Maharashtra Board Class 12 Biology Chapter 3 Inheritance and Variation PDF Download

Inheritance and Variation

Chromosomes and Mechanism of Inheritance

The transmission of genetic information from generation to generation is known as heredity or inheritance. The mechanism of inheritance was successfully investigated before chromosomes had been observed or genes were known.

Gregor Mendel, son of the peasant farmer, was born in Moravia in 1822. Gregor Mendel first gave the accurate explanation for the mechanism of inheritance by using hybridization technique.

Inheritance of the seven traits in garden pea plant were studied individually one at a time or in combination of two or three character at a time. He processed the data mathematically and statistically.

Mendel postulated the principles of heredity which then became the fundamental laws of heredity, as proposed by Correns (1900). He visualized that the traits as such are not inherited physically but by 'something' present inside the gametic cell. To this 'something', he coined term 'factors' that are responsible for expression of a particular trait/character. He proposed that factors are particulate in nature. The Mendelian factors are now termed as 'genes'. These factors occur in pairs in the parents and segregate from each other during gamete formation without blending/mixing.

Reasons for Mendel's Success

His experiments were carefully planned and involved large sample.

He carefully recorded the number of plants of each type and expressed his results as ratios.

Teacher's Note

Mendel did careful experiments with pea plants to find how traits pass from parents to children. Just like your mother's eyes or father's nose passes to you, Mendel found that plants also pass traits in the same way.

Exam Trick

Remember: Mendel is the father of genetics. He found that traits are passed by 'factors' (genes). Think of factors as carriers - they carry information like a postman carries letters from one house to another.

Points to Remember

Heredity means passing traits from parents to children.
Mendel used pea plants for his experiments.
Factors are units that carry information about traits.
Factors are now called genes.
Factors pass from parents to children without mixing.

Seven Traits in Pea Plant

Teacher's Note

These seven traits of pea plant are easy to see and count. Just like you can see the difference between tall children and short children in your class, Mendel could easily see these differences in pea plants.

Exam Trick

Remember the seven traits by thinking: Seeds look different (round/wrinkled), seeds have different colors, pods look different, and so on. Each trait has only two choices - just like your answer in exam has two choices (True or False).

Points to Remember

Pea plant has seven different traits.
Each trait has two opposite forms (tall or short).
Dominant traits are easily visible (round, yellow, tall).
Recessive traits hide when dominant traits are present.
Mendel studied one or two traits at a time.

Genetic Terminology

Character: It is a specific feature of an organism e.g. height of stem.

Trait: An inherited character and its detectable variant e.g. Tall or dwarf.

Factor: It is a unit of heredity, a particle present in the organism which is responsible for the inheritance and expression of a character. (factor is passed from one generation to the next through gametes). Factor determines a genetical (biological) character of an organism.

Gene: It is a particular segment of DNA which is responsible for the inheritance and expression of that character.

Alleles or Allelomorphs: The two or more alternative forms of a given gene (factor) are called alleles of each other. They occupy identical loci (positions) on homologous chromosomes. Allele is a short form of Allelomorph.

Dominant: It is an allele that expresses its trait even in the presence of an alternative allele i.e. in heterozygous condition only. Alternatively, the allele that expresses in F1 is called dominant. (It is an allele of a pair that masks the expression of other allele in F1 generation.)

Recessive: This allele is not expressed in the presence of an alternative allele (in heterozygous condition). It expresses only in the presence of another identical allele. It is an allele that does not express in F1 hybrid.

Phenotype: The external appearance of an individual for any trait is called phenotype for that trait. It is observable and is determined by different combinations of alleles. e.g. In pea, for the height of stem (plant) tall and dwarf are the two phenotypes (Tall is determined by TT or Tt and dwarf by tt).

Genotype: Genetic constitution or genetic make up of an organism with respect to a particular trait. It is representation of the genetic constitution of an individual with respect to a single character or a set of characters. e.g. pea tall plants can have genotype TT or Tt and dwarf has tt.

Homozygous (pure): An individual possessing identical alleles for a particular trait, is called homozygous or pure for that trait. Homozygous breeds true to the trait and produces only one type of gametes e.g., tall with TT and dwarf with tt.

Heterozygous: An individual possessing contrasting alleles for a particular trait, is called heterozygous. Heterozygous does not breed true for that trait and produces two types of gametes e.g. F1 generation hybrids (Tt). Heterozygous individual is also called hybrid.

Pure line: An individual or a group of individuals (population) which is homozygous or true breeding for one or more traits, constitutes pure line i.e. plant which breeds true for a particular character. It is a descendent of a single homozygous parent produced after self fertilization.

Monohybrid: It is heterozygous for one trait and is produced from a cross between two pure parents differing in single pair of contrasting characters e.g. Hybrid tall produced in a cross between pure tall and pure dwarf parents. It is a heterozygote for a single pair of alleles.

F1 generation: It refers to the first filial generation. It consists of all off-springs produced from a parental cross. Alternatively, it is first generation from a given mating between pure parents having contrasting characters.

Teacher's Note

These genetic terms help us understand how traits are passed. Think of alleles like two different recipes for the same dish - dominant is the recipe that wins, recessive is the one that loses when both are present together.

Exam Trick

Remember: Phenotype is what you see (appearance), Genotype is what is hidden inside (genetic makeup). Phenotype = Photo (like a photo shows your appearance). Use this trick to never forget!

Points to Remember

Gene is a small piece of DNA that controls a trait.
Alleles are two or more forms of the same gene.
Dominant allele always shows its effect.
Recessive allele hides when dominant is present.
Homozygous means both alleles are the same, heterozygous means they are different.

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