Reciprocal Cross: the test for a parent-of-origin effect
In short
A reciprocal cross is a pair of crosses in which the roles of mother and father are swapped: once strain A as mother with B as father, once the other way round. Comparing the two sets of offspring shows whether a trait depends on a particular parent.
What the comparison reveals
The logic is simple. If both directions give the same result, the trait is inherited through the nucleus, to which both parents contribute equally. If the results differ by direction, the trait sits with something one-sided: the sex chromosomes, or cytoplasmic inheritance, the genes outside the nucleus that only the mother passes on.
So the test cleanly separates three cases: pure nuclear inheritance, sex-linked inheritance and maternal influence. It is often the first, simplest handle before reaching for elaborate molecular explanations.
Classic examples
Two historical experiments show both outcomes. Around 1909 Carl Correns crossed the four-o’clock (Mirabilis jalapa) with variegated, white-and-green mottled leaves. With a green plant as mother the offspring came out green; in the other direction, with a variegated mother, they came out variegated. The trait followed the mother alone, a case of cytoplasmic inheritance through the chloroplasts.
Thomas Morgan found the other case in the fruit fly in 1910: eye colour behaved differently in the two cross directions because the gene sits on a sex chromosome. Gregor Mendel, by contrast, got the same ratios in both directions with his peas, a sign of pure nuclear inheritance.
The test in cannabis
This very tool undercuts the common assumption that aroma comes from the mother. Swap the parent roles in cannabis and the cannabinoid composition stays the same on average, with no consistent maternal or paternal effect (Vergara et al. 2019). If aroma were maternally inherited, the two cross directions would have to differ. They don’t.