PSY BEH 101D Lecture Notes - Lecture 4: Vanishing Twin, Twin, Zygote
15 Jul 2018
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Document Summary
About once in every 250 human conceptions, the zygote not only duplicates but splits apart completely, creating two, or four, or even eight separate zygotes, each genetically identical to that first single cell. Monozygotic twins : twins who originate from one zygote that splits apart very early in development. Because nurture always affects nature, even before birth, identical twins do not have exactly the same phenotype. Dizygotic twins (fraternal twins) are born 3x as often as monozygotic. Began life as two zygotes created by two ova fertilized by two twins sperm can look quite similar like siblings. Phenotypes may differ (about half are male female pairs) or they. Vanishing twin phenomenon may occur in about 12 percent of. Sometimes an early sonogram reveals two developing pregnancies organisms, but later only one embryo continues to grows. Almost every trait is polygenic (affected by many genes) and. Most genes are additive genes multifactorial (influenced by many factors)
