Complex sentence: The personal characteristics that encourage environmental responses, such as appearance, personality, and intellect, are not the same between different siblings and fraternal twins because of gene variations; therefore by this stage environmental preferences emerge that reflect both their similarities and differences.
Split into simpler sentences: The personal characteristics that encourage environmental responses, such as appearance, personality and intellect are not the same between different siblings and fraternal twins because of gene variations. As a result of gene variation, environmental preferences emerge that reflect both their similarities and differences.