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Recessive inheritance is when both matching genes must be abnormal to cause disease. Fish and amphibians, for example, have genetic sex determination but their sex can also be influenced by externally available steroids and incubation temperature of eggs. Sex determination and differentiation. The first 22 pairs are called autosomes. It can also occur when two fertilized embryo fuse, producing a chimera that might contain two different sets of DNA one XX and the other XY. Stevens and the Discovery of Sex Determination by Chromosomes". Sexual differentiation humans Development of the reproductive system gonads Mesonephric duct Paramesonephric duct.

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