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jirtle&weidman07 - Imprinted and More Equal Why...

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Imprinted and More Equal Why silence perfectly good copies of important genes? The answer may lie in a battle between mother and father staged in the genome of their offspring Randy L. Jirtle and Jennifer R. Weidman B eing a diploid life form has its ad- vantages. With two copies of each chromosome, diploid cells have a built- in insurance policy against tlie effects of mutation. If a gene on tine chromo- some has an error, there's another copy available, And for mtJst genes, one good copy is all you need. For this reason, geneticists have always been puzzled by the phenomenon of inipriiiling, in which swaths of DNA on one of a pair of chromosomes are silenced. The genes in these regions are excluded from the insurance policy, it's Uke flying a two- engine airpliine with only one engine. Gregor Mendel, the 19th-century motik who helped define genetics, nev- er encountered imprinted genes during I'A/Al.im\ Figure I. Imprinted genes—genes silenced on one of an animal's two chromosomes—appeared on the evolutionary stage with the advent of live birth, perhaps because of inherent conflicts in the reproductive strategies nf mothers and fathers. Similar patterns of imprinting in placenta! mam- mals and marsupials (such as these opossums) hint that some imprinting mechanisms have changed litHe since their last common ancestor some 180 million years ago. Mammals that do not give birth to live young, such as the egg-laying platypus, do not have imprinted genes. w u'w.a mcricanscientist.org 2007 March-April
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Tina imprint Tom egg sperm dominant alleles A dislikes spicy foods B normal risk of cancer C proficient at crossword puzzles recessive alleles 3 eats jalapeno doughnuts b high risk of cancer C not especiaily good at crossword puzzles female imprinted gene male imprinted gene Tim's genotype Aa Tim's phenotype dislikes spicy foods high risk of cancer proficient at crossword puzzles somatic cell Figure 2. Imprinting makes terms like doniinunt and recessive meaningless in tbe context of m-immalian iriheritiince. For an imprinted gene, the importanl attribute is which parent it come trom. In this cartoon, Tina and Tom are the parents of baby Tim. For a fictional spicy- food gene, the allele (gene variant) that leads Tina to dislike fiery edibles is dominant; the allele that has Tom craving hot peppers is recessive. Thus, baby Tim will share his mother's preference for mild meals. For two other imaginary traits, cancer predisposition and cross- word-puzzle proficiency, maternal and paternal imprints dictate which allele will manifest itself in the child. his studies. It's just as well he didn't, because imprintijig makes a hash of his lovely laws of inheritance. Mendel was the first to explain tlie relation between gciiotifpe —the genes that a^^ organism inherits—and plicnolypc —the traits that an organism shows. "For each char- acter, an organism inherits two genes, one from each parent," he stated. "If the two alleles (genes) differ, then (me, tlie
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