All genes in an organism do not follow Mendelian law of inheritance. There are some exceptions to it.
In some cases, a dominant allele does not fully express itself in the presence of recessive allele. We call this as incomplete dominance. For example, the colour of Snapdragon flower ( a decorative plant commonly found in nurseries) shows co-dominance. When a red colour (dominant trait) of the flower is cross-pollinated with the recessive White flower. The result is an intermediate pink colour in the F1 generation. In F2 generation we get Red: Pink: White in the ratio of 1:2:1. Here, the phenotypic ratio becomes the genotypic ratio.
In some cases both the allele expresses themselves. We call this as co-dominance. The ‘A’ and ‘B’ blood group Show Co-dominance. There is another blood group O. In O blood group, genes do not code any proteins. Genes for O blood group follow Mendelian law of inheritance.
Blood group: Alleles present
A: AA or AO
B: BB or BO