Paternity test identical twins

'Identical' twins with discordant karyotypes.

Nieuwint A, Van Zalen-Sprock R, Hummel P, Pals G, Van Vugt J, Van Der Harten H, Heins Y, Madan K.

Department of Clinical Genetics, University Hospital Vrije Universiteit, Amsterdam, The Netherlands. a.nieuwint@azvu.nl

A chromosomal abnormality in one of the fetuses of a monozygotic twin pregnancy is a rare phenomenon. In the prenatal unit of our cytogenetics laboratory we have recently come across two such heterokaryotypic twin pregnancies. In both cases ultrasound abnormalities were detected in one fetus of each twin pair. Chromosomal analysis showed that one twin pregnancy was discordant for trisomy 21 and the other for 45,X. Ultrasonographic examination suggested a monochorionic twin pregnancy in each case and DNA studies confirmed that both sets of twins were monozygotic. Both pregnancies were terminated. Biopsies taken from different sites of the placentas showed chromosomal mosaicism in both cases. There was no clear correlation between the karyotype found close to the site of the umbilical cord insertion in the placenta and the karyotype of the fetus. Sampling of amniotic fluid from both sacs is recommended in diamniotic twin pregnancies if one (or both) of the fetuses has ultrasound abnormalities, even if the twins are apparently monochorionic.

De novo 14484 mitochondrial DNA mutation in monozygotic twins discordant for Leber's hereditary optic neuropathy.

Biousse V, Brown MD, Newman NJ, Allen JC, Rosenfeld J, Meola G, Wallace DC.

Department of Ophthalmology, Emory University School of Medicine, Atlanta, GA, USA.

Monozygotic twin brothers, clinically discordant for Leber's hereditary optic neuropathy (LHON), had a heteroplasmic point mutation at position 14484 in the mitochondrial DNA that was not detected in their mother. Moreover, the mutation occurred on the rare European haplogroup X, rather than the haplogroup J commonly associated with the 14484 mutation. These data indicate that the 14484 mutation in this family was a new mutation, indicating that it was the de novo occurrence of a common, primary LHON mutation.

Discordant sex in one of three monozygotic triplets.

Dallapiccola B, Stomeo C, Ferranti G, Di Lecce A, Purpura M.

A case is reported of monozygotic triplets, discordant for phenotypic sex, in which the female presented at birth with the features of Turner's syndrome. Chromosomal analyses showed homogeneous 46,XY karyotypes in the lymphocytes of the three sibs, while a 45,X non-mosaic chromosome constitution was detected in skin fibroblasts of the female triplet. It is suggested that mitotic non-disjunction or anaphase lag occurring early during embryonic development accounted for the occurrence of monosomy X in one cell line of the affected triplet. Previous observations of monozygotic twin pairs discordant for chromosome constitutions are reviewed.

Originally posted by LURCH
This brings up another interesting question. If a woman mates with two identicle twins and conceives twins herself (a likely event, as multiple births have been linked to heredity), is there any way to determine if the new zygots are from one father or both? It would seem to me there isn't.

Theoretically, it could even be possible for "identical" twins to result from heteropaternal superfecundation.

The twins would be genetically identical, but not monozygotic, nor even from the same father.

Originally posted by LURCH

This brings up another interesting question. If a woman mates with two identicle twins and conceives twins herself (a likely event, as multiple births have been linked to heredity), is there any way to determine if the new zygots are from one father or both?

Originally posted by Moonbear
So it's even less likely that two offspring from two identical twins would both get the same complement of genes from the fathers.

Originally posted by Monique
That won't be possible since only half of the genetic material is passed on to the following generation. You'd have to calculate the chance that two genetically identical egg cells encounter two genetically identical sperm cells.

Originally posted by LURCH
How is it less likely? I thought it would be exactly equal in probability; very unlikely, but possible.