In a pedigree a circle that is all white represents a a normal female
b carrier female
c normal male
d diseased male

normal female

Explanation:

hope this helps

i'm thinking (a) to keep sand in place; ////////////

explanation: a sand fence or sandbreak, like a snow fence, is used to force windblown, drifting sand to accumulate in a desired place. sand fences are employed to control erosion, sand dune stabilization, keep sand off roadways, and to recruit new material in desert areas.