Answer :
I will indicate the black gene as "B" and the brown gene as "N".
The parents would be "B-N" since they are both heterozygous. If they were homozygous, they would be "B-B".
There are 4 combinations of outcomes for the offspring: B-B, B-N, N-B and N-N. Since 3/4 of the outcomes have black hair (because they have at least 1 black gene), that leaves 25% that will have brown hair.
25% of 20 is 5. (5 of the offspring should have brown hair.)
Hope it helps.
The parents would be "B-N" since they are both heterozygous. If they were homozygous, they would be "B-B".
There are 4 combinations of outcomes for the offspring: B-B, B-N, N-B and N-N. Since 3/4 of the outcomes have black hair (because they have at least 1 black gene), that leaves 25% that will have brown hair.
25% of 20 is 5. (5 of the offspring should have brown hair.)
Hope it helps.