One egg, two sperm: Rare twin type between identical and non-identical twins

According to a study published by The Journal of Human Genetics that coined the term and reported its very first case in the US in 2007, semi-identical twinning was described as being somewhere between identical and non-identical twins. Only two cases of semi-identical twinning have been recorded thus far, the most recent from Australia being reported in The New England Journal of Medicine in 2014. A 28-year-old primigravida at the time (2014) underwent a routine ultrasound scan at six weeks showing a single placenta and positioning of the amniotic sacs suggestive of a monozygous pair. However, a subsequent scan at fourteen weeks showed the fetuses were different genders, making it impossible for them to be identical twins.

Aside from identical (monozygotic), non-identical (fraternal/dizygotic), the lesser understood conjoined and polar body (half-identical), scientists have discovered a different kind of twinning: semi-identical or sesquizygotic twins.

Identical twins - when a single ovum (egg) is fertilized by a single sperm, then splits into two, producing two identical boys or two identical girls. They share 100 percent of their DNA. Twins occur more frequently now, but identical twins are much less common than non-identical.

Non-identical twins - occurs when two ova are released and fertilized by two different spermatozoa. They form either a boy-girl or same-sex pair of non-identical twins and share 50 percent of their DNA. Fraternal twins aren't any different from other siblings, only that they are born at the same time. They are much more common and are likely to occur in older mothers as they are found to release two ova during ovulation. IVF can also potentially produce twins as more than one ovum can be inserted into the uterus.

Semi-identical twins occur when a single egg is penetrated by two different sperm. This is a rare phenomenon as the ovum is biologically wired to attract and encapsulate a single sperm in preparation for fertilization. Dr. Fisk, a scientist at the University of New South Wales, stated that "Under odd circumstances such as these that two sperm manage to enter, the embryo now carries three sets of chromosomes (mother's XX + father's {XY + XY}) instead of two (mother's XX + father's XY) and chances of the resulting embryo surviving as a fetus become unlikely. When they do, however, the three sets of chromosomes split into two sets, thus forming the twins. "They end up sharing somewhere between 50 to 100 percent of their DNA. Clinical geneticist Dr. Michael Garrett goes on to mention that "In the process of the three sets of chromosomes splitting, cells from the first sperm/first XY chromosome will fuse with the egg to form a different proportion of paternal DNA from the second sperm/second XY chromosome fusing with the same egg. Semi-identical twins will possess 100 percent of their maternal DNA because they originated from the same egg, but share only a percentage of paternal DNA. Three sets of chromosomes cannot be divided into two to produce an equal number of genomic compositions. The Australian pair of semi-identical twins share about 89 percent of their genes."

It came to the scientists' attention that a proportion of semi-identical twins might have been mistaken for non-identical since they both singly stem from two different sperm. So they decided to look at 968 fraternal twins from different countries, in multiple laboratories, and in an abundance of genetic variants. Their results were conclusive - sesquizgous twins were unique and rare as they initially thought.

Considering semi-identical twins are an uncommon occurrence, chromosomal abnormalities or other forms of birth defects tend to increase in likelihood. Back in 2007, when the first semi-identical twins were discovered, scientists noted that one was a perfectly healthy male while the second twin identified as a true hermaphrodite. The 2014 Australian pair were otherwise both anatomically and developmentally healthy.

While the causes behind semi-identical twinning are neither adequately researched nor documented, one purported theory by biologist Michael Golubovsky is mentioned. He states that the ovum perhaps divides, but before becoming two separate entities, each half is fertilized by two different sperm distorting genes in the process. However, this explanation comes very close to another type - polar body (half-identical) twins, which suggests two completely different ova (no dividing involved) are fertilized by two uniquely different spermatozoa as well. At this point in research, not much can be done to understand further as the latter type of twins are only in theory, and no actual cases have been identified. Semi-identical twins' rarity and polar body twins' non-existence means there are no cases for genetic comparison and testing.