Suppose a polygenic system for producing color in kernels of a grain is controlled by three additive genes, G, M, and T. There are two alleles of each gene, G₁ and G₂, M₁ and M₂, and T₁ and T₂. The phenotypic effects of the three genotypes of the G gene are G₁G₁ = 6 units of color, G₁G₂ = 3 units of color, and G₂G₂ = 1 unit of color. The phenotypic effects for genes M and T are similar, giving the phenotype of a plant with the genotype G₁G₁M₁M₁T₁T₁ a total of 18 units of color and a plant with the genotype G₂G₂M₂M₂T₂T₂ a total of 3 units of color.
Assuming the threshold model applies to this kernel-color system, what proportion of the progeny of the cross G₁G₂M₁M₂T₂T₂xG₁G₂M₁M₂T₁T₂ do you expect to display colored kernels?

Question

Suppose a polygenic system for producing color in kernels of a grain is controlled by three additive genes, G, M, and T. There are two alleles of each gene, G₁ and G₂, M₁ and M₂, and T₁ and T₂. The phenotypic effects of the three genotypes of the G gene are G₁G₁ = 6 units of color, G₁G₂ = 3 units of color, and G₂G₂ = 1 unit of color. The phenotypic effects for genes M and T are similar, giving the phenotype of a plant with the genotype G₁G₁M₁M₁T₁T₁ a total of 18 units of color and a plant with the genotype G₂G₂M₂M₂T₂T₂ a total of 3 units of color.
Assuming the threshold model applies to this kernel-color system, what proportion of the progeny of the cross G₁G₂M₁M₂T₂T₂xG₁G₂M₁M₂T₁T₂ do you expect to display colored kernels?

Assuming the threshold model applies to this kernel-color system, what proportion of the progeny of the cross G₁G₂M₁M₂T₂T₂xG₁G₂M₁M₂T₁T₂ do you expect to display colored kernels?

Accepted Answer

Hi, everyone. Let's look at our next question. Celiac disease is an autoimmune disorder that affects the small intestine and is triggered by the consumption of gluten, which is found in wheat, barley and rye. What is the concept that can explain why some individuals with a genetic predisposition for celiac disease may remain asymptomatic throughout their lives while others may experience symptoms at various stages of their lives depending on the presence or absence of environmental triggers a penetrance. B heritability C polygenic inheritance or D threshold liability. Well, in this case, when you have the idea that you might inherit a genetic predisposition that might or might not get actually expressed in your lifetime, depending on triggers that are found is choice D threshold liability. And that involves that the idea that certain traits are only expressed when a certain threshold of genetic or environmental factors are exceeded in the case of celiac disease. That might be the amount of gluten a person person takes in. So you might inherit a genetic predisposition to celiac disease. But if there's very little gluten in your diet, naturally, you might never actually have the symptoms of celiac disease or perhaps there are other environmental factors that might lead to symptoms developing in some individuals but not in others. So let's look at our other answer choices to see why they're not correct choice. A penetrance might be an answer that might uh tempt us to choose it at first because penetrance involves the idea of inheriting a genotype that isn't always expressed. But in the case of penetrance, it is a factor that expresses the extent to which a trait inherited in the genotype is expressed in the phenotype. So someone has inherited the gene but may or may not express the trait or disorder. Now, there can be a little bit of overlap. Here, there might be a chance that you think something is an example of variable penetrance that there doesn't seem to be a reason why some individuals with this gene express the disease and others don't. But maybe you might come later to discover that some environmental factor is triggering it. In which case, you'd be looking at perhaps an example of threshold liability. But penetrance in general just refers to it as a variation and the extent to which a gene is actually expressed in the phenotype of people that inherit it. So choice A isn't quite correct since it doesn't refer to specifically something where there's a tipping point in genetic or environmental factors that affects the express of the uh the phenotype being expressed. Choice B heritability refers to the degree to which a trait is inherited versus environmental. And this is often determined in twin studies where you would look at identical twins who obviously have the same exact genotype and look at the chance when one identical twin is affected by something. What is the chance that the second twin will be affected by it? Also, if something is perfectly heritable, it will be 100% if one twin has it, the other one does. But if something is not necessarily just inherited but affected by environmental circumstances, you won't see that 100% correlation, but that's not what we're talking about. So that's why choice B is not correct. And finally, see polygenic inheritance is when there is more than one gene affecting a single phenotype. So a classic example is skin color in humans where multiple genes go into the final phenotype, the color of a person's skin. So you can see there's a little connection to variable expression of a trait uh in the sense that people don't just straightforwardly inherit um one skin color or the other, but that multiple genes inherited go into this single phenotype, but it's not what we're describing here. So that's why choice C is not correct. So again, what is this a concept that explains why some individuals with a genetic predisposition for a condition like celiac disease may remain asymptomatic while others experience symptoms depending on various triggers choice d threshold liability. See you in the next video

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Key Concepts

Polygenic Inheritance

Additive Gene Effects

Threshold Model