The law that states that one allele from each gene pair goes to a sex cell is called Mendel's Law of Segregation.

Here's a breakdown:

* Alleles: Different versions of the same gene. For example, the gene for eye color might have an allele for brown eyes and an allele for blue eyes.

* Gene Pair: Each individual inherits two copies of each gene, one from each parent.

* Sex Cell (Gamete): A reproductive cell, like an egg or sperm, which contains only one copy of each gene.

Mendel's Law of Segregation states that during the formation of gametes, the two alleles for a trait separate from each other, so that each gamete receives only one allele. This ensures that each offspring inherits one allele for each trait from their mother and one from their father.