What is the difference between dominant and recessive traits?

First of all it is necessary to define that a character is an observable physical characteristic, such as the color of a flower, while a trait is a particular shape assumed by a character, such as white or purple for the color of the flower. Now, according to Mendel's first law (or the law of dominance), that the hybrid individuals of the first filial generation manifest only one of the traits present in the parental generation, it means that if two pure lines of plants cross, one with purple flowers and the other with white flowers, the first filial generation produced will have exclusively purple flowers. However, if this first filial generation is left free to self-pollinate, it will produce a second filial generation, in which the trait "white" for the character "flowers color" reappears, which in the first filial generation seemed gone. This leads us to conclude that for the character "flower color", the "purple" trait is dominant over the "white" trait, which instead is recessive. In other words, in a cross between pure strains, a recessive trait does not manifest itself in the first filial generation, while a dominant trait is manifested in all the filial generations.

Obviously the molecular basis of this inheritance of dominant and recessive traits is to be found in our DNA, and more precisely in our genes. In fact, a gene is nothing more than a DNA sequence, encoding for a specific character, and the two or more alternative forms of the same gene are called alleles; therefore all genes owned by an individual constitute its genotype, while the observable and distinct characteristics that it determines constitute the phenotype of the individual. In these terms, considering the alleles of a single gene, an allele (a) is defined recessive when it is expressed phenotypically only in a homozygous genotype (aa), that is when the two alleles of the genotype are identical. Instead, an allele (A) is defined dominant when phenotypically is expressed both in a homozygous genotype (AA) and in a heterozygous genotype (Aa), therefore also when the two alleles of the genotype are different.