Heritability - Online Tutor, Practice Problems & Exam Prep

Heritability measures the proportion of variation in a population due to genetics, ranging from 0 to 1. Broad-sense heritability includes all genetic variance, while narrow-sense heritability focuses on additive variance. Artificial selection utilizes narrow-sense heritability to predict trait inheritance, crucial for breeding practices. Twin studies, particularly with monozygotic twins, help distinguish genetic from environmental influences. Concordant traits are shared by both twins, while discordant traits are not. Understanding these concepts is essential for grasping genetic variation and its implications in fields like agriculture and medicine.

Calculating Heritability

So heritability is a measurement, and what it's measuring is the proportion of variation in a population that's due to genetics. That's saying how much of a trait can actually be inherited. There's a ton of variation. How much of height can you actually inherit? And how much depends on the environment. It's important to remember, especially in news articles, that when people hear these heritability statistics, they often attribute them to something that they're not. It's a very specific measurement and only true for a certain population in a certain environment. It's not widely applicable to any individual of the same species; it's only in this population in this environment. What it measures is a range from 0 to 1. Most of the time, it's some kind of decimal point. The larger the value, the more the variation is explained by genetics. For instance, if you have a heritability of 0.65, what you're saying—this is important to understand—is that 65% of the overall variation in that population is explained by genetic differences found in individuals. It's not 65% of an individual's variation. So this is where some news sources may misinterpret the statistic, for example suggesting that much of your intelligence is dependent on genetic factors. It's not true. The statistic means that 65% of the overall variation in intelligence is due to genetic differences in individuals, but it says nothing about a particular individual at all.

There are two types of heritability. The first is broad sense heritability. This measures the contribution of genotypic variance to total phenotypic variance. Here's the formula: h² = v_g v_p , if you remember from our analyzing trait variance, this is the same exact calculation. If the h₂ is close to 1, this means that environmental conditions had little impact. If it's close to 0, environmental conditions had a major impact. So, an h₂ close to 1 means that it's mostly genetic, and very little environmental impact influences the trait. If we're going to calculate the broad sense heritability for each of these traits, what do you do? For body fat, the formula is 16.9 / 40.5, or about 0.42. This means 42% of the overall variance in the population for body fat is attributed to genetic factors. For body length, the calculation is 17.9 / 43.6, or about 0.41. These figures are very similar in terms of broad sense heritability.

The second type is narrow sense heritability. Narrow sense heritability represents the portion of phenotypic variation due to additive genotype variance. What is additive variation? Well, genetic variation is composed of two components: additive and dominance variation. Additive variation, denoted as V_A, is genetic variance caused by differences between alleles, like the differences between dominant and recessive alleles. Dominance variation, denoted as V_D, is variance caused by heterozygotes not being intermediates. The complete genetic variance, V_G, equals the sum of additive variance and dominant variance: V_G = V_A + V_D . Therefore, when calculating Narrow Sense heritability, h² = V_A V_P . For body fat using the additive value, the calculation is 7.66 / 40.5, or about 0.19 (19%). For body length, it's 5.12 / 43.6, or about 0.12 (12%). This shows that broad sense heritability includes more influential genetic factors than narrow sense, where only 12% of traits like body length are due to additive genetic variations, which involve the contribution of alleles that differ such as dominant and recessive. Let's now turn the page.

