Thomas Hunt Morgan · Gene mapping
Can you point to where in the body a gene sits?
That time, that place
When Mendel's paper resurfaced in 1900, that heredity comes in units was accepted. Where in the body those units sat, nobody knew.
Chromosomes had been named as candidates: the way they pair off and separate as a cell divides resembles how the units behave. But resemblance is not evidence, and Morgan himself was among the doubters.
Why this question
He kept flies in a cramped room at Columbia. Milk bottles and mashed banana were the whole of the equipment.
The choice of fly was practical: a generation turns over in two weeks and arrives by the hundred. Here was an animal you could do statistics on.
In 1910 a male appeared with white eyes instead of red. Bred on through generations, it showed an odd rule — white eyes turned up only in males. And what separates male from female is also a chromosome.
What was found
If a trait travels with sex, then what fixes that trait rides on the chromosome that fixes sex. It was the first evidence that the units of heredity sit on chromosomes.
Then a further step. Traits riding the same chromosome are usually inherited together, but now and then they come apart — and the rate of coming apart differs from pair to pair. If close neighbours rarely separate and distant ones often do, then the rate of separation is a distance.
In 1913 Sturtevant, still an undergraduate, spent a night laying six fly genes out in order along a line. The first genetic map.
A gene had gone from an abstract unit to something with coordinates.
Heredity had units, but where in the body they sat could not be said
White eyes travelling with sex, and the steady rate at which paired traits come apart
Genes acquired coordinates, and the fly became genetics' standard animal for the next century