In science education, we like to categorize - things are either "this way", or "that way". Simplifying the variety and pointing out the differences is a proven way to aid understanding, and "boiling things down" to their general features is usually also scientifically accurate - except when it isn't.
In our labs this term, we have talked about meiosis and chromosomes, how chromosomes are paired to give us our genotype, how genotype determines what gametes can be passed on, and how our alleles direct our phenotype. Much of those discussions, as is typical, are of a 'this-of-that' format - we either have this genotype or that one, this phenotype or that one. But exceptions to these general rules are interesting, and often enlightening.
Several weeks ago I sent you a news story about a bird that was spotted in Erie, PA that appeared to be a gynandromorph - a genetic mosaic of both male and female tissues. Gynandromorphs are rare, and form when something other than the normal pattern of gamete combinations, chromosome sorting, and cell division occurs during early embryonic formation.
This week, there is news of another, even rarer genetic variant, this time in humans: a semi-identical pair of twins. You are already familiar with identical twins, which form when a fertilized zygote splits into two genetically-equal cells that develop into separate individuals. Identical twins share 100% of their DNA.
You also know about fraternal twins, formed when two (separate) fertilized eggs, rather than one, develop at the same time. Fraternal twins share 50% of their DNA (on average), just like any two siblings born at different times from the same parents.
In this, case we have something different. Semi-identical (sesquizygotic) twins form when a single female egg is fertilized by not one, but two, male sperm. This early embryo divides and begins to develop into not one fetus, but two fetuses. These fetuses develop at the same time and are born as twins. In this case, though, they are predicted to share 75% of their DNA, because they share 100% of their mother's DNA and only 50% (on average) of their fathers DNA, which together average 75%. In the case reported here, the twins share a slightly higher value (89%).
It is likely that this unusual pattern of fertilization actually happens with some frequency, but that the early embryos do not typically survive. This chance occurrence will give geneticists a new data point in their ability to test hypotheses of genes and relatedness - no longer are all twins in their databases either 50% or 100% related, now there is a known 3rd possibility.
Have a great weekend -