Lesson – 3 GENETICS

Genetics :- 

  1. The branch of biology which deals with the study of heredity and variation is called genetics.
  2. The term genetics was first coined by Bateson ( 1905).
  3. This term is derived from the Greek language word “gene”.

Hereditary characters :- 

During the process of sexual reproduction various characters are transmitted from generation to generation by gametes. These characters are known as hereditary characters.

Heredity :- 

  1. The transmission of hereditary characters from parental generation to off spring is called as heredity.
  2. The term heredity was coined by Spencer (1863).
  3. During sexual reproduction variations occur among individuals of the same species due to crossing over during meiosis

MENDELISM / Life history of Mendel :-

  1. Gregor Johann Mendel ( 1822-1884) is called father of genetics because Mendel proposed the laws of inheritance in plants for the first time.
  2. Mendel was born on 22th July, 1822 in Silision village of Heinzendarf state of Austria.
  3. In 1842, after receiving a degree in philosophy he became a priest in the church of Brun town of Austria in 1843.
  4. Mendel performed hybridization experiments on garden pea (Pisum sativum) for seven years (1856-1863).
  5. In 1865, the findings of these experiments were presented in the form of a research paper to the Gregor Johann Mendel Brunn Society of Natural History.
  6. In 1866, these experiments were published in the society's annual proceedings entitled-"Experiments on plant hybridization,"
  7. Mendel was died on 6 January 1884.

Reasons for Mendel's success :-

  1. Mendel studied the inheritance of one character at a time
  2. Mendel maintained the statistical record of the experiments and analyzed them carefully.
  3. Mendel selected the plant material for his experiments carefully.

Selection of pea plant: - 

Mendel selected garden pea plant for his experiments because 
  1. Pea plant is annual, therefore, it was possible to study many generations in a short duration.
  2. It was easy to get pure line or homozygous plants by self pollination because of bisexual flowers.
  3. Artificial cross pollination can be easily done by emasculation technique.
  4. contrasting, character .re present in pea plant

Contrasting pair of pea plant :-

  1. S.No

    Character of plant

    Dominant

    Recessive

    1.

    Hight of plant

    Tall

    Dwarf

    2.

    Shape of seed

    Round

    Wrinkled

    3.

    Shape of pod

    Inflated

    Constrict

    4.

    Colour of immature pod

    Green

    Yellow

    5.

    Colour of seed

    Yellow

    Green

    6.

    Position of flower

    Axial

    Terminal

    7.

    Colour of flower

    Violet

    white

Rediscovery of Mendelism :-

  1. The laws of inheritance given by Mendel were neglected for about 35 years.
  2. The Hugo De Vries of Holland 
  3. Carl Correns of Germany
  4. Erich von Tschermak of Austria 
  5. rediscovered Mendel's laws 1900 while working separately.

Genetics terminology :-

It is very important to understand the following technical terms to understand the laws of inheritance

  1. Gene :- The factor that controls a character is called gene. Johannsen gave the name gene. For gene “factor” word is used by Mendel.
  2. Allelomorph or allele :- Alternative form of a gene which controls any one character is called allelomorph or allele. Eg.- Height of the plant is controlled by a gene which has two alleles T (Tallness) and t (Dwarfism).
  3. Homozygous :- When both alleles of a gene are similar, the condition is known as homozygous Eg.- TT or tt.
  4. Heterozygous: - When both alleles of a gene are dissimilar, the condition is known as heterozygous Eg.-Tt.
  5. Phenotype: - External appearance of an individual is called phenotype. Eg. - Tall plant which genotypically may be homozygous (TT) or heterozygous (Tt).
  6. Genotype: - Genetic constitution of an individual is called genotype Eg.-pure or homozygous tall(TT) and impure or heterozygous tall (Tt).
  7. Dominant characters: - The character which express itself in F1 generation is called dominant character.
  8. Recessive characters: - The character which does not express itself in F1 generation is called recessive character.
  9. Monohybrid cross: - Such type of cross in which single contrasting character (Trait) is considered is called monohybrid cross.
  10. Dihybrid cross: - Such type of cross in which two contrasting characters (Traits) are considered is called dihybrid cross.
  11. Trihybrid cross: - Such type of cross in which three contrasting characters (Traits) are considered is called trihybrid cross.
  12. Polyhybrid cross: - Such type of cross in which many contrasting characters (Traits) are considered is called Polyhybrid cross.
  13. Reciprocal cross: - Such type of cross in which “A” plant (TT) is considered as a male plant “B” plant (tt) is considered as female and in another cross if “A” plant (TT) is consider as female and 'B' plant (tt) is considered as male, called as reciprocal cross.
  14. Parental generation: - The plants which are crossed to obtain the offspring called as parental generation.
  15. Frist filial generation: - (F1 generation) The offspring obtained from parents is called F1 generation.
  16. Second filial generation: - (F2 generation) The offspring obtained from F1 generation is called F2 generation.
  17. Monohybrid ratio: - The ratio obtained from monohybrid cross is called monohybrid ratio.
  18. Dihybrid ratio: - The ratio obtained from dihybrid cross is called dihybrid ratio.