Artificial Selection

Okay, so now we're going to talk about artificial selection. And the reason we're going to talk about this is because this is actually an applicable reason to use the calculations we talked about before, heritability. So, what is artificial selection? That's the process of choosing specific individuals for some type of phenotypic breeding purpose. For instance, if a farmer like a dairy farmer, who produces cows for milk, wanted to increase the milk production of their cows, they would try to make specific individuals in order to increase that milk production. But before they start mating these cows, right, that's a huge undertaking to make these cows for bigger or for more milk production. They want to make sure that it's actually going to work. And so geneticists can actually tell you, you know, is this likely to increase milk production? And how they do that is they use Narrow Sense Heritability to predict the impact of breeding. Now, why Narrow Sense Heritability? Well, because Narrow Sense Heritability is measuring the likelihood that a trait is actually attributed to genetics and not inheritance. And it's also the ability because narrow sense heritability measures additive, right, then that is measuring the ability to inherit this dominant or recessive allele that's really going to impact the production of milk. So the higher the h2 value, so because h2 is a measure between 0 and 1 so that's going to be the closer to 1. So if you calculate this ht value, the closer to 1, then, the more likely that the breeder will observe a change. The more likely that that's going to result in potentially milk production. So there are 2 formulas. The first we talked about before, and that's just the Narrow Sense Heritability Formula. Right. This is what we talked about previously, but and sometimes you know this information. It's like on a test or quiz setting, you might be actually given this information, But if you're a breeder, you probably don't already just have the amount, additive variance that's contributed to the production of milk. Right? Like, you most likely don't have that. So there's a second formula that you can use to calculate narrow sense heritability, and that actually is this one. And it's r over s. And so, r is the mean of the offspring, so how much milk in volume the offspring produces versus the overall mean of the population that you started with, divided by the mean of the parent, so how much the mother produced essentially, versus the overall mean. So you do have to do at least one mating to get this. Right? And this is called the selection response, and the selection differential. And so, sometimes on I don't know which one you'll be given. Right? Like, if you're asked to do this on a test or quiz, you could be given this, or you could be given the means and asked to solve it this way. But essentially, the question would be, how likely is it that this trait could be inherited or that a breeder could select for this trait in the population. Either way, what you're looking for is an ht value close to 1. The closer to 1 it is, the more likely that it can be inherited. So if we go back to these numbers, which we talked about before. Right? We have body fat, had an h2 value of 0.18 and, we had body length as an h2 value of 0.11. I think this is right. Now, which of the following two traits will respond best to a selection by a breeder? Right? So it's the higher h2 value. So in this case, it would be this one, which would be body fat, because it's closer to 1. But really, to be honest with you, both of these are very low. Right? So you would need much higher h2 values before a breeder is like, okay, I will try to actually select for this trait. So make sure you understand this formula and this one and how you use that to determine whether or not a breeder, you know, whether or not that trait can be inherited or not for artificial selection purposes.

So with that, let's now turn the page.

Twin Studies

Okay. So now let's talk about twin studies. And the reason we talk about twin studies is the fact that humans can't be bred. Right? We can't be bred to determine in different environments and with different genetics to be able to determine what's genetic variation, what's environmental variation, what's additive genetic variation, dominance genetic variation. So how do we actually have to do this? We can't set up these control experiments. So what we do is we use twin studies. And twin studies are taking twins that are produced and studying them for genetic or environmental variation. So there are 2 types of twins. There are monozygotic twins, and how this happens is there's a single zygote. So there's a single sperm that fertilizes a single egg. But what happens is that it does this weird division thing. It mitotically divides and splits into 2 cells. And so when it does that, there are now 2 babies that are produced from these two cells, but they're genetically identical. So they have the same genetics. So, therefore, when you study them, all the variation that they have is environmental variation. That's kind of why, if you look at identical twins when they are babies, they look very similar. Right? Because they haven't had a lot of exposure to the environment. But if you look at those same twins and then they're in their seventies, they actually can look quite different even though they're identical. And that's because that environmental variation has built up throughout the years to make changes in, you know, potentially epigenetics or made them more likely to get a disease or various things, right, that impact phenotype. And so, monozygotic twin studies are extremely important in being able to identify how much environmental variation plays on human genetics. However, it's not ideal, and there's actually some genetic changes that occur very early in development. So an example of this is copy number variances can actually change a lot, in very early development and this is a way that monozygotic twins, which would otherwise be genetically identical, may actually not be 100 percent genetically identical because each twin could come, could actually carry different numbers of copy variants. And there have actually been cases of monozygotic twins who are genetically identical pretty much of one twin actually developing a genetic disease and the other one not, very early in their life because they had variations in copy numbers. So then it's not a perfect system, but it's really the best we have. The second type of twin is dizygotic twins. And this is actually 2 fertilizations. So there are 2 eggs and 2 sperm. And, these are, although they are twins, they are just as close genetically as any other sibling. So if you have a sibling that isn't a twin, they are just as close genetically to you as a fraternal or dizygotic twin. But because they often share the environment. Right? They are raised in a similar situation. We can kind of use that to, at least in part, study genetics. It's not perfect, obviously. The monozygotic twin is definitely the gold standard. But we can also use dizygotic twins, especially ones that have been separated and raised in different environments, to study various components of human genetic variation. And so, an interesting part of expression, of twin expression, is that sometimes traits aren't expressed equally. And so a concordant trait is a trait that both twins express either way. So they either both express it, or they both do not express it. The second type is discordant, and that is when one twin expresses a trait and the other doesn't. And this can happen in both monozygotic and dizygotic twins. And so just some fun vocab for you, as we finish out this topic. So with that, let's now move on.

