Linked genes
So far every characteristic has been treated as though it travelled independently. For most pairs of characteristics that holds. For some it does not, and the reason is a matter of simple arithmetic.
There are not enough chromosomes to go round
A human body cell has 46 chromosomes but tens of thousands of genes. Each chromosome must therefore carry a great many genes, laid out along its length like beads on one string.
Linked genes
Genes that lie on the same chromosome are said to be linked, and tend to be inherited together.
What that means in practice
- A gamete gets whole chromosomes, not pieces of them. Two linked genes therefore usually go into the same gamete and reach the same offspring.
- The characteristics they control then turn up together in the offspring far more often than chance alone would predict.
Linkage is why Mendel needed luck as well as care
The seven characters Mendel chose behave as though they are independent of one another. Had he happened to pick two strongly linked characters, his neat ratios would have come out wrong and the pattern would have been much harder to see.
Beyond the syllabus — who worked this out
Gene linkage was established by Thomas Hunt Morgan, working on the fruit fly Drosophila in the early twentieth century. By tracking which characteristics appeared together in thousands of flies, he was able to show not only that genes sit on chromosomes but roughly whereabouts along them.
The exception confirms the mechanism
If genes were free-floating factors, nothing would make two of them travel together. Linkage only makes sense if genes are physically attached to something that moves as a unit — which is some of the strongest evidence that Mendel's factors really are stretches of a chromosome.