If six fingers is a dominant human trait why do we have only five?
This is simple. There are just not many genes in the human population for
Look in any high school biology book for what is known as Hardy-Weinberg
equilibrium. These two scientists (separately) said that gene frequencies do
not change much unless something in the environment selects them over other
genes. In other words, unless 6 fingers somehow becomes an advantage, and
five-fingered people have less of an advantage, the frequency of six fingered
people in the population will not necessarily increase. This is the same
reason that recessive traits don't disappear from the population. Also, six
fingers is not considered attractive and they may not get as many mates.
Also, more people are born with six fingers than you might imagine but just
have them amputated shortly after birth.
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Update: June 2012