Hi Kadasuki,
Your brother is wrong about the potato gene. He is correct though that grocery store tomatoes can be tasteless. There's a few reasons for that.
One is farmers growing varieties that were bred to stay hard longer than normal to withstand shipping and stay saleable longer. This breeding has caused a loss of flavor.
Two is tomatoes shipped long distances are often subjected to cold for long periods. When tomatoes get cold, they lose flavor and texture too.
Three is some claim that the tomatoes are picked at the breaking stage(almost all green) and this causes flavor loss. I'm not sure it that one is entirely true.
Your best bet for a decent tomato is to buy local greenhouse ones especially during the cold months. Around here there are a couple that sell to the grocery stores. They are growing much tastier tomato varieties. I know some people are not so lucky and stores only carry long distance shipped ones.
If you are interested in knowing an easy way to locate others near you with an interest in heirlooms and seed saving, you can email me for tomato forum name that I can not write here(GW doesn't allow it.)
Remy