In a population of a beetle species, ratio of 3:1 shiny wing covers dull wing is observed. This ratio does not prove that the shiny allele is dominant. The ratio shows that the F2 ratio is obtained from a cross of heterozygous F1 organisms.
Similarly, two heterozygotes are crossed, each containing both alleles of a single gene. Then, we see that there is a 3:1 ratio where 3 have wild type phenotype and 1 has the recessive phenotype.
In addition, it is assumed that the two different characteristics are caused by two alleles of one gene. This single fact proves that the characteristic of shiny wings is more common in beetles than in dull wings.