Causes different genes to be activated in each

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causes different genes to be activated in each, results in very different set of phenotypic traits o Royal jelly silences key gene called Dnmt2, whose product normally adds methyl groups to DNA bee DNA is less methylated, many genes normally silenced in workers are expressed leading to development of queen characterstics o Injected small RNA molecules (small interfering RNA or siRNA) into bee larvae that inhibited expression of Dnmt3 lower levels of DNA methylation, many developed into queens: thus royal jelley brings about epigenetic changes (less DNA methylations) transmitted through cell division and modify developmental pathyway queen bee Anticipation is stronger or earlier expression of traits in succeeding generations Another genetic phenomenon not explained by Mendel’s principles is anticipation – genetic traits become more strongly expressed/expressed at earlier age as it is passed from generation to generation o Myotonic dystrophy – autosomal dominant muscle disorder, ancestors only mildly affected by the disease concept out of favor with geneticists because no mechanism, traditional genetics held that genes are passed unaltered from parents to offspring, attributed anticipation to observational bias Anticipation is now a legitimate genetic phenomenon: mutation causing myotonic dystrophy consists of unstable DNA region that can inc in size as gene is passed from generation age of onset and disease severity correlated with size, increase in size of region through generations produces anticipation – implicated in number of genetic diseases Expression of genotype may be affected by environmental effects Phenotype is the result of a genotype developing within specific environment: genotype sets potential for development, but how phenotype actually develops within limits imposed by genotype depends on environmental effects genotype may produce several different phenotypes depending on the environmental conditions in which development takes place Environmental effects on phenotype Phenotypic expression of some genotypes critically depends on the presence of a specific environment o Ex: Himalayan allele in rabbits produces dark fur at etremeties if rabbit is reared at temperature of 25 C or lower, expression is temperature dependent: enzyme necessary for production of dark pigment is inactivated at higher temperatures. Pigment restricted to nose, feet and ears because animal’s core body temperature is normally above 25C, enzyme is functional only in the cells of the relatively cool extremities o This is example of temperature-sensitive allele – allele whose product is functional only at certain temperatures (like vestigial wings) Environmental factors also play important role in expression of number of human genetic diseases o PKU – autosomal recessive allele that causes mental retardation. Disorder arises from defect in enzyme that normally metabolizes the amino acid phenylalanine. When this enzyme is defective, phenylalanine is not metabolized buildup causes brain damage in children. Simple environmental change (affecting child put on low-phenylalanine

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