In the context of genetics, what term refers to traits that are passed down primarily through the X or Y chromosome?

The correct term for traits that are passed down primarily through the X or Y chromosome is "sex-linked traits." This encompasses traits that are associated with genes located specifically on the sex chromosomes. In humans, for example, genes located on the X chromosome can lead to conditions like color blindness or hemophilia, which predominantly affect males due to their single X chromosome.

In contrast, the other options describe different genetic concepts. Polygenic characters refer to traits that are influenced by multiple genes, such as height or skin color, rather than being linked to a single chromosome. Codominance occurs when two different alleles are both expressed in a heterozygote, like in the case of AB blood type, where both A and B alleles are fully expressed. Incomplete dominance is a form of inheritance where the phenotype of a heterozygote is an intermediate of the phenotypes of the two homozygotes, such as in certain flower color expressions. These distinctions underscore why "sex-linked traits" is the most accurate term for the genes that are passed through the sex chromosomes.