Chapter 5: Principles of Inheritance and Variation


Branch of biology which deals with the study of heredity and variations. Study of heredity involves the mechanism by which characters pass from parents to offspring. Study of variations involves cause of heredity.

  • Father of Genetics: G. Mendel
  • Father of Modern Genetics: W. Bateson
  • Father of Experimental Genetics: Morgan
  • Father of Human Genetics and Biochemical Genetics: A. Garrod

Some Important Terms

Chromosome: The nucleoprotein structure which are generally more or less rod-like during nuclear division. The genes are arranged on the chromosomes in a linear fashion. Each species has a characteristic number of chromosomes.

Gene: The fundamental physical and functional unit of heredity, which carries information from one generation to the next. It is a segment of DNA. Mendel called them factors. Johannsen coined term 'gene'.

Allele: Alternative forms of a gene which are located on corresponding positions on homologous chromosomes. Alleles are gene controlling the same characteristic but producing different effects. They occupying corresponding positions on homologous chromosomes.

Homozygous: The organism having two similar genes for a particular character in a homologous pair of chromosomes is known as homozygous or genetically 'pure' for that particular character. Example: TT, RR, AA etc.

Heterozygous: An individual containing both dominant and recessive genes or traits or characters of a allelic pair is known as heterozygous. Example: Tt, Rr, AB etc. Term  homozygous and heterozygous are coined by Bateson.

Acquired Character: The alteration in the morphology or physiology of an organism in response to its ecological or environmental factors. These characters are usually not heritable.

Cross: The deliberate mating of two parental types of organisms in genetic analysis. Cross can be monohybrid, dihybrid or trihybrid. Individual produced by cross is known as hybrid.

Genotype: The genetic constitution or genetic make up of an organism for a particular character.

Phenotype: It is the external and morphological appearance of an organism for a particular character.


  1. Mendelian Inheritance
  2. Inheritance of One Gene
  3. Inheritance of Two Gene
  4. Mendel's Law of Inheritance
      (a)Law of Dominance
      (b)Law of Segregation
      (c)Law of Independent Assortment
      (d)Exception of Mendelian Inheritance
  5. Chromosomal Theory of Inheritance
  6. Sex Determination
  7. Mutation
  8. Genetic Disorders

Mendelian Inheritance

Gregor Johann Mendel was the pioneer of classical genetics. He was born in July 22, 1822 in Heinzendorf in Austrian Silesia. Mendel worked in Augustinian monastery as monk at Brunn city, Austria.Beside his other duties, he took keen interest in natural sciences. From 1851 to 1853, he studied mathematics and natural science in the university of Vienna. In 1856-57, he started his experiments of heredity on pea (Pisum sativum) plant. His experimental worked continued on pea plant till 1865. (Duration of work is 1857-1865). The results of his experiments were published in the science journal by name of "Experiments on plant hybridisation". Mendel was unable to get any popularity. No one understood him. He died in 1884 without getting any credit of his work. After 16 years of Mendel's death in 1900, Mendel's postulates were rediscovered.

Rediscovery of Mendelism

Mendel's research paper remained dormant and unnoticed by the scientific world until 1900. There three botanists who rediscover mendel's postulation. They republished Mendel's original paper in 1901 in Flora magazine. Bateson confirmed Mendel's work by a series of hybridization. Re-discoverer were:

  • Carl Correns (Germany) worked on Maize
  • Hugo deVries (Holland) worked on Evening Primerose
  • Erich von Tschermak (Austria) worked on different flowering plants.

Question 1: What is inheritance and heredity?
The process by which characters are transferred from one generation to another generation is called inheritance and phenomenon is heredity.

Question 2: Explain Mendel's experiment in detail?
Gregor Johann Mendel(1822-1884) Father of genetics. Mendel performed his experiments with garden pea plant(Pisum sativum) and he selected pea because of following reasons:

  • Annual, short life span.
  • Easily available, maintained and raised easily.
  • Seven characters he selected were easily identified.
  • He observed one trait at a time.
  • Pea plant can easily self pollinated as the pea plant is bisexual plant.
  • Pea plant can also cross pollinated.

