Elements of heredity and variation

Genetics is defined as the study of how inheritable factors are coded in the cells of an organism, how this coding is maintained during cell reproduction, how it is passed from one generation to the next, and how it is expressed in physical features.


The transfer of genetic characters in living organisms from parent to offspring is called HEREDITY. The difference among individuals of a species is called VARIATION.

Father of Genetics

Gregor John Mendel (1822-84) was a teacher and Austrian monk . He conducted many experiments on plant hybridization. He was specially using a garden pea plant (Pisum sativum). Mendel worked out the patterns of inheritance before any knowledge of genes, DNA and chromosomes were available.

Mendel did not get recognition or honour for his valuable work in his lifetime.

Mendel’s work was rediscovered by Karl Correns and Eric Von Tschermak, Hugo deVries (1900) independently. They named this Mendel’s laws or Mendelism.

W.Bateson found that the same laws applied to plants are applied to animals.

Characters selected by Mendel

The contrasting characters selected by Mendel are as follows:

  1. Stem Length:                            Tall/ Dwarf
  2. Flowers Positions:                     Axial/ Apical or terminal
  3. Flower Colours:                         Purple/white
  4. Seed Shape:                           Round/ Wrinkled
  5. Seed  Colour:                            Green/ Yellow
  6. Pod Colour:                            Green/ Yellow
  7. Pod Shape:                              inflated/ Constricted

Reason for Mendel’s success

Mendel was, to some extent, lucky, as he chose characteristics controlled by single genes.
He succeeded where his predecessors did not due to-

1.     His method of working-he worked with only one character at a time.
2.     He carried out experiments on the second and the third generation
3.     He built up sufficient amounts of accurate data to come to a  scientific conclusion.
4. They performed the crossing techniques with proper care during cross-fertilization. He emasculated the flowers early to avoid self-pollination and bagged the flowers after pollination to prevent the deposition of pollen from other plants.

Why Mendel select the Pea plant?

Mendel selected the pea plant because:

  • The pea plant can be quickly grown and maintained.
  • Pea plants have bisexual flowers(having both male and female parts).
  • Pea flowers are self-pollinating flowers(same flower anthers pollinate the same flower, thus are primarily pure line as they bear the same character generation after generation.
  • It is an annual plant(one season). Therefore many generations can be studied in a short period.

Some terminologies for Genetics

Character– The feature of an individual or an organism, i.e. hair colour, flower colour, height etc.

Trait– The variant of the character(distinguishing quality or characteristic belonging to a person)-e.g. black or brown hair colour.

Gene– A gene is an ordered sequence(linear sequence) of nucleotides located in a particular position or on a particular chromosome that encodes a specific functional product(Trait). It is the fundamental physical and available units of heredity.

Allele– The alternative(alternate, opposite) forms of a gene on the same locus (the position on the chromosome) are called alleles.

Genotype– An organism’s or individual’s genetic constitution(makeup).

Phenotype– The physical appearance of genotype / Observable characteristics of an organism(individual) produced by the organism’s genotype interacting with the environment.

Homozygous– An individual with(having) both identical(similar) alleles at one locus (position).

Heterozygous – An individual with different alleles(not similar to each other) at a locus (position).

Dominant– The trait expressed(shown) in the Heterozygous is called dominant.

Recessive– The trait(character) that remains hidden in the Heterozygous alleles is called recessive.

Mendel conducted 2 main experiments to determine the laws of inheritance. These experiments were:

  1. Monohybrid Cross
  2. Dihybrid Cross

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