This would be a recessive disease.
Father is dd
Mother is Dd (has to be because one child has the disease)
Parents cross is dd x Dd which gives rise to Dd (0.5) and dd (0.5). Each time they have a child they have a 50% chance of the child having the disease. In their case, only one of their 3 children is dd. The others though are carriers!
A group of tissue that work together is a organ
