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develop from a single zygote (fertilized egg) that splits in two, creating two genetic replicas
develop from separate zygotes
genetically no closer than brothers and sisters, but they share the fetal environment
Identical twins Fraternal twins Same sex only Same or opposite sex
Identical (monozygotic twins) Non-identical, fraternal (dizygotic twins) Identical twins reared together same genes (relatedness of 1.0) same environment Identical twins reared apart sane genes (relatedness of 1.0) different environment Fraternal twins reared together half genes the same (relatedness 0.5) same environment Fraternal twins reared apart half genes the same (relatedness 0.5) different environment Nature v Nurture: Twin Studies
Jack Yufe: raised by Jewish father in Caribbean Oskar Stohr: raised by catholic grandmother in Nazi Germany Nature v Nurture: Twin Studies If we had identical twins reared apart could we separate the effects of phenotype, of genotype, and of the environment. In some cases this has happened.
Nature v Nurture: Twin Studies Both like sweet liqueurs Store rubber bands on their wrists Read magazines from back to front Dip buttered toast in their coffee Have very similar personalities