By studying identical and fraternal twins and their families, we can estimate how genes and environment interact to influence character, strengths, vulnerabilities, and values. Both types of twins share common life experiences. Identical twins differ from fraternal twins in the number of genes they have in common.
Genes are the chemical units in the cells of the body.
They act as blueprints for guiding and directing growth. Rather than developing into one individual, the fertilized egg cell develops into two individuals with the same genetic makeup. Two different individuals, no more genetically alike than brothers and sisters develop from separate fertilizations. Most think that fraternal twins have two placentas and identical twins share one placenta, but this is not the case. One-third of identical twins separate within a few days of conception, before the placental tissue has begun to form.