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Complications to Mendel: Gene Interactions Lecture starts on next page From Genotype to Phenotype a. vaculolar pH = 6.6 b. vacuolar pH = 7.7 This is a single gene trait. The coloration of the flowers depends on the: • production of the appropriate anthocyanin pigments • presence of metal ions and co-pigments • the vacuolar pH in epidermal cells These wild-type (b) and mutant (a) morning glories produce exactly the same anthocyanin pigment in their petals. The phenotypic difference in the two plants is due to a single gene difference: the flower on the left has a recessive, loss-of-function mutation in a gene that codes for a Na+/H+ exchanger. This mutation results in a decrease in the vacuolar pH of epidermal cells causing the petals to appear purple rather than blue. Using genetical conventions, we would name the gene defined by this phenotypic variation the purple gene and assign the following allele symbols: p+ = blue p = purple 1 The art and genetics of color in plants and animals 2 Are these examples of color variation single gene traits? In other words, can a single gene (perhaps with multiple alleles and complications to dominance) explain color variation in budgie parakeets or in bell peppers? 3 The table below shows the results of crossing true-breeding lines of green, blue, yellow and white budgies. parents Green Blue Yellow White Green green green green green Blue green blue blue green Yellow green green yellow yellow White blue white green yellow Number of genes? Number of alleles? Dominance? Possibilities: • One gene: four alleles; dominance complete • One gene: three alleles; some incomplete dom • Two genes: two alleles; dominance complete • Three or more genes 4 P true-breeding yellow X true-breeding blue â F1 green X F1 green â F2 9/16 green 3/16 blue 3/16 yellow 1/16 white How many genes involved? Speculate about genotypes using elementary principles of combining colors 5 Additive Gene Effects • independent, additive contribution to phenotype: effects of the alleles of the two loci are essentially the sum of the independent gene actions • unmodified Mendelian ratio: AaBb X AaBb à 9:3:3:1 (two genes; each gene has two alleles, complete dominance) • essentially there is no gene interaction – the genotype at one gene locus doesn’t affect the expression/function of the alleles at a second gene locus Yellow and blue pigments are synthesized in independent biochemical pathways. A (mutational) disruption in one pathway does not affect the other pathway 6 See also this beautiful example of additive gene effects in corn snakes (Chapter 6 of text) Two genes independently controlling the synthesis of two different pigments; each gene has two alleles showing complete dominance 7 The complicated relationship between genes and phenotypes Coat Variation in the Domestic Dog Is Governed by Variants in Three Genes 2 OCTOBER 2009 VOL 326 SCIENCE Coat color and type are essential characteristics of domestic dog breeds. Although the genetic basis of coat color has been well characterized, relatively little is known about the genes influencing coat growth pattern, length, and curl. We performed genome-wide association studies of more than 1000 dogs from 80 domestic breeds to identify genes associated with canine fur phenotypes. Taking advantage of both inter- and intrabreed variability, we identified distinct mutations in three genes, RSPO2, FGF5, and KRT71 (encoding R-spondin–2, fibroblast growth factor–5, and keratin-71, respectively), that together account for most coat phenotypes in purebred dogs in the United States. Thus, an array of varied and seemingly complex phenotypes can be reduced to the combinatorial effects of only a few genes. See Figure on the next page: 8 - represents ancestral allele (found in wolves) + = variant allele 3 genes, 2 alleles (complete dominance) 7 out of 8 combos are shown here Which is missing? furnishings= moustache and large eyebrows 9 Additive gene effects: RECAP • independent, additive contribution to phenotype: effects of the alleles of the two loci are essentially the sum of the independent gene actions • unmodified Mendelian ratio: AaBb X AaBb à 9:3:3:1 • no interaction of the alleles – the genotype at one gene locus doesn’t affect the expression/function of the alleles at a second gene locus Gene Interactions: Specific alleles of one gene mask or modify (enhance, suppress or in some way alter) the expression of alleles of a second gene • complementary gene action • epistatic gene interaction • modifiers & suppressors 10 Gene Interactions: Specific alleles of one gene mask or modify (enhance, suppress or in some way alter) the expression of alleles of a second gene • complementary gene action • epistatic gene interaction • modifiers & suppressors 11 Complementary Gene Action Both