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Cell devopment in oppisite sex twins

Testosterone is an important hormone in the sexual differentiation of the brain, contributing to differences in cognitive abilities between males and females. For instance, studies in clinical populations such as females with congenital adrenal hyperplasia CAH who are exposed to high levels of androgens in utero support arguments for prenatal testosterone effects on characteristics such as visuospatial cognition and behaviour. The comparison of opposite-sex OS and same-sex SS twin pairs can be used to help establish the role of prenatal testosterone. However, although some twin studies confirm a masculinizing effect of a male co-twin regarding for instance perception and cognition it remains unclear whether intra-uterine hormone transfer exists in humans.
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A New Kind of Twin

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Twin - Wikipedia

The anomalies statistically associated with twin births are due to the re-arranged embryos of the monozygotics. Common knowledge further requires that dizygotic pairs are dichorionic; monochorionicity is exclusive to monozygotic pairs. These are fundamental certainties in the literature of twin biology. Multiple observations contradict those common knowledge understandings. The double ovulation hypothesis of dizygotic twinning is untenable. Girl—boy twins differ subtly from all other humans of either sex, absolutely not representative of all dizygotics.
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Washington State Twin Registry

A pair of twins born in January in Australia share all of their mother's genes, but only 78 percent of their father's, according to a new case report published yesterday Feb. It's unclear how many other semi-identical, or "sesquizygotic," twins are out there, but it's likely "extraordinarily rare," said lead author Dr. Michael Gabbett, the diagnostic genomics course coordinator at the Institute of Health and Biomedical Innovation in Brisbane, Australia. The first set of semi-identical twins was identified in in the U. Initially, the mother of the twins described in the case report thought that she was pregnant with identical twins, based on an ultrasound early in her pregnancy.
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But, as explained here , identical twins do not always look exactly the same. Using that definition, the answer is yes! Humans have 23 pairs of chromosome. One of these pairs of chromosomes, called the sex chromosomes X and Y , determine the sex of the person. Very rarely, a glitch in copying sex chromosomes can occur, either during the formation of sperm or egg cells, or during the divisions of the zygote after it is formed.
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