Why do siblings not display identical traits?

Siblings do not display identical traits primarily because they inherit different alleles from their parents. Each sibling receives a distinct combination of alleles due to the process of genetic recombination during meiosis. When their parents produce gametes (sperm and egg cells), the alleles for various traits are shuffled and randomly assorted. As a result, even though siblings share many of the same genes, they can inherit different forms (alleles) of those genes, which ultimately leads to variation in traits. This unique combination of alleles contributes to the individual differences observed among siblings, such as physical characteristics, behaviors, and susceptibilities to certain conditions.

While the other factors listed, such as the influence of genes from both parents, environmental factors, and genetic mutations can play roles in shaping traits and phenotypes, the direct reason for the variation in traits among siblings is rooted in the unique combination of alleles that each sibling inherits.