Zygosity
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Zygosity is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism.
Homozygous and heterozygous
North Holland Blue youngstock showing the dosage effect of the "barred" trait: periods of pigmented feather growth are shorter in the homozygous cockerel, giving a lighter overall impression; the periods are longer in the hemizygous females, giving a darker overall impression.
Twin
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Twins are two offspring produced by the same pregnancy. Twins can be either monozygotic, meaning that they develop from one zygote, which splits and forms two embryos, or dizygotic, meaning that each
Zygosity
…forms two embryos. Fraternal twins are dizygotic because they develop from two separate oocytes (egg cells) that are fertilized by two separate sperm. Sesquizygotic twins are halfway between monozygotic and dizygotic and are believed to arise after two sperm fertilize a single oocyte which subsequently splits into…
Identical twins can be difficult to visually distinguish especially when young, as demonstrated by brothers Billy and Bobby Mauch.
Adult fraternal twins
Fraternal twin brothers as young babies
Austrian TV host Mirjam Weichselbraun (right) and her fraternal twin Melanie look very similar but with a significant difference in height.