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The Broad-Sense Heritability Equation

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  • 0:04 Heritability
  • 2:09 The Broad-Sense Equation
  • 4:22 Heritability of a Trait
  • 6:03 Lesson Summary
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Lesson Transcript
Instructor: Kristin Klucevsek

Kristin has taught college Biology courses and has her doctorate in Biology.

Heritability describes how a phenotype is affected by genetic variation. Who you are is more than just your DNA - it's somewhat influenced by things, like what you eat and where you live. In this lesson, find out how geneticists account for variations using the broad-sense heritability equation.

Heritability

You've probably heard or said something like, 'I inherited my blue eyes from my mom' or 'Those big feet? Well, I got those from my dad.' Or, perhaps you've wondered while trying to shoot a basket, 'Why is everybody in my family six feet tall except me?' People talk about heritability, the ability for a trait to be inherited, all the time. People also think about why something wasn't inherited. In this lesson, we'll talk about how geneticists use heritability to help them understand how likely it is that a trait is 'passed on,' or really, how likely it is that it is expressed as a physical trait, or phenotype.

Heritability describes how a total phenotype is affected by genetic variation. Remember that a phenotype is the physical trait. Importantly, this genetic variation can be due to multiple genes and multiple alleles of these genes. In essence, we are talking about a phenotypic trait that is multifactorial. You may remember that multifactorial describes when multiple factors play a role in a trait. These factors can be both genetic and environmental. Therefore, whenever heritability of a trait isn't 100%, that's most likely because there are some environmental factors that come into play.

This is a good time to introduce the concept of penetrance, too. This is a term usually associated with a disease that is inherited. Penetrance is used to describe the percent of a population with a specific genotype that shows the expected phenotype. For example, 100% of individuals who inherit an allele for Huntington's disease will develop the disease phenotype. This is an example of complete penetrance.

In contrast, 80% of individuals who inherit the BRCA1 allele for breast cancer will develop this cancer phenotype. Why not 100%? Why is this genotype less penetrant, or incomplete? Science is working on answering that because the answers are likely to help us cure the disease. It helps to know that because this is a heritable disease, and likely a multifactorial disease, there's both genetic variation and environmental variation involved in the phenotype.

The Broad-Sense Equation

Okay, so we've established that phenotypes can be influenced by your DNA and the world around you. How can we use this knowledge to figure out the heritability of a trait? Well, not surprisingly, a little (or in some cases, a lot of) math can help us get to that answer.

Geneticists use the broad-sense heritability equation to measure the proportion of total phenotype that is affected by genetic and environmental variations. There are some variations on this equation itself, but a standard we can use is H^2 = V sub G / V sub G + V sub E, where H represents the heritability, V sub G represents variance due to genotype and V sub E represents variance due to the environment.

For the sake of this lesson, we won't get into too much of the math behind calculating V sub G, or V sub E or even H. Those numbers are calculated over long experimental observations. What we will do is talk about how this equation can be used, and what it means to genetics and what that means for your poor chance as a basketball star.

From looking at this formula, you can see that the range of heritability is from 0 to 1. For example, if V sub G (or the genotypic variation contributing to heritability) was 0, then there would be no heritability. This makes sense, as the genotype would have no influence and so the heritability would also be 0. You can't inherit something that does not have a genetic contribution.

In contrast, if V sub E (or the environmental variation) was 0, then heritability would be 1. This would mean that the environment made no contribution to the heritability; it was all genetic. If everyone in your family can touch the ceiling without standing on their tippy toes, and you are the odd man out, then perhaps your environment made a contribution to your stature.

For example, perhaps you were never fond of your vegetables and milk as a kid. I don't know. It's a possibility. Regardless, the heritability here would be somewhere between 0 and 1, as both genetics and the environment influence a trait like height. Further, we can simplify this equation because phenotype (P) is the result of both a genotype (G) and a phenotype (E), this equation can also be written as H^2 = V sub G / V sub P.

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