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Unformatted text preview: Lethal alleles Many so called dominant mutations are in fact semidominant , the phenotype of the homozygote is more extreme than the phenotype of the heterozygote. For instance the gene T (Danforth's short tail) in mice. The normal allele of this gene is expressed in the embryo. T/+ mice develop a short tail but T/T homozygotes die as early embryos. Laboratory stocks are maintained by crossing heterozygotes, T/+ x T/+ | | V T/T T/+ +/+ 1 : 2 : 1 ratio at conception 0 : 2 : 1 ratio at birth Incomplete or semi- dominance Incomplete dominance may lead to a distortion of the apparent ratios or to the creation of unexpected classes of offspring. A human example is Familial Hypercholesterolemia (FH). Here there are three phenotypes: +/+ = normal, +/- = death as young adult, -/- = death in childhood. The gene responsible codes for the liver receptor for cholesterol. The number of receptors is directly related to the number of active genes. If the number of receptors is lowered the level of directly related to the number of active genes....
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