Linkage

Mendel's Law of Independent Assortment works cleanly only when the two genes being followed sit on different chromosomes, because only then can they segregate into gametes independently of each other. Genes that happen to sit on the very same chromosome are physically tied together and so cannot assort independently; the first clear demonstration of this came in 1906, when William Bateson and Reginald Punnett crossed two homozygous sweet pea (Lathyrus odoratus) strains, one with purple flowers and long pollen grains and the other with red flowers and round pollen grains. Every F1 plant had purple flowers and long pollen, showing that purple/long is dominant over red/round -- an entirely ordinary Mendelian result so far. But when they testcrossed the F1 to the double recessive parent, the F2 did not appear in the expected 1:1:1:1 ratio; instead, purple-long and red-round plants (the two parental combinations) vastly outnumbered the other two classes. Bateson and Punnett concluded that the genes for flower colour and pollen shape must be sitting close together on the same homologous chromosome pair, so that they overwhelmingly travel together into the same gamete rather than shuffling freely. This tendency of genes on the same chromosome to stay together during gamete formation is called linkage, and how tightly two genes are linked depends directly on how physically close together they sit: genes that a …