We all know how babies are made (sorry to any parents out there now answering that question), but what happens when you get two sperm and one egg? In February the media reported on the second ever recorded case of semi-identical twins, so we’re going to explore what exactly happened.
The two instances of twins you will be familiar with, are identical and non-identical twins, but let’s have a biology recap. Non-identical twins occur when two eggs are released, two sperm fertilise them, and both fertilised eggs successfully embed. These are called “Fraternal” twins and will produce two completely different children which can be the same or different genders, dependent on the chromosome they receive from their father (i.e. X = female, Y = male). Identical twins however, come from one egg fertilised by one sperm and the fertilised egg then splits and embeds twice. Identical twins share exactly the same DNA and are thus the same sex.
What about the curious, and very rare, case of semi-identical twins? This is only the second ever recorded case, so not common at all, but interesting nonetheless! It is thought that semi-identical twins occur when one egg is simultaneously fertilised by two sperm cells. This results in the fertilised egg having three sets of chromosomes, and hence such embryos do not usually survive. However, if the egg splits successfully and embeds this can result in semi-identical twins because they share exactly the same DNA from their mother but different paternal DNA (as each sperm carries different genetic variations). In the most recent reported case, the Australian-born twins were non-identical, so the girl inherited an X chromosome from one sperm whilst the boy inherited a Y chromosome from the other. How did they find out that the twins were semi-identical and not just non-identical twins? The 6 week scan showed one placenta with two amniotic sacs, implying identical twins as they shared a placenta (whilst non-identical twins do not). However, at the 14 week scan (when you can determine the sex of a baby) the results showed one boy and one girl; it was impossible for non-identical twins to share a placenta so these twins were in fact semi-identical (or sesquizygotic, quite a mouthful).
Why does this not normally happen, and what stops us from all being twins? Usually only one egg is released per monthly cycle. However, rates of twin pregnancies (in the US) have risen by 75% in the last 30 years to 33 instances in every 1,000 births. Have you heard that rumour that most of us just eat our twin in the womb? Although this rumour isn’t true, the technical term is ‘Vanishing Twin Syndrome’; a twin will disappear due to miscarriage and is absorbed by the other twin, placenta or mother. This obviously happens early in a pregnancy and is likely to happen in 21-30% of multiple pregnancy cases, being likely to occur in women over 30. What stops us from having lots of twins? Scientists are still unsure as to what it takes to cause the splitting of an egg so early in a pregnancy, that it develops into identical twins. However, nature has devised some mechanisms to prevent cells having too much DNA.
In the case of semi-identical twins, if a cell had 3 sets of DNA it would not go on to create a living cell; it could not function. But what stops more than one sperm fertilising an egg (i.e. polyspermy) usually? Many sperm will come into contact with the egg and try to be the one to fertilise it. However, there are two mechanisms to prevent more than one sperm from fertilising an egg: Fast and slow block to polyspermy. When the first sperm attaches to the egg, in fast block, sodium channels in the egg membrane open, allowing Na+ ions to enter and depolarise the membrane, stopping any further sperm from fusing. Slow block involves the release of Ca++ ions when a sperm has bound to the end. This triggers a chain reaction resulting in a change of the egg membrane, meaning no more sperm can bind and fuse.
There are factors which can increase the chances of having a multiple pregnancy. For example, genetic factors (i.e. having twins on your maternal side), having had twins before, older mothers, fertility treatments and ethnicity. The number of twin pregnancies may be rising but they are still much less common than single child pregnancies. It is very unlikely that we will see more cases of semi-identical twins springing up as the conditions needed for two sperm fertilisation are unbelievably rare. However, it is a very interesting insight into a biological process.