Which of the following conditions is characterized by the presence of only one chromosome from a homologous pair?

The condition characterized by the presence of only one chromosome from a homologous pair is known as monosomy. This occurs when an individual has only one chromosome instead of the typical two in a homologous pair. For example, if a person has a missing copy of a chromosome that normally exists in pairs, it results in monosomy for that chromosome, which can lead to various genetic disorders depending on which chromosome is affected.

In the context of genetics, aneuploidy refers to any deviation from the normal diploid number of chromosomes and encompasses conditions like monosomy and trisomy, where an extra chromosome is present. Trisomy involves having three copies of a specific chromosome instead of the usual two, while nullisomy refers to the complete absence of both chromosomes of a homologous pair. These definitions highlight why monosomy is distinctly defined as having only one chromosome from a homologous pair.