Gametes, which are sex cells, carry the alleles. Why must a gamete carry one allele (represented by a letter) from each gene? Meaning, why can’t a gamete carry an “hh” instead of an “hs” or an “Hh” instead of a “HS?”

Due to the law of independent assortment

Explanation:

In diploid species like humans, all somatic (body) cells have two complete sets of chromosomes, one chromosome inherited from the maternal gamete and one chromosome inherited from the paternal gamete. Each one of the homologous chromosomes contains one copy of each gene, or allele, such that one allele is inherited from each parent. According to the law of independent assortment, maternal and paternal chromosomes are independently assorted during meiosis, which means that different genes localized on different chromosomes are randomly distributed into gametes. In consequence, alleles of different genes are transmitted from parents to offspring independently of one another.

a tetrad is the foursome during meiosis made by two homologous chromosomes that have each already replicated into a pair of sister chromatids.