The only way to confirm heteropaternal superfecundation in human twins is genetic testing.
It is very common that kittens or other animals from one litter can have different fathers. This happens when two or more eggs from the same menstrual cycle are fertilized with sperm from different fathers and is called heteropaternal superfecundation. While in some animal litters it is very obvious if an offspring is heteropaternal because of very specific external appearances, it is harder to determine this in human twins. Therefore the only way to confirm heteropaternal superfecundation is genetic testing.
Companies such as the Paternity Testing Corporation, Humangenetik and Ingenetix perform genetic tests by using approved molecular genetic analyses based on STR (Short Tandem Repeat) from DNA derived from blood samples or buccal swabs. Since STRs are highly variable locations in the genome that are made up of short repeated DNA sequences (2–5 nucleotides) its length varies between different people which results in a genetic fingerprint. The analysis spectrum most often contains 15 short STR marker systems (Penta E, D18S51, D21S11, TH01, D3S1358, FGA, TPOX, D8S1179, vWA, Penta D, CSF1PO, D16S539, D7S820, D13S317 and D5S818) as well as the Amelogenin marker, which is used for sex determination. The analyzed STR markers are located on different chromosomes in non-coding genomic areas and are therefore inherited independently of each other. If the alleged father’s DNA is included in testing as being that of the biological father, the probability of paternity is calculated and is said to be proven if the value exceeds 99.99 %.
A study by Wenk et al. determined that 2.4% of dizygotic twins whose parents were involved in paternity suits were heteropaternal. Another study suggested heteropaternity among twins to be as high as one pair in four hundred for white women in the United States. Although reported cases are very rare, the frequency of heteropaternity is probably underestimated as the incidence of heteropaternity depends on promiscuity in the population and the use of genetic testing.
By Mojca Jez, PhD, Researcher at Blood Transfusion Centre SI
