Is either one of your parents is twins? Or does your partner is a twin? Are you a twin? Does your family or your partner’s family have twin genes? How true is it that hereditary is linked to multiple births?
Well, the answer to these questions is “yes”. Twins do run in families, and if you want to have multiples, hereditary might play a significant role in it. Twin genes in a family increase your chances of having multiple births at a time, but it doesn’t guarantee that you won’t have twins if you don’t have genes. You can.
Multiple births are dependent on genes to an extent. If you or your partner comes from a family with several twins, you might be wondering about the science behind it?
The first thing to consider is ‘what kind of twins we are talking about’ because identical twins (monozygotic) are not related to genetics, but fraternal twins (dizygotic) are indeed related to genes. Genetics plays a vital role for fraternal twins as studies show that women who are fraternal twins or if they have twin genes in their family are 2.5 times more likely to give birth to twins.
However, the mother’s side of genetics plays a more vital role than that of the father’s. Fraternal twins are a result of two eggs being fertilized simultaneously, fertilized in just one. Husbands’ genes cannot make a woman release double the number of eggs.
For instance, if twin genes run in your family but are passed down to your son, and your son is the father, so his genes are on the wrong side of the family tree; now, if his wife gets pregnant with twins, it is likely that it was not due to the hereditary.
Now let’s assume that the ‘son’ we are talking about has a daughter, he might not increase the chances of his wife having twins, but he can pass the genes to the daughter or granddaughters. Therefore, a strong history of twin genes increases the likeliness of having twins.
Every woman has a chance of having twins given they are supported by other factors such as age, nutrition, environment, weight, etc., as these are some of the factors which affect the chance of having twins.
The twin gene
According to a study, the increased chance of having twins is due to hyperovulation, sometimes also called the “twin gene.”
Hyperovulation tends to release more eggs during ovulation which increases the chances of conceiving dizygotic or fraternal twins, so, in families where women have increased tendencies of hyperovulation, hereditary could be the reason that explains the increased presence of fraternal twins. However, since only women ovulate, this explains why mothers’ side of the gene plays such a vital role.
Why are identical twins not related to hereditary?
Eggs are the explanation why identical twins are not related to hereditary. The most crucial difference between identical and fraternal twins are that identical twins come from a single fertilized egg, whereas fraternal twins come from two different fertilized eggs. Identical twins, also known as monozygotic, result from splitting the single embryo into parts soon after fertilization.
This splitting is the reason why identical twins have similar DNA, and it is because they both are a part of one single egg. Since the splitting of embryos is a random event that happens by chance, it has no connection to genes whatsoever, but that is not the case with fraternals. Unlike the splitting of embryos, ovulation is more of a biological process that is controlled by genes.
Some women have different versions of genes known as alleles. These make them more likely to hyper ovulate, which means a higher chance of having multiples who may be fraternal twins, triplets, quadruplets, etc. such genes can be passed on from the parent to their children that is why it is suggested that the fraternal twins run in the family.
Studies were conducted to understand the difference between single birth mothers and mothers of multiples. Data of the number of mothers of fraternal twins were taken, and this data was compared to other women who didn’t have twins or had identical twins. Researchers searched for individual DNA bases known as nucleotide polymorphisms (SNPs). This SNPS varies from person to person.
Researchers found that the SNPs were more often found in mothers with fraternal twins than compared to others. Later they narrowed it down to two potential SNPs and reran an analysis; they found out that having one copy of each SNP improved the chances of having fraternal twins by 29%. The first SNP is near a gene called (FSHB) which involves follicle-stimulating hormone (FSH). the levels of this hormone keep changing as the eggs mature in the ovaries.
If these hormone levels are high for an extended period, then the ovaries might release multiple eggs, which could result in the birth of fraternal twins. The second SNP gene is called SMAD3. This gene might change how ovaries respond to FSH. Researchers plan to study whether having SMAD3 variant genes increases the chances of getting pregnant with twins.
Why are twins not constant in a family tree?
There is a myth saying that twins skip generations, which explains why twins are not constant in a family tree and making it look like they missed a generation. Let’s take an example:
Case 1- A lady, let’s assume her name Jane has two fraternal twins who are both boys. Now Jane may have passed the gene to the boys, but since the boys do not ovulate, they end up with single births as their having the gene does not increase their partners’ possibility.
But later, these both sons have daughters who may get the gene from their respective carry and carry on the line of twins. So, you see, the twins skipped a generation because of the fact above.
Case 2 – Jane has two fraternal daughters who have genes passed down from their mother will end up with multiples. In this case, twins didn’t skip as the girls carry the genes and also because they ovulate so the results are fraternal twins.
So, even though it might seem that twins skip a generation in the family, there is a high probability that they might not, or they might skip “generations” given that the conditions are not ideal.
Even though hereditary does indeed play its role, it is not necessary to have twins only if you have it. There are undoubtedly other factors that facilitate the birth of multiples.