Consider the genetic cross for absent-mindedness, which is a dominant trait. All of the offspring from this cross will be absent-minded. If two parents that are absent-minded have offspring that are not, what MUST be the genotypes of the parents?
Both parents must heterozygous having one dominate gene for absent mindedness and one gene that codes for normal brain function.
In a punnett square cross for one trait there are four possibilities
AA, Aa, aA aa
Where A stands for the dominate gene for absent mindedness
Where a stands for the recessive gene for normal brain function
AA, Aa, aA all result in an offspring that is absent minded.
aa results is an offspring with normal brain function.
A AA Aa
a aA aa
If one parent was homozogous for the trait all of the offspring would be absent mindedness
A AA AA
a a A aA