Twin Information

Twin Information

Twin Information

Twins are a fascinating and unique part of human biology, and at TwinsUK, we are dedicated to studying both identical and non-identical twins to better understand genetics, health, and disease. Below, you’ll find key facts about twins and answers to common questions.

What is Zygosity?

Zygosity explains whether twins are identical (monozygotic) or non-identical (dizygotic) based on the genetic similarity they share. Identical twins share 100% of their genes, while non-identical twins share around 50%, similar to regular siblings.

Types of Twins

  • Identical Twins (Monozygotic): Formed when a single fertilized egg splits into two. They share all their genes and are always the same sex.
  • Non-Identical Twins (Dizygotic): Result from two separate eggs being fertilized by two different sperm. These twins share around 50% of their genes and can be of the same or different sexes.

Zygosity Testing

The most accurate way to determine zygosity is through DNA testing, with genetic fingerprinting being 99% reliable. When DNA testing isn’t available, a ‘peas-in-the-pod’ (PPQ) questionnaire is used, offering a 95% accuracy rate.

Common Twin Questions

No, differences in environmental factors can cause identical twins to look different over time.

Yes, identical twins have unique fingerprints due to variations in foetal development.

Non-identical twins can run in families, especially on the mother’s side. Identical twins typically occur spontaneously.

Around 25% of identical twins exhibit opposite features, like hair parting or dominant hands. This may be linked to the timing of egg splitting.

Yes, genetically, they share the same amount of DNA as half-siblings.

Yes, environmental factors in the womb can lead to differences in height and foot size.

Arranging a Zygosity Test

If you are interested in a private Twin DNA Zygosity test you can contact the Multiple Births Foundation (MBF) on 020 3313 3519, or email info@multiplebirths.org.uk for an application pack or visit their website You will receive the results within 4-6 weeks. However, if we already have a blood sample from you then we are able to have this test carried out for you for a small fee. Please note there will be a considerable wait for the results as we only process the samples approximately once a year as they are outsourced to a private laboratory in a large batch.

What’s It Like Growing Up as a Twin?

Many twins report a strong bond with their twin sibling, often described as closer than that of other siblings. While some twins enjoy similar interests and have shared experiences, they also develop individual personalities, likes, and dislikes. The twin experience can be diverse, and no two twin relationships are exactly alike.

What Are Some Common Misconceptions About Twins?

  • Identical Twins Don’t Always Look Alike: Environmental factors and individual life experiences can cause identical twins to look different as they age.
  • Twins Don’t Necessarily Skip a Generation: This is a popular myth. Non-identical twins can be influenced by family genetics, but this does not always follow a predictable generational pattern.
  • Twins Don’t Have to Be Born on the Same Day: In rare cases, twins can be born on different days or even weeks apart.

Interesting Facts About Twins

  • Twin Studies Have Changed Health Research: Studies on twins have been crucial in helping scientists understand the role of genetics and environment in health and disease. Twins help researchers disentangle how much of a condition is inherited versus influenced by lifestyle and surroundings.
  • Fertility Treatments Increase Twin Births: The rise in twin births in recent decades is largely due to assisted reproductive technologies, such as IVF (In-Vitro Fertilisation), which increases the likelihood of multiple embryos being implanted and resulting in twin births.
  • Vanishing Twin Syndrome: Sometimes, one twin can disappear in the early stages of pregnancy, a phenomenon known as “vanishing twin syndrome.” This occurs when one twin miscarries while the other continues to develop normally.