• Question: can a boy and a girl be twins

    Asked by saima to Chris, Josh, Rebecca, Rob, Susan on 23 Jun 2015.
    • Photo: Susan Cartwright

      Susan Cartwright answered on 23 Jun 2015:

      Yes, of course, but almost always only non-identical (fraternal) twins.

      There are two ways to make twins. Either the mother produces two eggs, both of which are fertilised, or one fertilised egg splits into two embryos early in its development. In the former case (fraternal or dizygotic twins), two different sperm are involved, and they each have equal chances of carrying an X or a Y chromosome. So 1/4 of such twins are both boys, 1/4 are both girls, and 1/2 are one of each. However, in the latter case (identical or monozygotic twins), only one sperm was involved, so either both twins are XX (girls) or both are XY (boys).

      I remember reading about one pair of monozygotic twins who were of different genders. In this case, one of the embryos had managed to lose its Y chromosome, so it just had one X chromosome. As the gene that makes your body develop as a male is on the Y chromosome, children with this condition (Turner’s syndrome) develop as females, although their reproductive systems do not develop properly and they are not fertile. So in this pair of twins, one was a normal boy, the other was a girl with Turner’s syndrome, but all their other genes were identical. This condition is extremely rare: only a handful of cases have been reported.

    • Photo: Rebecca Dewey

      Rebecca Dewey answered on 24 Jun 2015:

      Absolutely – in fact, I went to school with triplets who were two boys and a girl!
      There are two main ways that multiple births (twins, triplets, etc.) happen:

      1) two eggs are fertilised at once by two different sperm. It is the chromosomes in the sperm that decide the gender of the child, so if they are different sperm, they could be the same gender as each other, or different! They will have separate placentas and separate amniotic sacs.

      2) one egg is fertilised by one sperm, and then the egg splits. This will always lead to identical twins of the same gender. This split can happen (a) in the first 3 days since conception, before the egg attaches to the wall of the uterus, therefore the twins will be identical, but will also have separate placentas and separate amniotic sacs. This happens in about 18 to 30% of cases of twins. (b) the split can happen in days 4-8 after fertilisation. The twins will be identical and share the same placenta, but will have separate amniotic sacs (about 60-70% of cases of twins). (c) if the split happens even later, in days 8-13 after conception, then the twins will share the same placenta and the same amniotic sac – this is when you sometimes see twins hugging or sucking each others thumbs during gestation (very cute!) but only happens in a very small number of cases (1–2%) and also can very very very rarely lead to siamese / conjoined twins.

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