Case A female patient came to the genetic counseling office two weeks ago to consult me about fertility. She was a patient with a balanced chromosomal translocation, 36 years old, with 2 recurrent spontaneous miscarriages. I explained her condition in detail and encouraged and suggested her to try third generation IVF (pre-implantation diagnosis) to solve her fertility problem as soon as possible. A few days later, she came back, depressed. “My husband said he looked it up on the internet and consulted a specialist, the chances of getting pregnant normally with this problem are not as high as you say, only one in ten”, “It’s so slim, he said he wants children and wants to divorce me”. 1/18 With the popularity of the Internet, there are a lot of online experts to search for. If you use the keywords “chromosomal translocation pregnancy probability” and do a search, figures such as 1/18 and 1/9 will pop up, including the opinions of many well-known experts. These two numbers mean that balanced translocation chromosomes can theoretically form 18 types of gametes, including one completely normal gamete and one chromosomal translocation gamete with the same parent, and the embryos obtained from these two cases are viable, adding up to 1/9. The remaining 8/9 (16/18) gametes are non-balanced chromosomes, and the embryos formed are not viable. 1/26? The number 1/18 is also used in a textbook edited by one of our famous geneticists. This theory is based on the tetrasomal segregation of translocation chromosomes. A more authoritative book, ISCN 2009 (International System of Human Cytogenetics Nomenclature 2009), describes it this way. Generally speaking there are two types of segregation of balanced translocation chromosomes, 2:2 and 3:1. According to the principle of proximity 1 of 2:2, four types of gametes can be formed, one of which is normal, one of which is identical to the parental translocation, and two of which are unbalanced. By the proximity 2 principle of 2:2 segregation, 6 types of gametes can be obtained, all of which are non-equilibrium chromosomes. By 3:1 segregation usually 8 gametes are obtained also all non-equilibrium chromosomes. This is the source of the popular 2/18 and 16/18 data. But yeah but, the ISCN book has a footnote in the 3:1 segregation section specifically stating that there can be another 8 segregations if the fragment is exchanged between the trophectodomain and the exchange site. So there are actually 18+8=26 gametes possible on the ISCN. 1/36? In strictly theoretical terms, none of the above is complete, and there is a 4:0 mode of segregation. This situation gives 10 gametes types. So theoretically speaking balanced translocation chromosomes can produce a total of 36 gametes. The actual phenomenon But ah, another but, whether 18 gametes, 26 or 36, the probability of each gamete occurring is different. Balanced translocation chromosomes are predominantly separated 2:2, which means that the probability of having perfectly normal and parentally identical translocated gametes is much higher than other types of gametes. The so-called 1/18 is one of the 18 possibilities, not the probability of 1/18. From the results of PGD (pre-implantation genetic diagnosis) in patients with balanced chromosomal translocations that I have contacted, the probability of having a transferable embryo (normal or karyotype identical to the parental generation) is much greater than 1/9. I have also consulted with several other colleagues in fertility centers on this issue, and they also indicated that They also indicated that the probability of transferable embryos is much higher than 1/9, but the specific value has not yet been calculated in a large sample and multicenter. Some scholars have indicated that the overall probability of normal pregnancy in families with chromosomal translocations is around 40-50%. Some scholars have also indicated that male patients with chromosomal balance translocations are more likely to produce normal gametes than female patients with chromosomal balance translocations. The author has seen cases in which the wife of a husband with a chromosomal translocation had four pregnancies and three healthy children. The semen of a normal male contains hundreds of millions of sperm in one ejaculation, and the lowest probability is that at least a few million of these hundreds of millions of sperm are normal. Imagine a fertilization site where hundreds of millions of sperm swim all the way to the egg, it should be the normal sperm that move faster and have better stamina to reach the egg first. Evidence When conducting genetic counseling, the less than accurate clinical experience of an individual or a few individuals often does not fully convince the patient, and some patients prefer to believe what the “experts” on Doula say. A careful review of other books shows that Human chromosomes clearly states that 2:2 segregation of translocated chromosomes is the main mode of segregation, while other modes such as 3:1 and 4:0 are very rare. The book “Practical Preimplantation Genetic Diagnosis” (Practical Preimplantation Genetic Diagnosis) contains statistics on embryos obtained from patients with balanced translocations in several fertility centers in Europe and the United States, and out of 1081 embryos tested, 276 (25.5%) were transferable (normal and balanced translocations). Of these, 29.8% of embryos with male balanced translocation factors were transferable; 23% of embryos with female factors were ultimately transferable. In conclusion: Patients with chromosomal translocations, abandon the concept of 1/18, the probability of producing normal gametes is much higher than 1/18. Have the confidence to get your own offspring through natural pregnancy or preimplantation diagnosis, and do not give up your marriage and family because of this small problem that can be solved by itself.