Reasons for success of Mendel:

  • Controlled pollinated as pea can both cross and self pollinated easily.
  • He used true breeding pea plant.
  • He kept record of his breeding experiments.
  • Analysed results by using law of probability.
  • Traits selected by him does not show interaction like incomplete dominance, linkage.

Question 3: Why work of Mendel does not recognized initially?
The work of Mendel was not recognized because of following reasons:

  1. His work was ahead of his time.
  2. Journals in which his work published had limited circulations.
  3. He failed to get similar result in Hawkweed.
  4. Mathematical approach in working out biological problems was strange to others.
  5. Minds of biologists were preoccupied with Darwin's theory of evolution.

Question 4: Explain about the following 1)Genes 2)Alleles 3)Dominant 4)Recessive ?
1)Genes: These are the units of inheritance. They contain the information that is required to express a particular trait in an organism.
2)Alleles: Genes which codes for a pair of contrasting traits are known as alleles i.e. they are slightly different forms of same gene(T and t).
Dominant: Characters which can express itself in both homozygous and heterozygous conditions.
4)Recessive: Characters which only express themselves in homozygous condition and remain masked in heterozygous condition.

Question 5: Explain laws of Mendel?
Mendel gave three laws on inheritance based on hybridization experiments, Three laws are as follows:

Inheritance of One gene

The Cross between the pea plants differing in single pair of contrasting characters is known as monohybrid cross. Mendel called the original parents the P Generation and the first generation offspring the F1 (filial first) Generation. The second generation offspring is known as F2 (filial second) Generation. F1 generation is obtained by cross fertilization while F2 generation is obtained by self fertilization. Phenotype ratio 3:1 and genotype 1:2:1.

Monohybrid Cros

Inheritance of Two gene

The Cross between the pea plants differing in two pair of contrasting characters is known as dihybrid cross. When a dihybrid cross between pea variety having yellow cotyledons and round seed with another variety having geen cotyledons and wrinkled seeds was done, the F1 seeds were yellow and round. In second generation phenotypic ration is 9:3:3:1 ratio. Genotypic ratio is 1:2:2:4:1:2:1:2:1.

Dihybrid Cros

  1. Law of Dominance: This law states that when two alternative forms of a character are present in an organism only one factor expresses itself which is called as dominant while the other that remains masked is called recessive.
  2. Law of Segregation: This law states that factors or alleles of a pair segregate from each other during gamete formation such that a gamete receive only one factor. They do not show blending. This law is also known as law of purity of gametes.
  3. Law of Independent Assortment: When two pairs of traits are combined in a hybrid, inheritance of one pair of characters is independent of the other pair of characters.

Sex Determination

Sex DeterminationSex determination is the method by which the determination between males and females is established. Sex determination may be chromosomal, cytoplasmic, Environmental and Genic balance mechanism. The genetic or chromosomal mechanism of sex determination is based on occurenece of two types of gametes, means on the basis of set of chromosome in zygote.
Male heterogamety: Means male produces two types of gametes(X or Y and X or O).
1. XX-XY- type (Ex- Human beings, Drosophilla)
2. XX-XO type (Grasshopper, cockroach)
Female heterogamety: Female produces two types of gametes(Z or W and Z or O)
1. ZZ-ZW type (Birds, reptiles)
2. ZZ-ZO type (Moths)
Haploid-diploid sex determination: In honey bees males are haploid having 16 chromosomes and the develop by the process of mitosis cell division and female are diploid having 32 chromosomes and they develop by meiosis division to form female gamete. Ex- Honey bee .

Dharmendra Gaur

Dharmendra Gaur (Msc. Zoology) aka DRGP. I love to learn and teach biology. By this blog I want to increase my knowledge and share my knowledge with others. Feel free to ask what ever you want to ask related to biology, I try my best to help you.

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