organisms differ among themselves.

It is termed by Hugo de Vries.

It may be induced artificially or occur spontaneously in nature.

It results in all heritable changes which alter the phenotype of individual in a broad sense.
It can be defined as the degree by which the progeny differs from its parent is known as variation.

Mendel’ s Experiment 
After performing experiments on a pea plants ‘ Gregor Mendel ‘ he discovered the fundamental laws of inheritance which are divided into three laws. Mendel selected seven characters of opposite traits of a  pea plants and on 14 true breeding pea plants varieties he conducted his experiment.

The population obtained by the result of plants crossing with contrasting characters is known as first filial generation or F1.

The F1 plants obtains progeny due to self fertilization which represents second filial generation or F2.

From the crossing of two individuals the plants obtained is known as hybrids and the process is known as hybridization.

Character	Recessive	Dominant
Stem height	Dwarf	Tall
Flower colour	White	Violet
Pod shape	Constricted	Inflated
Pod colour	Yellow	Green
Flower position	Axial	Terminal
Seed shape	Wrinkled	Round
Seed colour	Green	Yellow

Mendel’ s Laws of inheritance 
There are 3 laws proposed by Mendel.
1.Law of Dominance 
2. Law of Segregation or Law of purity of gametes.
3. Law of Independent assortment 

1. Law of Dominance:- In this law there is a unit which controls all the traits and the characters which is known as Factors. Alleles are the factors which are found in pairs.
When the alleles are found in same pair they are known as homozygous whereas if they occur in different pairs they are known as heterozygous.Homozygous can be either recessive or dominant and the Heterozygous will always be dominant.
Example :-
For the tallness the allele is dominant over the dwarfism allele.

2. Law of Segregation :-  In this law it states that when a pair of alleles are brought together in hybrid (F1) and they remain together  without blending but  separate complete during gamete formation.It occurs in such a manner that the pair of  alleles  recieves only one of the two factors from one another.

3. Law of Independent assortment:- In this law it states that in hybrid F1 when  two pairs of independent alleles are brought together then they show independent dominant effects.The law of Segregation operates in the formation of gametes. But, the factor assort themselves.

Punnett square 
It is the graphical representation in a genetic cross to calculate the probability of all possible genotypes of offsprings.

Inheritance of one generation
With the help of a monohybrid cross the inheritance of one gene using Mendel’ s law can be explained.It states that the cross between tall and dwarf is done and it results in tall hybrid plants in F1 generation or first filial generation. And later on , these progenies are self pollinated which results in the production of F2 generation or second filial generation where one dwarf and three tall plant will formed showing the ratio of 3:1.

Incomplete dominance
To Mendel’ s law of dominance some character are exceptions and shows incomplete dominance. The hybrid individuals are less or more intermediate in such cases between two parents and this phenomenon is called as incomplete dominance.The one allele is not completely dominant over the other allele. It results in a combined phenotype. The other name of incomplete dominance is mosaic or partial dominance.

The Mirabilis jalapa which bears pink flowers also the Marvel of Peru is commonly called as four O’clock plant. The dominant (red flowers) were crossed with recessive (white flowers)  and pink flowers in F1 generation.
In F2 Generation they occur in 1:2:1 ratio as red, pink and white flowers.The 1:2:1 is the genotypic and phenotypic ratio.

Codominance or Multiple Allelism 
For a single gene on the same chromosome in which three or more alternative forms of alleles are  present in a condition then this is known as Multiple Allelism and alleles are called multiple alleles.

A well known example of autosomal codominants is A, B ,O blood group in humans. Here the inheritance of ABO blood group is a gene I where I represent isohemagglutinin which remains 3 allelic expression they are IA ,IB and I. The IA is for blood group A and the codes for glycoprotein A and Gene IB is  for blood group B and the Code for glycoprotein B. ‘I’ gene does not produce glycoprotein and the person in a homozygous condition who have two alleles together will have O blood group. IA and IB gene are dominant over I gene but alleles are equally dominant and they both produce glycoprotein A and B which results in AB blood group. These types of alleles are called as co dominant alleles.

Inheritance of two genes
This type of inheritance requires two characters of same trait. To explain the inheritance of two genes Mendel has chosen two traits which involves shape and colour of the seed. With the help of a dihybrid cross it can be observed .Here the Y alphabet represent the colour of the seed which is dominant yellow, y represent green colour for Recessive and R represent the shape of the seed which is round ,r represent wrinkled shape of seed. RRYY and rryy is the genotype of the parents. The RY and ry gamete will produce F1 hybrid RrYy after fertilization.
For the Law of independent assortment the dihybrid cross is useful. The F1 ratio found to be 9:3:3:1 after Self fertilization of F1.

Chromosomal theory of inheritance 
This theory was given by Walter Sutton in 1902 which explains the linear structure of chromosomes in particular sites with genes and mentioned as loci and Boveri also studied this theory seperately and is also known by Boveri-Sutton Chromosome theory . 
1. On the chromosomes genes are found at specific location.
2. Chromosomes independently assort as well as segregate.
3. Number of chromosomes becomes diploid after fertilization.

Linkage and Recombination 
To under the process of linkage and recombination different experiments were conducted by T.H Morgan . To study about the genes that were sex linked T.H Morgan performed Dihybrid crosses in house fly (Drosophilia) . 
Example :- The brown bodied and red eyes males were crossed by yellow bodied and white eyes females by T.H Morgan and Self breed their F1 progeny which results in 9:3:3:1 ratio in F2 generation therefore, the genes do not segregate independently and it results that the genes are linked. The condition by which the genes show linkage physically is known as linkage.

Expressing of both characters of parents by organisms is known as recombinations.In Sexually reproducing organisms the phenomenon by which mixing of paternal and maternal character is responsible is called recombination.It is a condition by which genetic material is rearranged.

Sex determination

This process is defined as where the gender of a child can be revealed.To know the sex of a child the chromosome are responsible for it. Males have X and Y Chromosomes and females have XX chromosomes in humans.In female the egg will be having X chromosome identical but in males it will not be identical for sperms because of one X and Y chromosome. Here same type of chromosomes in females is known as homogametic and different types of chromosomes in males is known as heterogametic.

 

For mechanism of sex determination in insects is of XO type.  The x chromosome is present in eggs and in sperms there may be one or none X chromosome.Here different type of chromosomes in females is known as heterogametic and samet types of chromosomes in males is known as homogametic.

Mutations

It is a phenomenon by which it results in alternation of DNA sequences and changes the results consequently in the phenotype and genotype of an organism.

There are different types of Mutations are:-
Frame Shift mutation,Deletion,Duplication,Substituiton,Insertion

1.The addition of deletion of base which cause gross change in amino acids which shifts the reading frame of condoms at site of mutation is known as frameshift mutation.

2.In excess of normal complement the presence of part of a chromosome is called duplication.

3.The loss or absence of section of chromosome and also may involve one or more gene is called deficiency or deletion/

4.Insertion is the process in which the addition of DNA bases takes place.

Chromosomal mutation or aberration

In phenotypically the structural changes in chromosomes is known as chromosomal mutation or aberration.

It was analysed by H.J.Muller at Drosophila in 1928 and Barbara McClintock at Zea in 1930.

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