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The Punnett square below shows the genetic distribution of two parents, one with attached earlobes (aa) and one with unattached earlobes (AA).
A A
a Aa Aa
There are four possible combinations of alleles in the offspring: two with unattached earlobes (Aa) and two with attached earlobes (aa).
Thus, the chances of their offspring having unattached earlobes is 50%.