Ans. When in the heterozygous condition one allele masks or prevents the expression of the other, it is called the dominant allele.The allele which is not expressed is called Recessive. A dominant allele only suppresses the expression of recessive allele. It does not affect its nature.
Presentation
The dominant alleles are represented by capital letters and recessive alleles b' lower case letters.
Example
Albinism is a recessive trait i.e. it is produced when both alleles are recessive. In humans, allele 'A' produces normal body pigments while allele 'a' does not produce pigments, If genotype is AA or Aa, the individual will produce pigments. On the other.hand if genotype is aa,no pigments will be produced and the individual will be albino. In this example, we see that the allele 'A' dominates over 'a', because in Aa individual pigments are,produced and the effect of 'a' is suppressed by 'A'.