Why are the chances for a husband and a wife having either a boy or girl 50:50???

Is this mrna or rna need need the answer in a hurry

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What is point source pollution of an aquatic ecosystem? a) pollution of freshwater sources b) pollution from many sources that combine into one c) pollution that occurs over a wide area; water that runs off from fields, parking lots, and other land surfaces d) pollution that can easily be identified with a single discharge source, such as an open pipe which drains into a body of water