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NCERT has one sentence on him: Morgan's student Alfred Sturtevant used the frequency of recombination between gene pairs on the same chromosome as a measure of the distance between genes and "mapped" their position. Behind that sentence is a 1913 paper you can still check with a calculator. This post covers who Sturtevant was, how he made the map, his actual numbers, the mistakes students make, and what to learn from his way of working.
Who was Alfred Sturtevant?
His National Academy of Sciences memoir by Edward B. Lewis gives his dates as 21 November 1891 to 5 April 1970. He grew up on farms in Illinois and Alabama and went to Columbia University in 1908. As a boy he drew up pedigrees of his father's horses. After reading a book on Mendelism he worked out how coat colour in horses was inherited and sent it to Thomas Hunt Morgan, who encouraged him to publish and, in autumn 1910, gave him a desk in the "fly room".
That room was only sixteen by twenty-three feet, at times with eight people in it, including Calvin Bridges and Hermann Muller. Sturtevant took his doctorate in 1914, stayed at Columbia until 1928, then moved with Morgan to Caltech in Pasadena, where he remained. He received the National Medal of Science in 1968.
How did one night produce a map?
Morgan had suggested that genes close together on a chromosome are less likely to be separated by crossing over. The memoir says the germ of Sturtevant's idea came in conversation with Morgan, and quotes his own account from his History of Genetics: he "went home, and spent most of the night (to the neglect of my undergraduate homework) in producing the first chromosome map".
The logic, as the memoir puts it: if A to B is x units and B to C is y units, then A to C is x + y if B is in the middle, or the difference if it is not. If crossover percentages really behave like distances, they must add up.
What did the 1913 paper show?
The paper, The linear arrangement of six sex-linked factors in Drosophila, appeared in the Journal of Experimental Zoology. A free transcription is kept by the Electronic Scholarly Publishing project. He took "percent of crossovers" as "an index of distance", so 1 per cent crossing over is one unit, now called a centimorgan.
He used short neighbouring distances to build the map, measured from the yellow-body gene (his factor B) at 0.0:
white/eosin eye (C, O): 1.0
vermilion eye (P): 30.7
miniature wing (R): 33.7
rudimentary wing (M): 57.6
Worked example: rebuilding Sturtevant's map
One distance from raw counts. For vermilion and rudimentary he found 109 crossovers in 405 gametes: 109 ÷ 405 = 26.9 per cent, so P to M is 26.9 units.
Add the neighbours. B to (C, O) = 1.0. (C, O) to P = 29.7. So P is at 1.0 + 29.7 = 30.7. P to R = 3.0 (17 crossovers in 573 gametes), so R is at 30.7 + 3.0 = 33.7. M is at 30.7 + 26.9 = 57.6.
Test a pair not used. For B and P directly he counted 1,464 crossovers in 4,551: 32.2 per cent, close to the predicted 30.7.
A long distance. For B and M the map predicts 57.6, but he observed only 37.6 per cent. He explained the shortfall by "double crossing over": two breaks between distant genes put the end genes back together, so those flies look like non-crossovers.
What mistakes do students make with map distances?
Expecting more than 50 per cent. Observed recombination between two genes never goes above 50 per cent, however far apart they are on the map. Sturtevant's 57.6 is a sum of short distances, not one measurement.
Trusting long distances. Use the closest pairs, as he did, and add.
Forgetting the middle gene. If A–B = 5 and B–C = 3, A–C is 8 or 2 depending on order. Check with the third pair.
Counting the wrong flies. He used F2 males, whose single X shows the mother's crossovers directly.
Our post on linkage and crossing over has three-gene problems, and Thomas Hunt Morgan tells the fly-room story. If ordering genes is where you get stuck, the Fermi Problem Library has worked problems with a step-by-step solver that explains the reasoning.
What can you take from how he worked?
Turn a hint into a test. He saw that distances must add, and checked whether they did.
Explain the misfits. The long distances that did not fit led him to double crossing over.
Puzzles are practice. The memoir says he loved solving puzzles and treated research in much the same way.
Next step: using the map above, predict the crossover percentage between white and miniature, then compare it with the 37.2 per cent that NCERT quotes.
