FAQ - Twins Genetics
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"We just got our results and found out that we are identical. I think everyone thought we were except my mother's doctor. We have always looked just alike. I want to say thank you. It's like we received this wonderful gift, 50 years later than we may have liked, but still very much a gift. We just couldn't imagine any twins being..."

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These questions and answers provide detailed information about twins genetics. We discuss here various factors that influence the differential development of identical twins. We also discuss the probabilities of half-identical twins or polar body twins. Listed below is a selection of those that are most frequently asked. If you can not find the answer to your question, please contact us and we'll promptly provide you with an answer.

Are identical twins really identical?

Is the DNA of identical twins really identical?

What is the relative impact of genes vs. environment in explaining the differences between identical

What types of traits are considered to be genetically based?

What types of traits are considered to be environmentally based?

What are mirror image twins?

What are polar body or half-identical twins?

Is having twins affected by heredity (i.e. "twins run in the family") or is the twins gene, sex-link

What is the value of my participating in a twin study?

Can fraternal twins have different fathers?

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Are identical twins really identical?

With respect to the phenotype (i.e. physical characteristics), identical is not necessarily an ideal word to describe monozygotic (i.e."identical") twins. To begin with, there are a wide variety of environmental influences during development in the uterus that causes each twin to develop differently such as differential sharing of nutrients or vascular interactions in monochorionic placentas. Additionally, some soft tissues of the head and face tend to be influenced by the limited space in the human uterus which was primarily designed for one fetus. Therefore, head shape can be quite different between infant identical twins although these differences may only be transitory. Later in development, a wide variety of environmental factors can influence the likeness of identical twins. One twin's preference for a particular sport or physical activity or even intellectual activity (i.e. reading fantasy novels) can greatly impact their individual physical or personality development. For this reason, nearly all identical twins can be distinguished by parents or anyone who knows them well. It would preferable to refer to them as monozygotic twins in order to recognize their obvious individuality. On the other hand, monozygotic twins who are separated at birth and raised in different environments can sometimes exhibit genetically determined behaviors that influence their innate selection of their environment and when reunited later in life can resemble each other very strongly, even more so that some monozygotic twins who had shared the same essential environment. Likewise, monozygotic twins may subconsciously inherit the desire for more similar life experiences than regular siblings. See the next question regarding differences in genotype between identical twins. back to top

Is the DNA of identical twins really identical?

The best answer to the question is, "almost identical". With respect to the DNA, this refers to the differences in genotype between identical twins. The sequence of the DNA is nearly 100% identical although there are likely to be handful of spontaneous mutations that occurred early in development just after the original fertilized egg had split. Other twins genetic modifications can also occur but they are exceedingly rare. Factors such as DNA imprinting that ultimately control the way that genes are expressed can be more influential than the actual sequence of the DNA. Many environmental factors can influence the control of DNA expression which ultimately lead to changes in the the phenotype or physical characteristics of monozygotic twins. back to top

What is the relative impact of genes vs. environment in explaining the differences between identical

Indeed, the study of twins provides the best controlled studies that can address this question. For nearly all human traits, both genes and the environment work together in different ways. The mechanisms that cause one trait to be influenced more by genetic than environmental factors, and vice versa, is very difficult to untangle. For example, there is a great deal of evidence that identical twins tend to be more similar in height than fraternal twins. Since both identical and fraternal twins are raised in the same relative environment, this would suggest that genetic factors exhibit a lot of control over height. However, if you look at an immigration example where U.S.-born children of Asian immigrants are significantly taller than their parents, you can see that Western diets which are generally more protein rich can cause these children to be much taller than their parents. This would suggest that an environmental factor like diet is the primary controller over human height. back to top

What types of traits are considered to be genetically based?

Generally, traits such as height, weight, hair color, hair texture, and eye color have a consistent genetic basis. These traits are usually predictive of zygosity status. back to top

What types of traits are considered to be environmentally based?

The true answer is that all traits have an environmental basis. back to top

What are mirror image twins?

Mirror image twins are identical twins that result when a fertilized egg splits later in the embryonic stage than usual, typically from day 9 to 12. Mirror image twins tend to exhibit characteristics with reversed asymmetry (e.g., one twin is left-handed and the other is right-handed, or opposite-sided occipital hair whorls and hair partings). If this split happens much later than this, then the twins can be co-joined. A DNA-based zygosity test cannot determine if twins are mirror image twins. back to top

What are polar body or half-identical twins?

Polar body twins are believed to occur when the mother's egg splits in the normal course of ovulation and one half (referred to as the 'polar body') is also fertilized by a different sperm. In theory, this results in twins who share half their genes in common (from the mother) and the other half different (from the two sperm). They share some features of identical twins and some features of fraternal twins and are so-called half-identical twins. However, there are intervening genetic recombinations during ovum development which may make polar bodies different from each other and therefore not possessing the same maternal DNA. For this reason, and others related to costs and accuracy, DNA-based zygosity tests are not currently offered by us to determine if twins are polar-body twins. back to top

Is having twins affected by heredity (i.e. "twins run in the family") or is the twins gene, sex-link

With respect to multiple ovulation (i.e. dizygotic twins), there is some evidence that twinning is heritable, but neither family studies nor molecular-genetic data suggest that it is either autosomal or sex-linked. There is little evidence that suggest that identical twinning has any genetic basis. back to top

What is the value of my participating in a twin study?

As a twin, you have a unique opportunity to contribute to one of the oldest questions humans have about ourselves: "Does nature or nurture control our fate with respect to certain human characteristics?" Participating in a twin study will provide you and your twin the opportunity to do a fun activity with each other that you could do with no one else. You will learn about twins genetics and something about yourself in the process. Some studies provide monetary compensation for your time and some are volunteer only. Some studies may also provide complimentary zygosity testing. Here is a link for more information about enrolling in twin studies. back to top

Can fraternal twins have different fathers?

Yes. This called is heteropaternity and is possible when more than one sexual partner was present during the same ovulatory cycle. A zygosity test does not establish paternity. back to top


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