"Jonas and Wyatt Maines were born identical twins, but from the start each had a distinct personality.
Jonas was all boy. He loved Spiderman, action figures, pirates, and swords.
Wyatt favored pink tutus and beads. At 4, he insisted on a Barbie birthday cake and had a thing for mermaids."
Twins are not genetically identical, random mutations can be present in one twin and not anther, but even among identical parts of the genome, gene expression itself varies thanks to things like epigenetics.
The converse of this observation is that in the US puberty is starting earlier and earlier... Makes me wonder what hormone or hormone analogs are creeping into our daily intake to inspire such a thing.
Epigenetic mechanisms can easily be integrated into a model of phenotypic variation in multicellular organisms, which can explain some of the phenotypic differences among genetically identical organisms. MZ twin discordance for complex, chronic, non-Mendelian disorders such as schizophrenia, multiple sclerosis or asthma could arise as a result of a chain of unfavorable epigenetic events in the affected twin. During embryogenesis, childhood and adolescence there is ample opportunity for multidirectional effects of tissue differentiation, stochastic factors, hormones and probably some external environmental factors (nutrition, medications, addictions, etc.) (39,43) to accumulate in only one of the two identical twins (Fig. 2) (30,33).
This review weighs the evidence for and against the hypothesis that ovulation is regulated by a critical amount of body fat. The evidence supporting this hypothesis is correlative, and most of it stems from observations made in humans. On balance, the evidence from human studies does not support the hypothesis, however, and the results of animal studies argue strongly against it. In the latter regard, a variety of experimental approaches have been tried in both adult and peripubertal females of several species, and the results almost uniformly show little relationship between fatness and ovulation. There is no doubt that ovulation can be regulated somehow in relation to whole-body energy balance and that fat stores are an important component of energy balance, but there is no reason to accord body fat a direct causal role in regulating ovulation.
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