Can someone please help me with this science question (10 points)?

While a little rambling, your paragraph is basically correct. But there may be another possibility: couldn't one parent have had a mutation producing a dominant gene for this trait? The newly created dominant gene would not show up in the affected parent but could be passed down to any of the offspring producing the darkened trait. Any unaffected offspring would not display the trait. (But as you have not provided the question, this may not apply.)