Why is it possible for two healthy parents to give birth to a child with a genetic defect such as cystic fibrosis?

the answer would be 2, if crossing over doesn’t occur 2 out of 4 of the daughter cells will have that allele.

hope this : )

blood enters the heart through two large veins, the inferior and superior vena cava, emptying oxygen-poor blood from the body into the right atrium of the heart. as the ventricle contracts, blood leaves the heart through the pulmonic valve, into the pulmonary artery and to the lungs where it is oxygenated.