In order to find sources of genetic variations in humans, experts often look into the differences in genetic traits between identical twins and fraternal twins because of their availability. Identical twins share all of each other's genes, while fraternal twins only share half.

For instance, a study in May last year that gathered previous studies on twins found that 49 percent of the variation in human traits and diseases is heritable, while 51 percent is affected by environmental factors.

Researchers from the University of Queensland and the University of Amsterdam have found that about 69 percent of the resemblance in twins is due to additive genetic variation or the average differences in gene variants.

The team's discovery meant that risks for certain diseases and mental disorders were highly heritable for twins and non-twins in general, and that some human traits were also significantly affected by external factors.

One Twin Develops Cancer, The Other Might Follow

Now, a new study conducted in Europe has found that when a twin sibling is diagnosed with cancer, the other twin also has a high chance of developing any type of cancer.

The large-scale twin study, which is featured in the Journal of the American Medical Association, examined 23 different types of known cancer, and data that contained records of 200,000 same-sex twins.

The team of researchers from Harvard University, University of Helsinki, and University of Southern Denmark has discovered that as twins with any type of cancer often developed different types of the disease, there is a shared risk for developing the disease among some family members.

"Twins are no more likely to get cancer than non-twins. But what we saw is the risk for cancer increased if you have a relative who developed cancer," said study co-author Lorelei Mucci of Harvard's Chan School of Public Health.

High Risks For Heritability Among Families

Heritability of cancer among families is a way to measure a person's risk of developing cancer.

Mucci said past studies have shown estimates regarding the heritability and common cancer risks among families. These include risks for prostate cancer, breast cancer and colon cancer.

However, little is known about the risks for rarer cancers. Some studies were also too small, and the follow-up time was too short to be able to exactly estimate risks or heritability, she said.

In their study, Mucci and her colleagues evaluated health records for more than 80,000 identical and more than 120,000 fraternal twins from the registries of Finland, Norway, Denmark and Sweden. Follow-ups for the participants were at an average of 32 years from 1943 to 2010.

About 38 percent of identical twins were diagnosed with cancer, while the rate for fraternal twins was 26 percent. Among identical twins with one sibling diagnosed with cancer, the risk for diagnosis was 46 percent. For fraternal twins, the risk was 37 percent.

Overall, the risk for cancer heritability was 33 percent, the researchers said. Chances were significantly higher for melanoma, ovarian cancer, testicular cancer, prostate cancer, breast cancer, uterine cancer and kidney cancer.

Why was the risk of heritability high? Mucci said one reason was the shared genetic factors among family members and several environmental factors.

"When one sibling smokes, both siblings are more likely to smoke," said Mucci.

Genetic Profile Of Cancers

Researchers know that the genetic profile of cancers is much significant than its location in the body.

According to the American Cancer Society, a woman whose BRCA1 or BRCA2 genes--proteins that prevent the proliferation of cells--mutated has an 80 percent increased risk for developing breast cancer, as well as a 70 percent increased risk for developing ovarian cancer.

The implication of these cancers may be different for individuals, but experts believe similar genetic profiles are essential in improving cancer treatments and increasing survival rates.

University of Helsinki's Dr. Jaakko Kaprio, another member of the research team, said the study may be beneficial in patient education and cancer risk counseling.

Meanwhile, Mucci hopes to investigate further into the role of genetics in cancers to see if twins share some of the same genetic mutations. Her current project involves analyzing tissue samples of tumors from twin males with testicular cancer.

"We're interested in the genetic factors that determine what the tumor looks like," added Mucci.

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