Ask A Scientist Molecular Biology Archive

name         Beren A.
age          17

Question -   An extra finger is a dominant trait.Why are there not
more people showing this trait?
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It is actually relatively common.  Most infants have the extra digit removed
shortly after birth.  Also, just because a trait is dominant, it does not 
mean
it is the most prevalent form in the population.  In our shallow society,
many people would not "mate" with someone who had an extra finger, so their
chances to pass on their DNA may be less.

vanhoeck
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Dear Beren:

I am not certain where you found your information that "An extra finger is a
dominant trait", as I was not aware of that fact.  However, let us consider
that it is, in fact, dominant for the sake of argument.  Just because a
trait is dominant does not necessarily mean that it will convey a selective
advantage, which is really what is required for a trait to be passed on.  As
an extreme example, many genetic diseases are also dominant, but they often
convey a distinct selective disadvantage & are not passed on as readily to
the population as a whole.  I would guess that an extra finger might make an
individual
more clumsy, thus conveying a selective disadvantage.  For example, cats,
which sometimes have 6 toes, might be too clumsy to compete for prey in the
wild, so a dominant gene for the extra toe would gradually be eliminated
from the population by the premature death of those individuals.  In fact,
natural selection works most efficiently to eliminate or retain dominant
traits that are expressed more frequently than recessive.  Once these traits
are depleted, it is difficult for them to reestablish themselves without the
intervention of a strong, new, selective advantage.  In the cat example, the
6th toe is not a great disadvantage or advantage for domestic cats that do
not need to hunt for their food, which is probably why it appears at a
noticeable, but not a high, frequency.  If a 6th toe were to become
necessary to enable domestic cats turned wild to catch a certain type of
rodent, which was the only available food source in a particular area, then
it would be strongly selected for in that geographic area.

Hopefully you at least get the general idea - selective advantage is more
important than dominance in spreading a particular trait through a
population, known as "penetrance".  Thanks for the good question,

Jeff Buzby, Ph.D.
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