Nsnails, the shell may either coil to the right or the left. coiling to the right is dominant. a homozygous recessive female snail is crossed with a homozygous dominant male snail. all of the offspring have shells that coil to the left. why do all of the offspring have left-coiled shells

So first off homozygous means that both alleles are the same
If coiling to the right is dominant and to the left is recessive let capital c (C) represent right coiling and let lower case c (c) represent left coiling.
The female who is homozygous recessive must have both recessive alleles
The male who is homozygous must have two dominant alleles
So the cross will be between a CC and a cc
So we set up a Punnett square

We take a look at all the combinations and arrange them

So we find that there is a 25% chance of the offspring having left coiling

The light is great source of energy