What is the physical expression of an organism's genes called?

The correct answer is the phenotype, which refers to the physical expression or characteristics of an organism, resulting from the interaction of its genotype (the genetic makeup) with the environment. The phenotype encompasses observable traits such as morphology, development, biochemical properties, and behavior.

In contrast, the genotype is the specific set of genes an organism carries, which determines its potential traits but does not account for environmental influences. Haploidy refers to having a single set of chromosomes, which is important in genetic contexts but doesn't directly relate to the physical characteristics expressed. Karyotype involves the number and appearance of chromosomes in the nucleus of a cell, which is a structural aspect rather than a description of physical traits. Therefore, phenotype accurately captures the observable characteristics that manifest from genetic information.