Genetic evidence for monozygotic (identical) twinning in mammalian species is rare in the literature, Here we report what may be the first pair of
monozygotic twins identified in a wild caniform carnivore, the grey wolf (Cams lupus).
a) The concordance rate for systemic sclerosis is nearly 100% among
monozygotic twins.
In studies of male twins,
monozygotic twins (single egg) are more likely than are dizygotic twins (two eggs fertilized by two different sperm) to be concordant for homosexuality.
Since epigenetic mechanisms in gene expression do not involve changes in DNA sequence, and involve environmental inputs, epigenetics may explain many clinical aspects of the idiopathic mental disorders (15) like a high degree of discordance of
monozygotic twins for these disorders; the relatively late age of their onset; differences between the sexes in the rate and course of illness; fluctuations in the course of illness; and parent-of-origin effects, i.e., the phenomenon that in some idiopathic mental disorders risk to offspring depends on the sex of the affected parent.
For
monozygotic twins, the risk is similar to the age-related risk in a singleton gestation.
Most doctors believe that conjoined twins start off like other
monozygotic twins, but the zygote doesn't split completely, so the embryos remain joined as they grow.
Higher prevalence rates are seen in first-degree relatives (5%-15%),
monozygotic twins (70%-75%), and patients with other autoimmune disorders.
I consider the argument, thought to clinch the moral case for use of a human embryo solely as a means, that (i) only a human individual can be a person, (ii) because it can happen at any time before formation of the primitive streak that an embryo splits into
monozygotic twins, no embryo in which the primitive streak has not formed is a human individual, and therefore (iii) no embryo in which the primitive streak has not formed is a person.
"Although ovarian transplantation between
monozygotic twins will be rare, the demonstration that ovarian function can be restored and that natural conception and successful pregnancy can be achieved after transplantation of ovarian tissue may have broader implications for preserving fertility in young women, such as those who require potentially sterilizing treatment for cancer," the authors said in the report.
If major psychiatric disorders have a strong genetic component,
monozygotic twins will be highly concordant for the disorder; dizygotic twins will show some level of concordance but not to the same extent as the
monozygotic twins.
Take the case of
monozygotic twins who share exactly the same genes and were formed in the same womb at the same time.
Depending on the time of developmental division,
monozygotic twins may have a monochorionic placenta or a dichorionic placenta.