Question

In an eight–cell embryo of an organism, the cells of the upper tier are aligned directly over the cells of the lower tier. Which organism is it likely to be?

earthworm
snail
starfish

Answer 1

starfish
All normal eggs from the mother have an X chromosome while usually half the sperm cells from the father have a Y (male) chromosome and the other half have an X (female) chromosome and it is therefore the father's sperm that dictates which gender the embryo will take.

If the egg is fertilized by a sperm with a Y chromosome then the embryo will be a genetic male due to XY chromosome combination.
If the egg is fertilized by a sperm with an X chromosome then of course the embryo will be a genetic female due to XX chromosome combination.