Почему многие признаки наследуются на первый взгляд не в соответствии с законами Менделя? Какой возникает вопрос?

Can they run?
Can he jump?
Can she dance?
Can she sing?
Must she sing? 
Would she like to sing?
Can they  swim?
Would they like to swim? 
Must they swim?
Would she  like to ski? 
Can she ski?
Must we do?
Can we go?
Can we help Oleg ?
Must we help Oleg?

Can they run?
Can he jump?
Can she dance?
Can she sing?
Must she sing? 
Would she like to sing?
Can they swim?
Would they like to swim? 
Must they swim?
Would she like to ski? 
Can she ski?
Must we do?
Can we go?
Can we help Oleg ?
Must we help Oleg?