Why are some alleles dominant and some recessive?

Part 1: Introduction of dominant and recessive allele
The allele which masks the effect of other alleles is called the dominant allele. It is represented by the capital letter. The allele which gets masked by the dominant allele is called recessive allele. It is represented by the lowercase.

Part 2: Reason for dominant and recessive alleles

• Dominant allele encode for a protein or an enzyme that is functional and shows its effect. For example: T allele encodes for an enzyme that is responsible for the synthesis of a hormone. This hormone then helps in increase in plant height.
• Recessive allele is unable to show its effect because it does not encode for an enzyme at all or the enzyme is not functional or enzyme has very less activity. For example: t allele is unable to encode for functional enzymes that synthesise the growth hormone in plants due to which plant’s height does not increase significantly.