IT'S GENETIC
'(<a href="http://www.j-archive.com/media/2005-03-28_DJ_09.jpg" target="_blank">Sarah of the Clue Crew points to a Punnett square on a chalkboard.</a>)  Shown here, this would be the expected ratio of offspring exhibiting the upper case dominant trait to those exhibiting the lower case recessive trait'
3:1