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The Laws of Gregor Mendel can help predict progeny when there is a situation where there is a dominant and a recessive gene, or blending inheritance, incomplete dominance or sex linkage. However, when a trait is due to multifactorial inheritance and is a polygenic trait, prediction becomes less precise due to the additivie effects of multiple genes as well as an environmental component. However, when two individuals are crossed in a dominant/recessive situation, only one dominant gene is needed in the offspring to produce a phenotype that displays the trait. Two recessive genes would be needed for the recessive trait to appear. In blending inheritance, as in Japanese four oclock flowers, if the offspring inherits a red and a white allele, the effects of both combine to produce a pink flower. In sex-linked inheritance, the X chromosome carries the allele. Therefore, a female offspring can have three possiblilities: XX, XX-, and X-X-. The X- symbol represents a sex linked gene on the X chromosome for something such as colorblindness or hemophilia. A female who is XX is normal, XX- a carrier, and X-X- an affected female. In males, there are only two possibilities XY and X-Y. Since males are hemizygous for their X chromosome, whatever they inherit from their mother will appear in the offspring. XY is a normal male and X-Y is an affected male. These are some predictions that can be made using Mendel's Laws.
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