Final answer:
The best description for why red hair and freckles usually occur together is linkage, as the genes for these traits are often located close to each other on the same chromosome and are inherited together.
Step-by-step explanation:
The phenomenon where people with red hair usually also have freckles can best be described by linkage. This genetic concept refers to genes that are located close to each other on the same chromosome, which are often inherited together. Due to linkage, characteristics such as red hair and freckles frequently appear together because the genes controlling these traits are physically near each other on the chromosome and tend to be passed on as a group during the process of inheritance.
Linkage contrasts with independent assortment, where genes on different chromosomes are inherited independently of each other, and codominance, where two different alleles are fully expressed in the phenotype. In the case of red hair and freckles, the close proximity of genes contributes more directly to their common association than independent assortment or codominance. Moreover, pleiotropy, which describes a single gene influencing multiple traits, does not fully account for the pattern observed in individuals with red hair and freckles, making linkage the most accurate description.