Advanced Placement (AP) Biology Practice Exam

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Which genetic term describes an organism with only one copy of a chromosome?

Haploid
Diploid
Monosomic
Aneuploid

The correct answer is: Monosomic

The term that describes an organism with only one copy of a chromosome is "haploid." In organisms that are haploid, such as certain fungi, algae, and gametes (sperm and egg cells in animals), each cell contains only one set of chromosomes, which is crucial for sexual reproduction. When two haploid gametes fuse during fertilization, they create a diploid zygote, restoring the two sets of chromosomes typical of the species. While "monosomic" refers to an organism or cell that lacks one chromosome from a typical diploid set, it specifically applies to a situation with a diploid organism missing one chromosome of a homologous pair. "Aneuploid" describes a cell with an abnormal number of chromosomes, such as having an extra or missing chromosome but does not specify that there is only one of a particular chromosome type. Therefore, "haploid" is the most precise term for an organism with just one copy of each chromosome.