What is the primary characteristic of sex-linkage?

The primary characteristic of sex-linkage is that it involves alleles located on sex chromosomes, which often leads to traits that are expressed differently in males and females. This is primarily due to the presence of certain genes on the X or Y chromosomes. For instance, in many organisms, including humans, the X chromosome carries numerous genes that can influence traits, while the Y chromosome carries fewer genes.

Because males have one X and one Y chromosome, any allele present on the X chromosome will be expressed regardless of whether it is dominant or recessive. In contrast, females have two X chromosomes, which means they can be homozygous or heterozygous for alleles on the X chromosome. Consequently, conditions such as color blindness or hemophilia, which are linked to the X chromosome, often exhibit more frequent expression in males than in females. This differential expression based on sex is a hallmark of sex-linked traits.

Understanding this principle of sex linkage helps explain why certain genetic conditions can appear to be more common in one sex, revealing the complexities of inheritance related to sex chromosomes.