About Twins

Why are Twins so Special to Science?

By studying identical and fraternal twins and their families, we can estimate how genes and environment interact to influence character, strengths, vulnerabilities, and values.

  • Identical (MZ) twins are genetic duplicates.
  • Fraternal (DZ) twins share 50% of their genes.
  • Both types of twins share common life experiences.
  • These facts plus information we collect from you about how alike or different members of a twin pair are allow us to determine how genes and environment influence…
    • falling in love
    • divorce
      Twins
    • aging
    • personality development
    • vocational choices
    • talents and abilities
    • attitudes
    • body characteristics
    • health
    • coping with stress
    • brain waves and other physiological responses

How Are Twins Made?

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.

Identical, or monozygotic (MZ), twins have the same genetic makeup.

  • Shortly after a single egg cell has been fertilized by a single sperm cell, the fertilized egg cell (or zygote) splits in half.
  • Rather than developing into one individual, the fertilized egg cell splits and develops into two individuals with the same genetic makeup.

Fraternal, or dizygotic (DZ), twins share 50% of their genes.

  • Two different individuals, no more genetically alike than brothers and sisters, develop from separate fertilizations. Fraternal twins develop from two separate eggs; each fertilized by a different sperm. The only difference between fraternal twins and regular siblings is the timing - fraternal twins develop during one pregnancy.

Mirror Twins: A Case of Two Opposites

Mirror twins are a special kind of identical twin that are essentially "mirror images" of each other. There are different degrees to which these twins mirror each other, but some common things are opposite handedness, hair whorls on opposing sides, mirror image teeth, and in extreme cases, they can even develop their organs on opposite sides of the body. Mirror twins can even potentially develop mirrored health issues, such as tumors in opposing lungs or mirror image dental problems.

Since mirror twins do not have DNA that differentiates them from other identical twins, there is no medical test that can be done to prove whether you are a mirror twin or not. However, one thing to keep in mind is that though mirror twins are identical (they share the same DNA) they have important differences from each other that can be used to distinguish between identical (monozygotic) and fraternal (dizygotic) twins.


Therefore, if you are left handed and your same-sex twin is right handed, this does not automatically mean that you are fraternal; you could in fact be identical mirror twins.