Explain how two phenotypically normal parents can give birth to a child with a genetic disorder.

Phenotype only tells you so much about the genotype.

Explanation:

Both the parents are carrying two different alleles. For example Gg. As long as you have one G, then you will not display the genetic disorder. If you have two gg, then you will display the genetic disorder. The parents are carriers by having a g. There was a 25% that their child would inherit a g from each of them and have gg as their genotype and a genetic disorder as their phenotype.

he forelimbs below belong to four species that descended in a direct line from the ancestral mammal shown above. of the four descendants shown, which species most likely appeared earliest in evolutionary history?