Quantitative Trait Loci (QTL). I know, I know, there I go again with these three worded acronyms. Don’t worry, I’ll explain what a QTL is, but first here’s a primer: every living thing on earth has DNA, and that DNA is organized into individual chromosomes that reside within a cell’s nucleus. Humans have 46 chromosomes, dogs have 39, and the ciliated protozoan Oxytricha trifallax has 1900 (!).
Now, within our chromosomes we have specific stretches of DNA (genotype) that account for a certain physical characteristic (phenotype). Maybe your chromosome #19 has a small region which has DNA that effects height. The differences in this region could explain why you’re so tall and why your younger brother got the shorter end of the stick (pun intended; this is only an example. In no way should you go ahead and blame chromosome 19 for your height troubles!).
I'm not trying to say physical characteristic differences between individuals are the due to single region variances in your chromosomes, but we can say that a certain stretch of DNA (the locus) correlates with a variation in a characteristic (the quantitative trait). That is, a stretch of DNA has been shown to influence quantitative traits (things you can measure), like height or weight.
When it comes to plants, we can actually find QTLs and determine how we can use them to our advantage. After discovering a QTL, scientists must then figure out why this change is taking place at a molecular level. This is the scope of the paper “A Rare Allele of GS2 Enhances Grain Size and Grain Yield in Rice” published in Molecular Plant in 2015. Scientists discovered a new QTL named GRAIN SIZE ON CHROMOSOME 2 (GS2). They found that grain plants with a specific mutation of GS2 enhanced total grain weight and yield. This mutation enhanced expression of the protein Growth Regulation Factor-4 (osGRF-4), which accumulates in the nucleus. Here, it is hypothesized that osGRF-4 acts as a transcriptional activator for growth genes (transcriptional activators: proteins which bind to DNA in the nucleus to enhance their expression).
The take away here is, inserting this mutation of GS2 into rice cultivars could lead to enhanced grain size and overall, yield. However, concerns are raised about grain quality: the study saw a correlation between the mutation of GS2 and poorer grain quality. Only time will tell what the discovery of GS2 has in store, and whether it will find a way to a grocery store near you.
Hu, J., Wang, Y., Fang, Y., Zeng, L., Xu, J., Yu, H., … Qian, Q. (2015). A Rare Allele of GS2 Enhances Grain Size and Grain Yield in Rice. Molecular Plant, 8(10), 1455–1465. doi:10.1016/j.molp.2015.07.002