MTFS Twin Info and Frequently Asked Questions

How Are Identical (MZ) and Fraternal (DZ) Twins Different?babies

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 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.

Can doctors tell if twins are identical or fraternal?

Most think that fraternal twins have two placentas and identical twins share one placenta, but this is not necessarily the case.

One-third of identical twins separate within a few days of conception, before the placental tissue has begun to form. Each embryo then grows it's own placenta. So if your doctor said that you and your twin were fraternal because you had two placentas, the doctor may be incorrect.

Though fraternal twins have their own separate placentas, sometimes the two fertilized eggs implant close to each other in the uterus, which can result in their placentas fusing. The two fused placentas look like one placenta, causing them to be mistaken for identical twins.

This is a fairly common mistake; as many as twenty percent of all twin births are misidentified as identical or fraternal. This confusion is one reason why we take special steps, such as sometimes taking blood, to determine if twins are identical or fraternal.

How common are twins?

In the United States, Canada, and Great Britain about 1 out of every 83.4 births is a twin birth. Reported frequencies throughout the world vary from about 1 in 70 to 1 in 145 births; however, source records from many areas are unreliable. In North America DZ twins are more frequent than MZ twins, the relative proportions being 71.8 to 28.2; similar proportions hold among other populations of European origin. Excluding the use of fertility drugs, approximate figures for the frequency of multiple births are 1 in 80 births for twins, 1 in 6,400 for triplets, 1 in 512,000 for quadruplets, 1 in 40,960,000 for quintuplets, and so on.

"Multiple Birth." Encyclopedia Britannica from Encyclopedia Britannica Online.

How did we get chosen to participate? Can just anyone volunteer?

Using Minnesota public birth record information, we attempted to locate same sex male and female twins born during targeted years (roughly between 1971- 1994). Once the families were located, they were recruited to participate based on eligilbility criteria such as distance living from the University and the ability to participate in the full day intake assessment. The families needed to meet eligiblity criteria to ensure that participants would be representative of Minnesotans in general. We are not currently recruiting any new families for our studies and are unable to accept volunteers.

Why do you use twins?

Because identical (MZ) twins carry identical genes, they have been used extensively in medical and psychological research. By comparing sets of MZ twins to carefully matched control sets of fraternal (DZ) twins (the "twin method"), researchers have attempted to elucidate the relative importance of heredity versus environment in the development of certain diseases, in the formation of personality, and in intelligence.

Can you explain again how you determined the twins were identical or fraternal (zygosity)?

Twin zygosity was determined using the following three methods: (a) parents completed a zygosity questionnaire that included questions regarding the twins’ physical similarity and frequency with which family members and others confused them; (b) research staff determined zygosity after rating twins on physical similarity (e.g. eye color, ear shape, etc.); and (c) zygosity was determined using an algorithm based on ponderal index (height/weight ratio), cephalic index (head width/head length ratio), and fingerprint ridge count. In the event that the above three methods disagreed, DNA was collected by either drawing blood or performing a buccal swab. The samples were then sent to Memorial Blood Center where analyses were carried out on the nuclear DNA. Families were informed of the results.

To assess the efficacy of our twin identification procedure, we have performed a study during which we assessed 50 twin pairs with all four methods. We found that, when the three methods involving behavioral and physical measures agreed, the DNA analysis always confirmed this agreement.

Why do you use twins who are specifically age 11 or 17 at intake?

We wish to study the differences between twins who are at various developmental stages in their lives: late childhood, early adolescence, and late adolescensce. Then we can compare these differences and similarities over time.

Can I get results?

Unfortunately, staff, time, and budget constraints do not allow us to give out individual results based on your assessments. We do, however, mail out some personality feedback to each participant who completes the personality questionaire. General findings and research published from the overall study data can be found by accessing the research page. With longitudinal studies, it takes time to gather, analyze and publish the data; therefore, research and data from the MTFS is continually being produced and published.

When will the study end?

The purpose of the study is to follow twins as they develop through adolescence and adulthood. Hopefully, our funding will continue so we can continue to explore the important environmental expericences that shape our participants' development. At this point, we don't have a set endpoint. We hope there are interesting and exciting research oppportunities as our twins grow older. As always, though, you are free to stop participating at any time.

Why do you need a step-parent's information, since he/she isn't the biological parent?

We collect as much data as possible on each family member to determine both genetic and environmental influences within each family. The step-parent provides valuable information about the daily environment in which the twins live.

What if one twin doesn't want to participate?

Because this is a longitudinal study, each individual participant is important to us and our research. We hope each of our twins and their parents will continue to participate in each phase with us. However, we can still learn valuable information on how an individual develops and changes over time from each of our participants, even if a twin pair can not participate together. Likewise, parents can still share valuable information with us about themselves.

How will participating benefit us?

Each participant receives monetary compensation for participating in any phase of the study. Also, we mail some personality feedback to you based on questionaires you fill out at your visit. Your participation also benefits society by allowing a better understanding of human behavior.


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The U of MN is an equal opportunity educator and employer. This page last updated: September 4, 2007 2:09 PM