Explanation

According to law of segregation, the heredity character in the form of alleles segregate from each other during gamete formation, i.e, each gamete carry only one allele of each gene. This is also called law of purity of gametes. When tall and dwarf plants are crossed only tall plants are produced in $F_{1}$ generation. By selfing of these $F_{1}$ plants tall and dwarf plants produced in 3 : 1 ratio.