Law of dominance: -

  1. According to this law, when a cross takes place in between homozygous contrasting plants, the character which express itself in F1 generation is called dominant and the character which does not express itself In F1 generation is called recessive.
  2. Example — When a pure or homozygous tall (TT) plant is crossed with pure or homozygous dwarf (tt) plant then in F1 generation all plants ( 100%) were tall (Tt)

Law of segregation or Law of purity of gametes: - 

This law is also based on results of Mendel's monohybrid cross.
  1. According to this law “During gamete formation from hybrid or heterozygous of Fl generation, both alleles separate or segregate to each other, therefore, it is called as law of segregation and each gamete carries one allele for each character, therefore, it is also called as law of purity of gametes”
  2. Example- If homozygous tall (TT) plant is crossed with homozygous dwarf(tt) plant then in F1 generation all plants were hybrid or heterozygous tall (Tt).
  3. In heterozygous condition, both alleles are not contaminated with one another, at the time of gamete formation both alleles segregate from each other and enters in different gametes, Due to this segregation dwarfism (tt) character reappear again in F2 generation
  4. phenotypic ratio obtained from F2 generation is 3 : 1
  5. Genotypic ratio obtained from F2 generation is 1: 2: 1

Back cross: - 

If F1 generation (Tt) is crossed with any one parent TT or tt , then it is called as Back cross.
It is of two types

Out cross: - 

If F1 generation (Tt) is crossed with dominant parent (TT), called as out cross.               
The offspring obtained from this cross were all tall plants, out of these 50% were homozygous tall (TT) and 50% were heterogynous tall


Test Cross:- 

  1. If F1 generation (Tt) is crossed with recessive parent (tt), then it is called as test cross.
  2.  The phenotypic and genotypic ratio of offspring obtained from this cross was same i.e. 1:1 50%heterozygous tall (Tt) and 50% homozygous dwarf (tt) plants are obtained 

Law of independent assortment: - 

This law is based on results of Mendel's dihybrid cross. 
  1. According to this law – “If two or more than two contrasting characters are considered then there is not any effect of inheritance of one character on the inheritance of another character or alleles of each character are not only separate but alleles of different characters behave independently or they assort independently, therefore, it is called as Law of independent assortment.”
  2. Example— If homozygous yellow round seeded (YYRR) plant is crossed with green wrinkled (yyrr) seeded plant then In F1 generation all plants were yellow round seeded (YyRr).
 Genotype of F2 is YYRR : YyRR : YYRr : YyRr : yyRR : yyRr : Yyrr : Yyrr : yyrr
                                   1      :     2   :     2   :    4    :    1     :    2    :    1   :   2    :  1
Phenotype of F2 is  Yellow rounded : Green rounded : Yellow wrinkled : green wrinkled
                                           9                   :           3                :         3                   :         1

Importance of Mendel's law: -

  1. It is important to find dominant characters in living organisms because many harmful and lethal recessive genes are not able to express themselves in the presence of dominant gene.
  2. Gene concept was proved by Mendel's law of segregation. 
  3. According to law of segregation one gene has two alleles and they control two contrasting characters.
  4. With the help of Mendel's laws, we know about the new characters develop in hybrid off spring
  5. Useful characters can be brought into same species and harmful characters can be remove by the method of hybridization.
  6. With the help of Mendel's laws disease resistant and high yielding varieties of crop plants can be developed.
  7. A branch pf science Eugenics related with the improvement of human race is based on Mendel’s law