Genotype is the *genetic make-up * of an organism . It describes about the nature of each allele .
Phenotype is the morphology of an organism .
The genotype is expressed as phenotype when the information encoded in the genes is used to make protein and RNA molecules.
Consider an example :
Here we have crossed a heterozygous ( the two alleles show dominant and recessive relationship vix T and t ) male gamete with a heterozygous female gamete .
We get the progenies as
1 ) TT homozygous ( the two alleles are of same type ) : TALL PLANT
2) Tt heterozygous :TALL PLANT ( since T is dominant over t )
3) Tt
4) tt homozygous :DWARF PLANT
Here the Phenotype (morphology) of the plant shows you : 3 : 1 ratio (Tall plants :Dwarf plants)Just consider how the plant heights look to you externally)
However if you consider the genotypic ratio i.e genetic make up you consider the nature of genes i.e heterozygous or homozygous .
According to that we have a ratio : 1 :2 :1 (homozygous tall :heterozygous tall :homozygous short )
Note : Recessive alleles will always express themselves in homozygous conditions.
Also it is not necessary that Phenotypic ratio is always different from Genotypic ratio. We consider there studies in Non – Mandelian Inheritance examples of which are
1) Incomplete dominance
2) Codominance e.t.c