The gametes are always pure for a particular character (allele) and its examples

Mendel an Austrian monk performed a series of experiments on different varieties of Pea plant (Pisum sativum) in his garden at Brunn and published results in 1865 and 1869 in the journal of local Natural History Society. The science of heredity is called Mendelism. Mendel noticed in Pea seven pairs of contrasting characters, three of which are as follows

(1) Stem tall (6 to 7 ft :) or dwarf (3\4 to 1 1/2 ft :)

(2) Ripe seeds smooth or wrinkled

(3) Colour of cotyledons yellow or green.

He considered each pair of contrasting characters separately and as result of his experiments formulated two laws, the law of segregation and the law of independent assortment.

Law of segregation or Purity of gametes

This means that the genes or factors representing a pair of contrasting characters when brought in the cells offspring and later become separated in gametes so that each male or female gamete carries only one gene for the member of the pair but never both.

Mendel crossed a true breeding tall pea plant with a true breeding dwarf pea plant. He brought about the exchange of pollen between these pure varieties of pea plants and raised the seeds which were collected and grown. He found that in first hybrid generation (F₁ = first filial generation) all the plants were tall like one of the parents. Mendel called the characters which appeared in the hybrid dominant and that which was suppressed recessive. The tall hybrid plants of F₁ generation were then self fertilized and the resulting seeds were grown. It was discovered that in the second hybrid generation (F₂) so obtained the tall and dwarf plants were in the ratio of three to one. 

That is out of every 100 plants 75 were tall and 25 dwarf. He further found that the dwarf plants of F₂ generation produced only dwarf plants when self fertilized. In other words they bred true and were pure for the dwarf character (pure recessive). Similarly one third of tall plants of F₂ generation produced only tall plants when self fertilized. That is they bred true and were pure for the tall character (pure dominants) when the tall hybrid of F₁ generation forms its gametes the two genes (T) and (t) segregate or separate and half of the male and female gametes (pollen grains and egg cells according to Mendel) receive the gene T and the other half the gene t. Thus each gamete carries T or t but not both. In other words a gametes pure with respect to one or the other number of the pair of contrasting characters.

The zygotes from which pure tall and hybrid tall arise are respectively known as homozygotes and heterozygotes.

The four o clock plant (Mirabilis jalapa) furnishes an excellent example.

This species consists of two varieties, one with white flowers and the other with red flowers. When a true breeding red flowered plant is crossed with a true breeding white flowered plant the F₁ hybrids have all pink flowers. When these hybrids are self fertilized they produce three types of plants, half bearing pink flowers (hybrids) one fourth bearing white flowers (pure recessive) and one fourth bearing red flowers (pure dominants).