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Heritability is the proportion of variance in a particular trait, in a particular population, that is due to genetic factors, as opposed to environmental influences or stochastic variation. That’s just a general definition to give you a feel for it. Actually we need to be more rigorous than that. There are two definitions of heritability. A common simplification in all sorts of genetic studies and models is to assume that all alleles and all genotypes act independently of each other – this is called an ‘additive model.’ So for instance, if one allele of a particular SNP gives you a 1 cm increase in height, then being homozygous for that SNP should give you a 2 cm increase in height. Clearly, this model doesn’t allow for dominant or recessive effects, even though we know these abound. It also doesn’t allow for gene-gene interactions, where maybe that SNP only gives you a 1 cm increase in height if paired with another SNP. For these reasons, the additive model is a huge simplification, but a useful one. Now for the two definitions of heritability: ‘narrow sense heritability’ (h2) is defined as the proportion of trait variance that is due to additive genetic factors ‘broad sense heritability’ (H2) is defined as the proportion of trait variance that is due to all genetic factors including dominance and gene-gene interactions. Both kinds of heritability are incredibly tricky to estimate and to interpret. In terms of estimation, a big problem is that people who share parts of their genome tend to share parts of their environment too. Problem: THC percentage per plant is regarded as the phenotype of the plant. In the population as a whole, the average THC percentage per plant is 5.8 percent. Plants whose THC percentage averages 6.2 percent are selected and mated to produce the next generation. The narrow-sense heritability of THC percentage in this population is estimated to be 20 percent. A) What is the expected average THC percentage among the plants after two generations of selection? B) Is this continuous improvement sustainable? Why is it unreasonable to expect continuous crop “improvement” through artificial selection? (What limits artificial selection)? #Genetics #phenotype #genotype #NarrowSenseHeritability #BroadSenseHeritability #breeding #selection
