The heritability of a trait is determined by how much of it is inherited from the parents and how much is created by the environment. Dr David Scobie explains how heritable some traits are, some environmental factors that impact on this and how they measure heritability.
Terms to discuss: heritability, genetic, environment, trait
Questions to consider:
Brainstorm and discuss environmental factors that may affect inheritance of different traits.
Dr David ScobieAgResearch
The heritability of a trait is how much of it you receive from your parents. So in sheep, the heritability of 4 legs is about 100% – most sheep end up with 4 legs. Traits like the short tail, that is about 70% heritable, and what we mean when we say that is 70% of tail length, that information is received from the mother and father of that lamb, and that determines 70% of its tail length.
Then on top of that we have 30% of environment. So if it had a better life in utero or shortly thereafter, its tail might grow slightly longer or slightly shorter. A trait like dagginess has a fairly low heritability of about 0.2 or 20%. That means that 20% of dagginess is determined by the 2 parents, but it is so low because there is all sorts of things that can affect dagginess, like lush feed or high protein levels or internal parasites and bacteria, and those sorts of things can cause scouring, and that will cause dags.
To measure heritability, you have to go out and measure large numbers of animals in different circumstances and get large numbers of offspring per sire, large numbers of progeny from any 1 individual, which will tell you, alright, this ewe had 5 lambs over 5 years, and they tended to have short tails irrespective of who she was mated to. Whereas with a ram, we could produce 300 offspring in 1 year, and we would get a picture very quickly of how short the tails of the offspring of that ram were going to be.
And then you can take that information, measure it, put it into the computer and use a whole lot of complicated algorithms to work out the heritability. It’s just an estimate, it just tells you if it’s close to 100% then that trait is very heritable or if it’s close to 0 then its hardly heritable at all. With the traits we've been looking at, we expect around about 5–7 years to get most of your flock looking like what we're talking about.
Because tail length is so heritable and you can see it in the lamb when it’s born, we can breed that trait in very very fast because we can use that… so we get a decent ram lamb, we can use him when he is 5 months old. With the bare belly trait, it tends not to show up until that animal is about 18 months old, so that slows us down by a whole year, and tail length is very heritable, highly heritable, and so it comes into the flock very quickly, whereas the bare bellies are about 0.3, 30% heritable, and so you can only drag that one along much more slowly.
Acknowledgements:
Dr Clive Dalton, Woolshed 1
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