Why does a constant growth rate produce a j-shaped curve?

Neeed now for 1. eye color is a polygenic trait. what is the possibility of getting a green (aabb) eyed child, if parent 1 has brown eyes (aabb) parent 2 has blue eyes (aabb) 2.albinism is a recessive trait. having normal pigment is the dominant trait. what is the possibility of having an albino child if parent 1 is heterozygous normal parent 2 is heterozygous normal 3.in rabbits black fur is dominant over white fur. what is the probability of getting a rabbit with black fur if parent 1 is heterozygous black parent 2 is homozygous white