William Bateson and Reginald Punnett used the sweet pea (Lathyrus odoratus) in genetics studies in the early $20^{th}$ century. Pure-breeding plants that produced purple flowers and long pollen grains were crossed with pure-breeding plants that produced red flowers and round pollen grains. The resulting offspring all produced purple flowers and long pollen grains. Two of the $F_{1}$ generation plants were crossed. The table shows the ratio of phenotypes in the $F_{2}$ generation.

William Bateson and Reginald Punnett used the sweet pea (Lathyrus odoratus) in genetics studies in the early $20^{th}$ century. Pure-breeding plants that produced purple flowers and long pollen grains were crossed with pure-breeding plants that produced red flowers and round pollen grains. The resulting offspring all produced purple flowers and long pollen grains. Two of the $F_{1}$ generation plants were crossed. The table shows the ratio of phenotypes in the $F_{2}$ generation.

Flower colour	Pollen grain shape	Number of plants
purple	long	4831
purple	round	390
red	long	393
red	round	1138

What is an explanation for these experimental results?