The principle of inheritance and variation in the class 12 biology syllabus is a very important topic with a good weightage in the competitive exams. This chapter explains the basic concepts of inheritance and theories put forward by biologists. In this chapter, students will learn the laws of heredity, incomplete dominance, the chromosomal theory of inheritance, Sex determination, mutation, Mendel’s law of inheritance, and much more. The idea of inheritance pattern was developed after Mendel’s work and his contributions in the field of genetics. This chapter provides an insight into the essence of modern biology - which is genetics.
Let’s have a look at the notes of chapter principles of inheritance and variation in this article.
An elephant gives birth to only an elephant baby, a mango seed only gives rise to a mango tree, human beings give birth to only young human beings. This kind of continuation of species by giving birth to individuals with similar heredity or traits is because of inheritance. Thus, heredity is a process of transmitting heritable traits to young ones by their parents. The process of heredity can easily be described as the process of transferring characteristics or traits from parents to offspring or their unique individuals. This transfer can be conducted through either asexual reproduction or sexual reproduction. The traits that continue the legacy are found in genes on the chromosomes within the body of an individual. Genes are chemicals that carry genetic information about how to code proteins. Genes are the carriers of instructions that could have a similarity to one of their two parents.
Sir Gregor Mendel performed various experiments to study and know more about inheritance, heredity, genetics, and variation. Also known as the Father of Genetics was the first one in his field to conduct experiments on Inheritance, Genetics, Heredity, and Variations.
What is Variation?
The difference in DNA among individuals is called a variation in genetics. Mutation, genetic recombination, crossing over, environmental changes, and other such ways lead to genetic variation. It is the impact of these on the way genes are expressed.
Mendel’s Law of Inheritance
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Mendel performed experiments on Garden pea in the back of his garden to establish the principle of heredity. For this, he took 14 true-breeding pea plants with seven distinguishable characters that had two opposite traits.
Mendel called genes “factors” that pass to the offspring from the parents.
And the factors (genes) that coded for a pair of opposite traits were called “alleles”.
Based on his observation, Mendel gave three laws of inheritance.
1. Law of Dominance
In the case of the formation of the heterozygote, one of the alleles is dominant. And, the dominant allele is expressed in the phenotype. So, when homozygous tall with (TT) alleles are crossed with dwarf (tt) plants, all the resulting offspring plants are tall and have the tall dominant trait with genotype Tt.
2. Law of Segregation of Genes
At the time of gamete formation during meiosis, each allele separates. So, the characters are passed to gametes without blending. Also, heterozygotes produce different kinds of gametes while homozygous only have one type.
3. Law of Independent Assortment
Mendel proposed another set of rules which was based on the observations he made on dihybrid crosses which are known as Mendel’s Law of Independent Assortment. This particular law of Mendel states that “when two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters”. In simple words, this law explains the independent separation of genes. It means that alleles inherited by progeny from different traits are acquired independently. A simple way to understand this law is with the help of a Punette square when carried out with two pairs of genes during the stage of meiosis and egg-production and pollen in the F1 generation of the RrYy plant.
The process of Mutation is a result of alteration in DNA sequences which then results in the changes in the phenotype and genotype of an entity. The mutation also brings variations in DNA along with the recombination. The chromosomal alteration has its negative effects and causes aberrations or abnormalities. These aberrations or abnormalities are usually observed in the cancer cells, according to the studies. Another type of mutation that takes place as a result of changes in a single base pair of DN is called a point mutation. For example insertions and deletions of base pairs of DNA along with sickle cell anaemia leads to frameshift mutations.
Mentioned below are some of Mendelian Disorders:-
Sickle cell anaemia