What do you call a cell that contains only a single set of chromosomes or genes?

A cell that contains only a single set of chromosomes is referred to as haploid. In a haploid cell, there is one complete set of chromosomes, which is represented as "n." This is typical of gametes (sperm and egg cells) in organisms that reproduce sexually, as these cells fuse during fertilization to form a diploid zygote that has two sets of chromosomes (one from each parent).

In contrast, diploid cells, which contain two sets of chromosomes, are represented as "2n" and are common in somatic cells of many organisms, including humans. Homologous refers to chromosomes that have the same structure and carry the same types of genes, but may have different variants. Tetrad is a term that describes a structure formed during meiosis when homologous chromosomes pair up, but it does not pertain to a single set of chromosomes.

Understanding that haploid denotes a single set of chromosomes is crucial in genetics, particularly when considering processes such as meiosis and the formation of gametes in sexual reproduction.