if a trait is 80% heritable and you want to guess whether or not Bob has that trait then you’ll be 80% more accurate if you know whether or not Bob’s parents have the trait than if you didn’t have that information.
I think this is more or less correct for narrow-sense heritability (most commonly used when breeding animals) but not quite right for broad-sense heritability (most commonly used with humans). If you’re talking about broad-sense heritability, the problem is that you’d need to know not just if the parents have the trait, but also which genes Bob got or not from each parent, as well as the effect of dominant genes, epistatic interactions, etc.
Assuming you’re talking about broad-sense heritability, I think a better way of looking at it would be to say that you’ll be 80% more accurate if Bob has an identical twin raised by a random family and you know if that twin had the trait. This isn’t quite right either, but I think it’s valid if you assume that phenotypic traits are the sum of genetic effects and environmental effects and also that genetic effects are independent of environmental effects.
Of course, few people have identical twins raised by random families, and most phenotypes probably aren’t additive in genetic and environmental effects, and those effects probably aren’t independent! Which… is a lot of caveats if you want to know practical applications of heritability numbers.
I think this is more or less correct for narrow-sense heritability (most commonly used when breeding animals) but not quite right for broad-sense heritability (most commonly used with humans). If you’re talking about broad-sense heritability, the problem is that you’d need to know not just if the parents have the trait, but also which genes Bob got or not from each parent, as well as the effect of dominant genes, epistatic interactions, etc.
Assuming you’re talking about broad-sense heritability, I think a better way of looking at it would be to say that you’ll be 80% more accurate if Bob has an identical twin raised by a random family and you know if that twin had the trait. This isn’t quite right either, but I think it’s valid if you assume that phenotypic traits are the sum of genetic effects and environmental effects and also that genetic effects are independent of environmental effects.
Of course, few people have identical twins raised by random families, and most phenotypes probably aren’t additive in genetic and environmental effects, and those effects probably aren’t independent! Which… is a lot of caveats if you want to know practical applications of heritability numbers.