The short version is: think about how heritable something like "number of arms" would appear in a study like this. It would appear that variance in arm count is 100% environmental, since the only people who don't have two arms lost one in an accident of some kind. Arm count is therefore not controlled by genes!
The short version is: think about how heritable something like "number of arms" would appear in a study like this. It would appear that variance in arm count is 100% environmental, since the only people who don't have two arms lost one in an accident of some kind. Arm count is therefore not controlled by genes!