In a dihybrid cross, when would the proportion of parental gene combinations be much higher than non-parental types, as experimentally shown by Morgan and his group?

According to the mendelian dihybrid cross, the parental combinations are dominant in F1generation. The non-parental combinations are higher than parental combinations in F2generation because the genes of one trait are not inherited together with another trait and the alleles segregate independently. As per Morgan's experiment when the genes are linked, they are inherited together in F1as well as F2generation leading to increased percentage of parental combination in both the generations.