Answer: alleles
Explanation:
"An allele is a variant form of a gene. Genes come in different varieties, called alleles. Somatic cells contain two alleles for every gene, with one allele provided by each parent of an organism."- Nature .com
Aplastic anemia refers to a disorder that takes place when the body loses the tendency to generate enough amount of new blood cells. Aplastic anemia is a rare and serious disorder that can develop or arise at any age. In the condition, the person becomes fatigued and possess a greater threat of developing infections and uncontrolled bleeding.
Thus, in the given case, the nurse should appoint a roommate who is free from any kind of infectious disease or any other thing, which can stimulate an infection.
When the effects of genes add up to make the phenotype, they are called additive genes.