It has long been understood that inheritance is a factor in non-identical (DZ) twinning. However, until recently researchers have been unsure exactly why.
The main factors involved in DZ twinning are believed to be ‘super-ovulation’ (the tendency to release multiple eggs during ovulation), maternal age and the role played by assisted reproductive technologies.
In a recent study, scientists from eight countries compiled data from nearly 2000 mothers of non-identical twins and compared it to that of nearly 13,000 other study participants.
They identified two genes that increase the chances of mothers producing DZ twins. One of these genes is related to egg stimulating hormones – along with an earlier age of menstruation, earlier age of first child and an increased number of children. The second gene is believed to play a role in improving the environment for egg maturation – this finding, in particular, is an exciting development as it may help identify why some women respond better to in vitro fertilisation than others.
In fact, this research has important implications for fertility treatments, including improved outcome prediction and possible new avenues of fertility treatment.
The study revealed that having these genes increase the chance of producing DZ twins by 18-19 percent. A mother who has both genes will increase her odds by 29 percent.
But what about identical twins? While the DZ twinning rate varies quite significantly between races with Africans most likely to produce DZ twins at 16 per 1000 births and Asians less likely at 4 per 1000, the identical (MZ) twinning rate is consistent amongst all races at 4 per 1000.
In the past, researchers have believed that MZ twinning is a random occurrence with no genetic basis. However the research organisation, Twins Research Australia, has many families with a number of identical twins throughout generations – and this is not unique to its experience. It has been suggested that familial twinning may indeed occur although it is quite rare.
One of the co-investigators involved in the recent genetics of DZ twinning study, Professor Nick Martin, identified other genes that are likely to be connected with identical twinning but the findings could not be confirmed. Geneticists believe that familial MZ twinning could result from a change in a gene that controls cell-to-cell connections, leading to an increased likelihood of the embryo splitting prior to implantation in the womb.
More research into the factors associated with both DZ and MZ twinning – including the differences between twinning rates amongst races - will help us understand more about the types of mechanisms involved.
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