Final answer:
In eukaryotes, only diploid or somatic cells, which have two copies of each chromosome, can use homologous recombination to fix a double-strand break; haploid cells, such as gametes, cannot.
Step-by-step explanation:
In eukaryotes, only cells in diploid or somatic states have two copies of each chromosome, so in many cells homologous recombination (HR) can't be used to fix a double-strand break (DSB) in DNA. Diploid cells, designated as 2n, contain two matched sets of chromosomes, one set inherited from each parent. These cells include most body or somatic cells. On the other hand, haploid cells, designated as n, contain only one set of chromosomes and are exemplified by human gametes (sperm or eggs). During the cell cycle, somatic cells can undergo DNA repair processes such as HR because they have a copy of each chromosome to use as a template for repair.