Cystic fibrosis is a recessive gene disorder. the pedigree chart for a family known to have cystic fibrosis is shown below. a half shaded circle is shown connected by a straight line to an unshaded square. underneath the half shaded circle are the letters aa which represent the genotype for the first parent. underneath the second parent the letters, aa, appear representing the genotype for the other parent. below this is another row of circles and squares. this 2nd row represents the offspring. starting from the left a half shaded circle is shown connected to an unshaded square, followed by an unshaded square, and then an unshaded circle. each of the circles and squares, offspring, are connected by a horizontal straight line that connects to the parent group above. the genotype is written under each of the offspring in order from left to right; aa, aa, aa, aa. what percentage of the offspring have observable disease? 0% 25% 50% 75%