Green and Pale Seedlings: Inherited or Environmental?
A student in Nagano crosses two pea plants that both have normal green leaves, transferring pollen from one to the other. She collects 240 seeds and plants them all in the same tray of compost. 178 seedlings come up green; 62 come up pale yellow-white and die within two weeks, before they form true leaves. She also notices that seedlings on the shaded side of the tray are paler than those in full light, but most of those darken to normal green within a week. She wants to know whether the pale yellow-white seedlings are 'caused by the shade' as well. A classmate argues that because both parent plants were green, all the seeds should have produced green seedlings, so the 62 pale ones must be a counting error. The student has plenty of seed left from the same cross but no pale-leaved plant still alive.
1. Show that the 178 : 62 result is consistent with a single gene with two alleles, state the genotypes of the parents, and explain why the pale seedlings die.
Read the worked answer
Ratio 178 ÷ 62 ≈ 2.9 : 1, close to 3 : 1. Expected from 240 seeds: 180 green : 60 pale; observed 178 : 62 - a deviation of only 2 in each class, easily explained by chance.
Parents: both green but each carrying the recessive allele - let G = green (dominant), g = pale. Both parents are Gg. A Gg × Gg cross gives GG : Gg : gg = 1 : 2 : 1, i.e. 3 green : 1 pale.
Why the pale ones die: the pale seedlings are gg. The dominant G allele is needed to make chlorophyll, so gg plants cannot photosynthesise and cannot make their own glucose. They survive only while the seed's stored food lasts, then die before forming true leaves. This is why the dead seedlings are not a counting error - they are the expected 1/4 of the offspring, and their phenotype is a real result, not a mistake.
The classmate's expectation ('green parents → all green') is only true if both parents were GG.
2. The student also sees paler seedlings on the shaded side of the tray. Distinguish this from the yellow-white seedlings and describe a test using her spare seed that would settle the matter.
Read the worked answer
Two different causes give a superficially similar look:
- The yellow-white seedlings (gg): the paleness is inherited, appears in the same proportion whatever the light, and the plant cannot recover because the genetic instruction for chlorophyll is missing.
- The shaded seedlings: the paleness is environmental. Chlorophyll synthesis needs light, so a plant in dim light becomes pale; when it receives light it greens up. Its genotype is normal (GG or Gg).
Test: sow spare seed from the same cross in two trays of identical compost, one in full light and one kept dark, and compare after two weeks. If the genetic explanation is right, roughly 1/4 of the seedlings in full light will still be yellow-white and will die - light cannot rescue them. The shade-grown seedlings will be pale in both light conditions and, importantly, the shaded ones will green up when moved into the light. A single tray cannot separate the two causes, because genotype and position in the tray vary together.
3. The student wants to cross one of the surviving green seedlings with a pale plant to find out whether the survivor is GG or Gg. Judge whether this plan can work and, if not, propose a better one.
Read the worked answer
The plan cannot work. A test cross needs the recessive parent, but the gg (pale) plants die before they form true leaves, so none is alive to flower. There is no pale parent available for the cross, and this is a real biological constraint, not a technicality.
Better options:
- Self-pollinate the green survivor (or cross it with itself using its own pollen) and count the seedlings. If it is GG, essentially all offspring are green. If it is Gg, about 1/4 of the offspring are pale and die, so expect roughly a 3 : 1 ratio - this needs enough seed, since with only 8–10 offspring a 3 : 1 ratio can easily be missed.
- Cross it with a known Gg plant (a green plant whose parent was pale). GG × Gg gives all green; Gg × Gg gives about 1/4 pale. Knowing the parentage is what makes this possible.
The judgement to state explicitly: the information needed is the ratio among a decent number of offspring, so the result is statistical, and small numbers give no firm conclusion.