A picture of a little katydid (Amblycorypha oblongifolia) crossed my time line the other day.
Katydids, named after the sound of their chirp, can be distinguished from grasshoppers and crickets by their long back legs, very long antennae and rhomboid shaped body that is tented like the roof of a house.
A katydid looks like a walking leaf. Nearly all of them are green, but a few are bright pink or even orange and yellow. The colors are variations of a single gene. So it appears at first sight that these rare color morphs must be recessive traits, while the common green must be the dominant variation (known as “allele”), right? Wrong!
When two pink katydids were crossed, their offspring were not all pink as would be expected if pink was a recessive allele. Instead, a few green katydids appeared, with a pink:green ratio of 3:1.
The Punnett square is used to figure out what happened in the cross. Remember that the gametes or sex cells (sperm in male and egg in female) have all their chromosomes halved, so each gamete carries only one color variant. When the sperm fertilizes the egg, the gene variants pair up in different combinations of Pink (P) or Green (G). Only the GG combination gave green katydids. Even one P was enough to result in a pink katydid.
Each of the pink katydids must have carried the recessive green trait, which was masked or silent. When mixed together, a few offspring carried two green alleles and appeared green.
But why are most of the katydids found in nature green?
This is due to natural selection. Pink katydids are conspicuous in nature and easily found by predators. More green katydids survive because they are nicely camouflaged among the leaves.
Have you ever seen a pink katydid? If you do, be sure to congratulate it on surviving the odds of natural selection. And think about how not all dominant traits are “good” for survival of the species.