Final answer:
Genetic variation accounts for the differences in traits among siblings, resulting from the unique combination of genes inherited from their parents. Even with identical twins having the same genome, differences can arise due to gene expression and environmental influences. Blood type is an example where genetics determine traits based on allele combinations.
Step-by-step explanation:
Genetic variation is a difference in the same trait and is why siblings don't look exactly alike. This variation is due to the unique combination of genes each sibling inherits from their parents. While identical twins can share the same genome, slight differences might still be noticeable as a result of differential gene expression or environmental factors during development.
Siblings receive 50% of their genes from the mother's egg and the other 50% from the father's sperm. These genes, which exist as various alleles (gene versions), combine to form the individual's genotype. The physical expression of these genes, or phenotype, can result in siblings having different characteristics, such as hair color, blood type, or height. For example, blood type can be influenced by the presence of the A, B, or O alleles inherited from the parents. These differences in alleles lead to the trait variation that makes each person unique, even among siblings.
It's important to note that genetic differences must have some basis for being passed on, or selection cannot act upon them. Environmental factors may also influence traits, but without a genetic component, these influences won't affect the traits of the next generation.