John and Karen are deaf due to genetics. NATURE|VOL 431 | 21 OCTOBER 2004 |www.nature.com/nature See article: http://fire.biol.wwu.edu/trent/trent/Naturedeafdesign.pdf 12 Why then could they have only normal children? A mutant gibberellin-synthesis gene in rice New insight into the rice variant that helped to avert famine over thirty years ago. Nature 416: 701 April 18, 2002 The chronic food shortage that was feared after the rapid expansion of the world population in the 1960s was averted largely by the development of a high-yielding semi-dwarf variety of rice known as IR8, the so-called rice ‘green revolution’. The short stature of IR8 is due to a mutation in the plant’s sd1 gene, which encodes an oxidase enzyme involved in the biosynthesis of gibberellin, a plant growth hormone. Gibberellin is also implicated in green-revolution varieties of wheat, but the reduced height of those crops is conferred by defects in the hormone’s signaling pathway. There are various reasons for the dwarf phenotype in plants, but gibberellin (GA) is one of the most important determinants of plant height. To investigate whether the sd1 gene in semi-dwarf rice (Fig. 1a) could be associated with malfunction of gibberellin, we tested the response of this mutant to the hormone. We found that sd1 seedlings are able to respond to exogenous gibberellin, which increases their height to that of wild-type plants In the early 1970s, a Dr. Rutger, then in Davis, Calif., fired gamma rays at rice. He and his colleagues found a semi-dwarf mutant that gave much higher yields, partly because it produced more grain. Its short size also meant it fell over less often, reducing spoilage. Known as Calrose 76, it was released publicly in 1976. Today, Dr. Rutger said, about half the rice grown in California derives from this dwarf. Right: wild-type Left: semi-dwarf (sd1) mutant (more resistant than wild-type to damage by wind and rain and respond better to certain fertilizers) 13 Arabidopsis dwarf mutant These two plants have the exactly the same genotype: they are homozygous for the same recessive, loss-of-function mutation The plant on the left has been treated with an application of gibberellin. The plant on the right is untreated. You have three different (independently isolated) dwarf strains of Arabidopsis: Mutant Strains 1, 2 & 3 • Each dwarf strain has a recessive, loss-of-function mutation • Treatment of each mutant strain with gibberellic acid restores normal height suggesting that they have the same primary defect • You cross dwarf strain 1 X dwarf strain 3 and the untreated F1 are dwarf • But when you cross dwarf strain 1 X dwarf strain 2 the untreated F1 are wild-type in height (non-dwarf) • How to explain: dwarf X dwarf = wild-type? 14 Dwarf plants can result from a mutation affecting gibberellin production as well as mutations affecting any step in the cellular response to this signal gibberellin biosynthetic pathway 15 These mutations show complementation: Complementation: the production of wild-type F1 progeny when crossing two parents showing the same recessive mutant phenotype M1 X M2 wild-type F1 progeny (alleles symbols: uppercase = functional allele; lowercase = loss-of-function) mutant 1: aaBBCC X mutant 2: AAbbCC ê F1 AaBbCC wild-type ê self What phenotypes will appear in the F2? Will the progeny ratios be in 1/4’s or 1/16’s or 1/64’s 16 Genes are assorting independently 3/4 A- 1/4 aa 3/4 B- 1/4 bb all CC (not segregating) 9/16 3/16 3/16 1/16 A-B- CC aaB- CC A- bb CC aabbCC Since a homozygous recessive mutation in either A or B results in the mutant phenotype 9/16 wild-type 7/16 dwarf = modified Mendelian ratio 17 18 Dwarfism in plants and deafness in humans are examples of genetic heterogeneity Genetic (or locus) heterogeneity: Mutations in any one of several genes may result in identical phenotypes (such as when the genes are required for a common biochemical pathway or cellular structure) Heterogeneous trait or genetic heterogeneity: a mutation at any one of a number of genes can give rise to the same phenotype Although a single gene difference causes the phenotypic difference between the dwarf and the wild-type plants, this does not mean that normal height is the result of the action of a single gene. It means simply that only one gene differed* between the dwarf and wild-type plants under consideration * carried alleles with functional differences 19 Optional interesting growth-hormone-related genetics FDA approved genetically modified salmon: addition of Chinook growth hormone to smaller species http://opinionator.blogs.nytimes.com/2011/03/17/frankenfish-phobia/?nl=opinion&emc=tya1 20 See also optional reading on loss-of-function mutations in growth hormone receptor in humans: Equadorean villagers may hold secret to longevity http://fire.biol.wwu.edu/trent/trent/larondwarfism.pdf 21