Linkage is a phenomenon in genetics where two or more genes located on the same chromosome tend to be inherited together during meiosis, rather than assorting independently. This occurs because the physical proximity of genes on a chromosome makes them less likely to be separated by crossing over.

Example: Thomas Hunt Morgan's work with Drosophila melanogaster (fruit flies) famously illustrated linkage. He observed that genes for body color (e.g., grey vs. black) and wing size (e.g., normal vs. vestigial) were often inherited together in patterns deviating from Mendelian predictions, because they resided on the same chromosome.

Why it's an exception to Mendel's Second Law:
Mendel's Second Law, the Law of Independent Assortment, states that during gamete formation, alleles for different genes assort independently of one another. This law holds true for genes located on different chromosomes or for genes very far apart on the same chromosome.

Linkage is a direct exception because linked genes do not assort independently. Instead, they tend to move as a block during meiosis, remaining together in the same gamete. This results in parental combinations of alleles appearing in the offspring more frequently than recombinant (non-parental) combinations, which contradicts the 1:1:1:1 ratio of gametes and the 9:3:3:1 phenotypic ratio in a dihybrid test cross expected by independent assortment. The closer the genes are on a chromosome, the stronger the linkage and the less frequently recombination (crossing over) occurs between them.