Ref: DNA for beginners.org, access Aug 1, 2017
When Mendel proposed that each trait is determined by a pair of genes, it presented a potential problem. If parents pass on both copies of a gene pair, then offspring would end up with four genes for each trait. Mendel deduced that sex cells — sperm and eggs — contain only one parental gene of each pair. The half-sets of genes contributed by sperm and egg restore a whole set of genes in the offspring.
Mendel found that different gene combinations from the parents resulted in specific ratios of dominant-to-recessive traits. The results of a cross between two hybrid parents — each carrying one dominant and one recessive gene — were key to his synthesis. For example, a cross between two yellow-seed hybrids produces three times as many yellow seeds as green seeds. This is Mendel’s famous 3 to 1 ratio.