A working draft of the maize genome, unveiled by the same group of scientists in 2008, indicated the plant had 50,000-plus genes. But when they placed the many thousands of DNA segments onto chromosomes（染色体） in the correct order and closed the remaining gaps, the researchers revised the number of genes to 32,000.

"Sequencing the corn genome was like driving down miles and miles of desolate（荒凉的） highway with only sporadically（偶发地，零星地） placed sign posts," says co-investigator Sandra Clifton, Ph.D., of Washington University. "We had a rudimentary（基本的，初步的） map to guide us, but because of the repetitive（重复的） nature of the genome, some of the landmarks were erroneous（错误的，不正确的）. It took the dedicated efforts of many scientists to identify the correct placement of the genes."

Interestingly, plants often have more than one genome and corn is no exception. The maize genome is composed of two separate genomes melded into one, with four copies of many genes. As corn evolved over many thousands of years, some of the duplicated genes（双链基因） were lost and others were shuffled around. A number of genes took on new functions.

Corn is the third cereal-based crop after rice and sorghum（高粱） – and the largest plant genome to date – to have its genome sequenced, and scientists will now be able to look for genetic similarities and differences between the crops. "For example, rice grows really well in standing water but corn doesn't," explains co-investigator Robert Fulton, of Washington University. "Now, scientists can compare the two genomes to find variations of corn genes that are more tolerant to wet conditions."

The United States is the world's top corn grower, producing 44 percent of the global crop. In 2009, U.S. farmers are expected to produce nearly 13 billion bushels（蒲式尔，英斗） of corn, according to the U.S. Department of Agriculture.

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