CHAPTER 3. INHERITANCE AND VARIATION

Introduction:

1) One of the most important sub divisions of biology is genetics.

2) It is the science of transmission of characters from one generation to other.

3) It is also the mechanism of action of hereditary units which brings about variations in offsprings.

4) The term genetics was first used by Bateson in 1905.

5) Thus, genetics is science of heredity and variations.

6) Heredity indicates ‘like begets like’ whereas variations explain differences between two individuals.

7) Genetics: The branch of biology which deals with the study of heredity and variations is known as genetics.

8) Heredity: Heredity is the transmission of genetically based characters from parents to their offsprings.

Inheritance

    The process by which characters of the traits pass from one generation to next is known as inheritance.

Variations

    Variations is the degree of differences in the progeny and between the progeny and the parents.

Heredity

1) Heredity deals with the phenomenon of like begets like i.e. members of each species breed true to their type.

2) In other words, heredity operates within confines of a species.

3) Pre Mendelian concepts of Heredity:

    Pre Mendelian concepts of inheritance are also called as theories of blending inheritance.

    The important pre-Mendelian concepts are as follows:

a)    Moist vapour theory:

    Pythagoras (580-500 BC) proposed that every part of an organism emit some kind of vapour. These vapours are ultimately responsible for development of different organs of the organism.

b)    Fluid theory:

    Empedocles (504-433 BC) thought that every part of the body produces a fluid. This fluid then form offsprings. A defect in this, result in the formation of defective organs.

c)    Reproductive blood theory:

    Aristotle (384-322 BC) thought that an embryo is produced due to mixing of reproductive bloods of the two parents. It is pure in case of males and impure in females. As a results, male contributes more characteristics.

d)    Performation theory:

    Malphighi (1673) believed that a miniature individual of extremely small size is present in sperm or egg. It was named homonculus. Homonculus can be male or female. Homonculus grows into a new individual after it receives nourishment in the womb of female.

e)    Particulate theory:

    Maupertius (1689-1759) proposed that various parts of the body produce minute particles for reproduction. An individual is formed when the particles from male and female unit.

f)    Theory of Pangenesis:

    Darwin (1868) proposed that every cell produces a gemmule or pangene. The pangene carries parental traits as well as variations. This accumulates in reproductive cell and passes to embryo.

4)    Objections to blending inheritance:

a) Unisexual organisms are not able to blend characters.

b) If blending inheritance is true then children with intermediate colour should be produced from dark coloured and fair coloured parents.

c) Many individuals show ancestral characters which are never found in intermediate generations. This is known as atavism.

d) Characters may remain hidden in one or more generations and appears in a generation after a gap.

e) Kolereuter stated that traits have particulate nature and remain discrete.

f) Naudin (1862) concluded that a repeated crossing of hybrids their parental types appear in the offsprings showing that hybrids contain the traits of both the parents though they may not be visible externally.

Basic Features of Inheritance

    Prior to Mendel, Breeders like Kolereuter, Goss, and Naudin showed six basic features of inheritance which are as follows:

1) Traits have alternate forms.

2) Traits are represented by discrete particulate entities in the individuals which do not get blended or modified.

3) One alternate of a trait may be exhibited more often than the other.

4) An alternate form of a trait may remain hidden for one or more generations and then reappear in the unchanged state.

5) Particulars forms of two or more traits may occur together in one generation and separate in subsequent generations.

6) Out of the two alternate traits present together in an individual, only one is expressed.

Q.1: Define heredity and variations.                             

Ans.: 1] Heredity: - the transmission of characters from one generation to another generation is called as heredity.

2] Variations: - the differences that occur in closely related organisms are called as variations.

Q. 2:  Define the following terms: -                                   

Ans: A] Genes: - gene is the smallest hereditary unit through which characters are transmitted from one generation to another generation.

B] Genetics: - the branch of biology that deals with the study of heredity and variation is called as genetics.

Q.3: - Explain ‘like begets like’ with suitable example.              

Ans.: -1] def: - the fundamental principles of biology which explains the similarities between the parents and the off springs is called as like begets like.

2] Every individual is produced from pre-existing organism of same type.

3] All coming generations are produced through the process of reproduction

4] The individuals produced by asexual reproduction exhibit similarities in their characters.

5] The off springs produced through sexual reproduction show variations and resemblances in characters.

6] For example: - cat produces kittens, mango seed produces mango plant only.

Q. 4: Why Mendel selected pea plants for his experiment?          Ans.: 1] pea plant is annual herb and easily available.

2] Flowers are naturally self-pollinating and can be cross-pollinated.

3] The plants are true breeding or homozygous in nature.

4] The flowers are large, conspicuous bisexual and can be easily operated.

5] The progeny obtained is fertile.

6] Large numbers of contrasting characters are present in pea plants.

Due to all above reasons Mendel selected pea plants for his experiments.

Q. 5:  What are the various contrasting characters selected by Mendel?

Ans.: 1] Mendel selected seven pairs of contrasting characters from pea plants.

2] The selected characters are expressed in two forms as dominant and recessive.

3] Each pair of character includes one dominant character and one recessive character.

4)  The characters can be given as follows

Q. 6:  Why was Mendel successful in his experiments on pea plants?

Ans.: 1] selection of pea plant was important reason.

2] Mendel selected and studied single character at a time.

3] Mendel used principles of mathematics and statistics for interpretation of results.

4] Mendel kept his records in definite pattern which helped to trace the characters in several generations.

5] Mendel’s luck played major role in his success.

6] Luckily, there was no linkage or incomplete dominance in characters selected by Mendel.