The chapter principles of inheritance and variation are based on the principles or laws of inheritance. It highlights the important points of inheritance, such as heredity, genetics, genes, etc. It deeply explains the concept of heredity, inheritance, and its variations. 

Several related questions deal with a study of biology scientifically, known as genetics. It is a subject that also deals with inheritance along with the different characters from parents to the offspring. The variation is the degree to which the progeny differs from the parents. 

Let us look at the most important topics of Chapter 5, principles of inheritance and variations. The important topics are listed as follows:

• Mendel’s Laws of Inheritance
• Incomplete Dominance
• Chromosomal Theory of Inheritance
• Sex Determination
• Mutation
• Genetic-Disorders

The details of the different topics of NCERT Solutions Class 12 biology chapter 5 are as follows:

 

Mendel’s laws of inheritance:

Mendel conducted experiments such as artificial pollination and cross-pollination over the breeding pea lines. An actual breeding line has continuously undergone self-pollination and shows stable character inheritance for various generations. It took 14 true-breeding trees, out of which seven have different characters with two opposite traits. Male genes are called "factors", and opposite genes are called alleles. 

Gregor Johann Mendel is called the "father of genetics". Which gave three laws of inheritance based on his observation:

• Law of dominance:
  This law of Mendel states that in a heterozygote, one character will hide the presence of other characters for the same traits. Only one allele is dominant and expressed in the phenotype. 

• Law of segregation of genes:
  At the time of gamete formation, each allele separates during meiosis. Homozygotes produce a single kind of gametes, and heterozygotes have different kinds of gametes. There is no blending of the passing of different gametes and characters also. 

• Law of independent assortment:
  This law of Mendel’s states that the alleles of more than two different genes get sorted into the gametes independently of one another. We can say that the allele gamete received for one gene does not influence the allele received for another gene.

 

Incomplete dominance:

The incomplete dominance means the dilution of the dominant allele with the recessive allele and, as a result, in the new heterozygous phenotype. It is also called a form of gene interaction in which both alleles of a gene at which locus are partially expressed. For example, In red rose, the red colour allele is dominant over the allele for white colour. 

 

Codominance:

It is a concept where both the alleles express themselves together in an individual is called co-dominant.

 

Chromosomal theory of inheritance:

Sutton and Boveri is the father of the chromosomal theory of inheritance, which states that chromosomes are the vehicles of genetic heredity. The behaviour of chromosomes involves segregation, assortment, independence, linkage, etc. Some essential points of this theory are:

• Chromosomes occur as a homologous pair. 
• The two gene alleles are located at the same site on the homologous pair of chromosomes. 
• It separates during meiosis in the process of gamete formation.
• It segregates and assorts independently. 

 

Sex determination:

The determination system of biology that determines the development of sexual characteristics in an organism is called sex determination. There are unique systems of sex determination that are present in the different organisms. Hermann Henking first developed the sex-determination system. Firstly, he observed the X chromosome and named it X body. The chromosomes which determine the development of sexual characters are called the sex chromosomes, and the others are called the autosomes. The production of different kinds of gamete by a male called male heterogamete and a female called female heterogamete. 

Table showing sex determination in different species:

 

Mutation:

The process in which any changes in the sequence of DNA occur is called a mutation. A mutation can be the primary cause of cancer. The mutation is a gene that gets inherited from one generation to another. It changes the genotypes and phenotypes of an organism. Not all mutations are harmful. There are four types of mutation:

• Germline mutation occurs in gametes and is any noticeable variation within germ cells. 
• Chromosomal alterations: In that mutation, the changes in the chromosome structure occurred. 
• Point mutations: There is a substitution in the single base pair of DNA in such a mutation. 
• Frameshift mutations: There is an insertion or deletion of one or more pairs of bases in DNA under this mutation. 

 

Genetic disorders:

Genetic disorders occur when a mutation affects your genes or an alteration in chromosomes. Such conditions are inherited in the human being at the time of mutation. A pedigree analysis helps determine the risk of getting a genetic disorder at the time of offspring by studying the inheritance pattern of a particular trait. There are two groups of genetic disorders such as:

• Mendelian disorders:
  The mendelian disorder is caused due to alteration in a single gene. It follows the same inheritance pattern as per Mendel’s law. It also determines the type of trait. Rather it is dominant or recessive. 

• Chromosomal disorders:
  Chromosomal disorders are caused due to excess, abnormal, or absence in the arrangement of chromosomes. These disorders are further classified into two categories, aneuploidy which happens due to failure of segregation of chromosomes during the anaphase of meiosis. And polyploidy happens due to the failure of cytokinesis. It is mostly found in plants.