Jewish law prohibits marrying close relatives, including parent-child, sibling, and aunt/uncle-niece/nephew, as being related by blood (inbreeding or consanguinity), even historically, was regarded as dangerous, even though genes were unknown when the Torah was written.
These prohibitions were detailed in Leviticus 18. The patriarch Abraham sought a wife for Isaac in his mother’s family so that she would have the same values as him and not a woman living in a sea of idol worshipers.
The Torah permits marriages between cousins and even, according to some interpretations, between an uncle and niece, but these are discouraged today, thanks to modern medical understanding of genetic risks.
Consanguinity is still common among many Arabs and Jews in Israel and around the world, resulting in their offspring having genetic diseases or becoming carriers of these defects.
Now, a new model developed at Rabin Medical Center-Beilinson Campus in Petah Tikva (part of Clalit Health Services) could dramatically change how genetic compatibility is assessed between partners and offer a precise solution to a concern shared by many Jewish couples around the world.
Wrongly identifying genetic risks
The findings highlight a major flaw in the current global standard for evaluating the risk of hereditary diseases that is widely used by clinics worldwide; it doesn’t account for genetically homogenous populations like the Jewish community.
As a result, many couples are wrongly recognized as being at genetic risk, leading to anxiety, confusion, and, in some cases, termination of pregnancy after amniocentesis (removing fluid from the uterus).
The new model shows that what is often labeled as “abnormal” in one population may in fact be perfectly normal in another, according to Prof. Idit Maya, acting director of the hospital’s Genetics Institute and head of the research team.
In an interview with The Jerusalem Post, she explained that the universal threshold used today was originally developed for genetically mixed populations such as those in the US or Europe.
But applying that same measure to communities with shared ancestry – like Jews, Druze, Circassians, or other ethnic communities – can produce misleading results.
“Measuring the genetic closeness of Jewish couples using the same yardstick as American couples is a mistake,” she continued.
“In populations like ours in Israel, some level of genetic relatedness is expected. This is the result of generations marrying within the group. If we don’t take that into account, we end up with false red flags that can lead to heartbreaking decisions, including unnecessary terminations.”
Maya’s research was triggered by repeated cases of Jewish couples receiving alarming genetic reports from overseas clinics that were unfamiliar with the unique genetic background of Ashkenazi Jews.
To put her theory to the test, Maya and her team – in collaboration with Prof. Lena Sagi-Dain, chair of the Israeli Society of Medical Genetics – analyzed 15,000 genetic samples from the Rabin Medical Center, including blood and embryo data.
Based on this comprehensive study, they developed a new algorithm that adapts risk assessment to the genetic profile of each population. Rather than using a rigid, one-size-fits-all global cutoff, the new model allows for a personalized “true risk threshold.”
This distinction makes it possible to differentiate between normal genetic similarity – for example, among Ashkenazi, Bukharan, or Christian Arab couples – and concerning true closeness that could indicate inherited syndromes and diseases linked to “genetic imprinting.”
“Instead of automatically flagging any similarity that crosses an arbitrary line, our model adjusts to the couple’s ethnic-genetic background and issues a warning only when there’s a real reason to be concerned,” added Maya, who earned her MD degree at the Hebrew University Medical Faculty, did her internship at the Rabin Medical Center-Hasharon Branch, and specialized in medical genetics at Beilinson.
THE ISRAEL Society of Medical Genetics has already adopted the model, which is expected to be included in Israel’s updated clinical guidelines.
“This is a big step forward in personalized medicine – not just for individuals, but for entire communities,” Maya said. “Our model will reduce unnecessary concerns, prevent false alarms, and in some cases, even save pregnancies that might otherwise have been terminated. It’s a meaningful shift for the benefit of our patients.”
There were up to 20 million Jews at the start of the 20th century. But long ago, the genetic pool was much smaller. Jews married Jews; Ashkenazim almost always married Ashkenazim; Hassidim married Hassidim, usually from the same branch of Hassidim; while Jews of Lithuanian origin married those like them.
“As a result, there can be hundreds of genetic mutations causing genetic diseases among Jews, and we are discovering new ones all the time,” Maya noted. “I usually ask patients where their grandparents were born and if they had any genetic diseases to identify potential problems.
“Families who lost many relatives in the Holocaust have difficulty answering,” she said but suggested that in two or three decades, the situation will improve.
“There will be fewer marriages within a group and more marriages of Ashkenazim and Sephardim (who have some different genes). Genetic problems will be diagnosed earlier thanks to more knowledge and better technology, and treatments will improve and be given at an earlier stage.
“We can test gene sequences before a woman and her husband have a baby to see if there’s something rare. Then embryos without the defect that would cause disease would not be implanted.”
Instead of testing separately, “it’s now possible to do a panel of dozens of genes together, and it’s included in the health system’s basket of health services. When the tests were separate, it was more expensive,” Maya said.
Asked whether there are enough genetic specialists in Israel, the Rabin Medical Center physician said that “there are only about 100, and most are women because they earn less due to the lack of private medicine in the field, and lawsuits are higher against geneticists than any other specialty. Specialization in genetics takes many years.”
She suggested that “all doctors in Israel should learn genetics. Neurologists should look at genetics for neurological diseases, and cardiologists should learn about genes involved in heart disease.
“We are changing the curriculum at Tel Aviv University to include this, and it should be in all medical schools. With artificial intelligence, medical students don’t have to memorize everything as in the past. What I learned about genetics decades ago is not relevant.”
AI will make things easier for doctors, she concluded, “to help us with things we don’t know – but we have to communicate, and AI won’t help doctors to do amniocentesis. There are mistakes in AI.
Medical students have to develop the tools so they can understand the clinical picture of each patient. There is no book listing all possible genetic mutations. Students must be able to read journals critically and to educate themselves.”