5.  Principles of Inheritance and Variation 


  1. Genetics: Study of inheritance, heredity and variation of characters or Study of genes and chromosomes.

  2. Inheritance: Transmission of characters from parents to progeny.

  3. Heredity: Resemblance b/w offspring and their parents.

  4. Variation: Difference between parents and offspring.

  5. Clone: The group of organisms produced by asexual reproduction. (The individual of a clone is called rametes).

  6. Offspring: The organism derived by sexual reproduction.

  7. Alleles (Allelomorphs): The alternative forms of a gene. E.g. T (tall) and t (dwarf) are two alleles of a gene responsible for the character height.

  8. Homozygous: The condition in which chromosome carries similar alleles for a character. Also known as pure line (True breeding). E.g. TT, tt, YY, yy etc.

  9. Heterozygous: The condition in which chromosome carries dissimilar alleles for a character. E.g. Tt, Yy etc.

  10. Dominant character: The character which is expressed in heterozygous condition. It indicates with capital letter.

  11. Recessive character: The character which is suppressed in heterozygous condition. It indicates with small letter.

  12. Phenotype: Physical (Visible) expression of an individual.

  13. Genotype: Genetic constitution of an individual.

  14. Hybrid: An individual produced by the mating of genetically unlike parents.

  15. Haploid (Monoploid): An individual or cell containing a single complete set of chromosomes.

  16. Diploid: An individual or cell containing two complete haploid set of chromosomes.

  17. Punnett square (Checker board): A grid that enables to calculate the results of simple genetic crosses.

  18. Cross: Deliberate mating of 2 parental types of organism.

  19. Reciprocal cross: Two way cross of the same genotype in which the sexes of both parents are reversed.

  20. Trait: A phenotypic characteristic of an inherited character.

  21. Wild type: The species variety showing normal phenotype.

  22. Father of genetics: Gregor Mendel


​Hybridization experiments on garden peas (Pisum sativum)

  1. Mendel selected 7 pairs of true breeding pea varieties

  2. ​Characters 

  3. ​Dominant 

    1. Stem height 

    2. ​Flower colour 

  4. ​Recessive

    1. ​Violet 

    2. ​White

  5. ​Flower position 

    1. ​Axial 

    2. ​Terminal

  6. ​Pod shape 

    1. ​Inflated 

    2. ​Constricted

  7. ​Pod colour 

    1. ​Green 

    2. Yellow

  8. ​Seed shape 

    1. ​Round 

    2. ​Wrinkled

  9. ​Seed colour

    1. Yellow

    2. Green


  1. Monohybrid cross: A cross involving 2 plants differing in one character pair. E.g. Mendel crossed tall and dwarf pea plants to study the inheritance of one gene.

Steps in making a cross in pea:

  1. Selection of 2 pea plants with contrasting characters.

  2. Removal of anthers (emasculation) of one plant to avoid self pollination. This is female parent.

  3. Collection of pollen grains from the other plant (male parent) and transferred to female parent (pollination).

  4. Collection of seeds and production of offspring.

Monohybrid phenotypic ratio:

Tall: 3 Dwarf: 1          = 3:1

Monohybrid genotypic ratio:

Homozygous tall (TT): 1 Heterozygous tall (Tt): 2 & Homozygous dwarf (tt): 1= 1:2:1

  1. Mendel made similar observations for other pairs of traits and proposed that some factors were inherited from parent to offspring. Now it is called as genes.

  2. ​The F1 (Tt) when self pollinated, produces gametes T and t in equal proportion. During fertilization, pollen grains of T have 50% chance to pollinate eggs of T & t. Also, pollen grains of t have 50% chance to pollinate eggs of T and t.

  3. 1/4th of the random fertilization leads to TT (¼ TT).

  4. 1/2 (2/4) of the random fertilization leads to Tt (½ Tt).

  5. 1/4th of the random fertilization leads to tt (¼ tt).

Tt x Tt

Binomial expression = (ax + by) 2

Hence (½ T + ½ t) 2 = (½ T + ½ t) (½ T + ½ t)

= ¼ TT + ¼ Tt + ¼ Tt + ¼ tt

= ¼ TT + ½ Tt + ¼ tt

  1. Mendel self-pollinated the F2 plants. He found that dwarf F2 plants continued to generate dwarf plants in F3 & F4. He concluded that genotype of the dwarfs was homozygous- tt.

