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 Gregory A Petsko Genome Biology , 2003, DOI: 10.1186/gb-2003-4-6-112 Abstract: I think the genuine sense of loss that accompanied his untimely death - his obituaries literally spanned the country, from the San Francisco Chronicle and Los Angeles Times to the New York Times and Boston Globe - was a reflection on more than the quality of his work, outstanding though that was. He was the geneticists' geneticist, a man whose work on the simple single-celled fungus Saccharomyces cerevisiae, budding yeast (also known as brewer's or baker's yeast), had important implications for understanding the behavior of all eukaryotic cells. He himself was living testimony to the power of genes: his father, Irwin Herskowitz, was a famous fly geneticist at Indiana University. He was also living testimony to the limitations of that power: he had an identical twin, Joel Herskowitz, who went not into genetics but into pediatric neurology and is free of the disease that killed his brother.Ira started off fast: his graduate work on 'phage lambda, a virus that infects bacteria, was classic. But like most of the 'phage people who shaped the early days of molecular biology, he moved on to the study of a free-living organism. As a young faculty member at the University of Oregon he switched to yeast genetics because he felt, as did several others at that time, that the cell biology of 'higher' eukaryotes such as Homo sapiens could best be illuminated through an understanding of this model organism. Yet he never felt that was its only value: he genuinely loved the organism he studied, and he celebrated its beauty and surprising complexity in every conversation he had, every seminar he gave, and every paper he wrote.His best-known work concerned yeast mating. Yeast cells come in two sexes, denoted a and α. Interestingly, like some types of frog, yeast cells are able to change sex under certain conditions: type a can switch to α or vice versa. Ira showed how the yeast cells could flip from one mating type to another by reshuffling their DNA, a process involving the endonucle
 Mathematics , 2008, DOI: 10.1073/pnas.0901678106 Abstract: We present a computer-aided, yet fully rigorous, proof of Ira Gessel's tantalizingly simply-stated conjecture that the number of ways of walking $2n$ steps in the region $x+y \geq 0, y \geq 0$ of the square-lattice with unit steps in the east, west, north, and south directions, that start and end at the origin, equals $16^n\frac{(5/6)_n(1/2)_n}{(5/3)_n(2)_n}$ .