A chicken breeder has a population of chickens where the average number of eggs laid per hen per month is 34. The narrow-sense heritability is 0.75. With this information is it likely that a breeder could select for an increase in eggs per hen laid each month?

• A. No, breeders never know whether they can select for a trait
• B. No, the breeder will need to know the broad-sense heritability to determine whether selection could cause an increase in eggs?
• C. Yes, because the narrow-sense heritability is 0.75, this means selection is likely to occur

The narrow-sense heritability of the number of seeds per flower is 0.9. The mean of the population is 6.0 seeds per flower. A flower breeder crosses one flower with 7 seeds to another plant with 9 seeds. What is the expected number of seeds per flower in the offspring of this cross?

• A. 5
• B. 6
• C. 7
• D. 8

Heritability calculations were calculated for a variety of different traits. Which of the following traits would respond best to selection?

• A. H₂ = 0.8, h₂ = 0.3
• B. H₂ = 0.3, h₂ = 0.3
• C. H₂ = 0.9, h₂ = 0.8
• D. H₂ = 0.5, h₂ = 0.9

Here’s what students ask on this topic:

What is heritability and how is it measured?

Heritability is a measure of how much of the variation in a trait within a population is due to genetic differences. It ranges from 0 to 1, where a value closer to 1 indicates that most of the variation is due to genetics, and a value closer to 0 indicates that most of the variation is due to environmental factors. There are two types of heritability: broad-sense and narrow-sense. Broad-sense heritability (H²) includes all genetic variance, while narrow-sense heritability (h²) focuses on additive genetic variance. The formula for broad-sense heritability is H² = V_G / V_P, where V_G is the genetic variance and V_P is the total phenotypic variance. Narrow-sense heritability is calculated as h² = V_A / V_P, where V_A is the additive genetic variance.

What is the difference between broad-sense and narrow-sense heritability?

Broad-sense heritability (H²) measures the proportion of total phenotypic variance in a population that is due to all genetic factors, including additive, dominance, and epistatic variances. The formula is H² = V_G / V_P, where V_G is the total genetic variance and V_P is the total phenotypic variance. Narrow-sense heritability (h²), on the other hand, measures the proportion of phenotypic variance that is due to additive genetic variance alone. The formula is h² = V_A / V_P, where V_A is the additive genetic variance. Narrow-sense heritability is particularly useful in predicting the response to selection in breeding programs.

How are twin studies used to understand heritability?

Twin studies are a key method for understanding heritability because they allow researchers to separate genetic and environmental influences on traits. Monozygotic (identical) twins share 100% of their genes, while dizygotic (fraternal) twins share about 50% of their genes. By comparing the similarities between monozygotic and dizygotic twins, researchers can estimate the heritability of traits. If a trait is more similar in monozygotic twins than in dizygotic twins, it suggests a genetic influence. Twin studies also help identify environmental effects by comparing twins raised together versus those raised apart.

What is artificial selection and how does it relate to heritability?

Artificial selection is the process of intentionally breeding individuals with desirable traits to enhance those traits in future generations. It is commonly used in agriculture and animal breeding. Heritability, particularly narrow-sense heritability (h²), is crucial in artificial selection because it predicts the likelihood that a trait will be passed on to offspring. A higher h² value indicates that the trait is more likely to be inherited. The formula for narrow-sense heritability is h² = V_A / V_P, where V_A is the additive genetic variance and V_P is the total phenotypic variance.

What are concordant and discordant traits in twin studies?

In twin studies, concordant traits are those that both twins either express or do not express. For example, if both twins have the same eye color, the trait is concordant. Discordant traits are those where one twin expresses the trait and the other does not. For instance, if one twin has a genetic disease and the other does not, the trait is discordant. Studying concordant and discordant traits helps researchers understand the influence of genetics and environment on various traits.