Backcross and Testcross

  1. Backcross: Crossing of F1 hybrid with its any of parent.

  2. Testcross: Crossing of an F1 hybrid with its recessive parent (Test cross ratio=1:1). It is used to find out the unknown genotype. (See figure in T.B. Page: 75)

  3. Mendel conducted test cross to determine the F2 genotype.

Mendel’s Principles or Laws of Inheritance

  1. First Law (Law of Dominance)

    1. Characters are controlled by discrete units called factors.

    2. Factors occur in pairs.

    3. In a dissimilar pair of factors one member of the pair dominates (dominant) the other (recessive).

  2. Second Law (Law of Segregation)

    1. “During gamete formation, the factors (alleles) of a character pair present in parents segregate from each other

    2. such that a gamete receives only one of the 2 factors”.

    3. Homozygous parent produces similar gametes. Heterozygous parent produces two kinds of gametes each

    4. having one allele with equal proportion. Do not use T for tall and d for dwarf because it is difficult to remember whether T & d are

The concept of dominance

  1. In heterozygotes, there are dominant and recessive alleles.

  2. The normal (unmodified or functioning) allele of a gene produces a normal enzyme that is needed for the

  3. transformation of a substrate. The modified allele is responsible for production of

    1. The normal/less efficient enzyme or

    2. A non-functional enzyme or

    3. No enzyme at all

  4. In the first case: The modified allele will produce the same phenotype like unmodified allele. It becomes dominant.

  5. In 2nd and 3rd cases: The phenotype will dependent only on the functioning of the unmodified allele. Here, the modified

  6. allele becomes recessive.


  1. 1. Incomplete Dominance

  2. It is an inheritance in which heterozygous offspring shows intermediate character b/w two parental characteristics.

  3. E.g. Flower colour in snapdragon (dog flower or Antirrhinum sp.) and Mirabilis jalapa (4’O clock plant).





Chapter 1 – The Living World 

Chapter 2 – Biological Classification 

Chapter 3 – Plant Kingdom 

Chapter 4 – Animal Kingdom 


Chapter 5 – Morphology of Flowering Plants 

Chapter 6 – Anatomy of Flowering Plants 

Chapter 7 – Structural Organisation in Animals 



Chapter 8 – Cell: The Unit of Life 

Chapter 9 – Bio-Molecules 

Chapter 10 – Cell Cycle and Cell Division 


Chapter 11 – Transport in Plants 

Chapter 12 – Mineral Nutrition 

Chapter 13 – Photosynthesis in higher plants 

Chapter 14 – Respiration in Plants 

Chapter 15 – Plant Growth and Development 


Chapter 16 – Digestion And Absorption 

Chapter 17 – Breathing and Exchange of Gases 

Chapter 18 – Body fluids and circulation 

Chapter 19 – Excretory Products and their Elimination 

Chapter 20 – Locomotion and Movement 

Chapter 21 – Neural Control and Coordination 

Chapter 22 – Chemical Coordination and Integration 


Unit-VI Reproduction

Chapter 1 : Reproduction in Organisms 

Chapter 2 : Sexual Reproduction in Flowering Plants 

Chapter 3 : Human Reproduction 

Chapter 4 : Reproductive Health 

Unit-VII Genetics and Evolution

Chapter 5 : Principles of Inheritance and Variation 

Chapter 6 : Molecular Basis of Inheritance 

Chapter 7 : Evolution 

Unit-VIII Biology and Human Welfare

Chapter 8 : Human Health and Disease 

Chapter 9 : Strategies for Enhancement in Food Production 

Chapter 10 : Microbes in Human Welfare 

Unit-IX Biotechnology  

Chapter 11 : Biotechnology Principles and Processes 

Chapter 12 : Biotechnology and its Applications 

Unit-X Ecology and Environment 

Chapter 13 : Organisms and Populations 

Chapter 14 : Ecosystem 

Chapter 15 : Biodiversity and Conservation 

Chapter 16 : Environmental Issues 

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