 We use maps every day to figure out how to walk from home to school and then from school to soccer practice. Scientists also use maps to navigate through the enormous amount of DNA in a cell, three billion basefares in humans. The DNA is organized into several long strands called chromosomes. Within each chromosome are genes, regions of the DNA that specify traits like eye color or blood type. But how are these traits ordered in the DNA? Which genes are close together and which are far apart? To answer this question, you need a map. Back in the early 1900s, two scientists named Alfred Sturdovent and Thomas Hunt Morgan made the first map of genes in the fruit fly Drosophila. But before we get into Morgan and Sturdovent's research and its implications, let's quickly review what was known at the time about how traits are inherited. Fifty years earlier, Gregor Mendel, a monk who performed experiments on pea plants, noted that traits like whether the plant's flowers were white or purple in color or whether the peas were round or wrinkled in shape assorted independently. If you breed a white flower round pea plant with a purple flower wrinkled pea plant, you can get any combination of these traits in the offspring plants. Thomas Hunt Morgan studied two traits in Drosophila, eye color, which could be either red or white and the length of the wings, long or short. If he bred a white-eyed long winged fly with a red-eyed short winged fly, the offspring almost always resembled one of the parents. They were either white long or red short. Unlike the experiments from Mendel, these traits were not assorted independently. Instead, these traits were what he called linked to each other. So what was going on? Morgan guessed that the genes coding for these traits must lie on the same chromosome. So when sperm or eggs are formed, the two genes from the same parent always went into the same sperm or egg cell. They stayed linked together. In contrast, if they were on different chromosomes, then the genes and their traits should sort independently, as described by Mendel. Interestingly, however, Morgan also saw that occasionally some of the offspring from the white long to red short breeding had a different appearance from the parents. He would sometimes get white short or red long offspring flies. How could this have occurred if the genes are linked? Morgan's hypothesis, which we now know is correct, was that maternal and paternal chromosome pairs can exchange pieces of DNA with each other. This exchange is called crossing over or recombination. If the crossover point occurs in between the gene for eye color and the gene for wing length, new combinations of traits like white short or red long could be found in the offspring. So what does this have to do with maps? Morgan's student named Alfred Sturdevant reasoned that the frequency of crossover could be used to infer the approximate distance between genes on a chromosome. And if we know distance between genes, we can make a map. How did he do this? Let's look at the chromosome pair below. If the genes for eye color and wing length are very far apart from each other on the chromosome, it is very likely that recombination could occur at a spot in between the two genes. This means it would be likely to get new combinations of offspring that were distinct from their parents. In contrast, if the eye color and wing length genes were very close to each other on the chromosome, it is much less likely that the chromosomes would cross over in between the two genes. So in this case, we would not expect to get many offspring that had different appearance from the parents. Then Sturdevant analyzed six different genes that he knew were all found on the same chromosome. He determined the percentage of crossover events that occurred by looking at specific features in the offspring when he bred different types of flies. The higher the percentage of crossing over, the farther apart the two genes were on the chromosome. And the lower percentage of crossing over, the closer the two genes were. By this method, Sturdevant deciphered the order of the six genes on the chromosome and approximately how far apart each one was from the other. This amazing finding established that chromosomes are linear and that the genes are organized within each chromosome in defined positions. The Sturdevant-Morgan findings have had long-term implications. Scientists have used similar genetic mapping strategies to pinpoint the location of and then clone a gene of interest. On a grand scale, scientists constructed genetic maps of thousands of genes in Drosophila and humans, and then took this concept one step further to sequence the entire 3 billion base pair human genome. The genome sequence is the ultimate high-resolution map. Even now, scientists use genetic mapping to find a connection between a genetic disease that runs in families and gene mutations that might underlie the disease.