Are calculated genotypic frequencies necessarily the same as observed genotypic frequencies?
Observed allelic and genotypic frequencies are not exactly the same as calculated ones as explained below.
The phenotypic and genotypic ratios are calculated on the basis of Mendel's Laws of inheritence, The calculations are based upon the law of probability, assuming that every type of gamates produced by one plant has equal chances to fuse with every type of gametes produced by other plant.
However, in actual practice this may not be exact. Thus the observed results will be closer to the calculated results, not exactly the same.
Let us take example of pea plant. A homozygous tall pea plant is crossed to homozygous dwarf pea plant.
All the plants in F1 are heterozygous tall. These plants are allowed to interbreed freely to get F2 generation.
Both the plants crossed produce 2 types of gametes , i.e. having allele T and t. Male gamete carrying T allele has equal chances to fuse with female gamete carrying T allele or t allele.
Similarly, male gamete carrying t allele has equal chances to fuse with female gamete carrying T allele or t allele.
In F2 generation tall and dwarf plants will be in the ratio of 3:1. This is F2 phenotypic ratio.
Amongst the tall plants,1 will be homozygous (TT) tall and 2 heterozygous (Tt) tall . All dwarf plants will be homozygous dwarf (tt). Thus F2 genotypic ratio will be 1;2;1.
Actual observation shows different ratios in different experiments. However, all these will be closer to the calculated phenotypic or genotypic ratios.
For example, in actual observation tall and dwarf plants produced may be in the ratio of 360: 110 or 350:110 or 355:120.
These observed phenotypic ratios are not exact 3:1 as calculated, but all closer to this ratio.
Similarly observed genotypic ratios are closer to the calculated 1:2:1 ratio.