Polygenic Traits Overview & Examples
What Is a Polygenic Trait?
A common word pair used in genetics is ''human traits''. To understand what is actually occurring with human traits such as eye color and height, it is important to understand the words ''trait'' and ''genes''. Traits are specific characteristics of an organism and genes are a functional unit of heredity that make up a sequence of DNA. Variation in the quantity of phenotypes for a certain trait depends on quantity of genes regulating that trait. A single gene will control a single-gene trait. So, what is a polygenic trait?
A polygenic trait can be described as a trait that is regulated by numerous genes and not just one gene, and each of these genes will have numerous alleles. A polygenic trait definition in biology can also refer to the phenotype being regulated by numerous genes, and each gene will exhibit persistent distribution. In humans, for example, height, eye color, and hair color all exhibit persistent distribution that can be shown by a bell-curve graph.
The modified form of a gene that usually determines traits is called an allele. The combination of those two alleles designates the organism's genotype. The genotype traits are seen through the phenotype, which physically shows the genes and their properties. Dominant and recessive alleles are an outcome of the genotypes and phenotypes. Recessive alleles are seen in the absence of the dominant alleles and the completely dominant alleles are those that are displayed in the phenotype. For example, for the hair color of an organism, an organism inherits a dominant allele B for black hair and a recessive allele b that is genetically heterozygous (Bb) for the characteristic. As one allele is dominant (B) over the other (b), the organism will have black hair.
The definition of polygenic inheritance is also related to the number of genes. Polygenic inheritance is what happens when single inherited phenotypic traits are controlled by two or more different genes. One of the traits exhibited by polygenic inheritance is incomplete dominance. Incomplete dominance is when the alleles for the gene have an additive effect and therefore one allele does not mask or dominate another. The phenotype of the offspring will be a combination of the phenotype of the parents. For example, if a red homozygous flower (RR) is crossed with a white homozygous flower (rr), the resulting offspring will have pink colored flowers with genotype (Rr). This shows incomplete dominance.
Polygenetic Traits and Alleles
The traits with the most variation are polygenic. This means that more than one gene controls them, and the interaction of all the genes involved causes the observed appearance. People don't just come as short or tall, or heavy or light; they have a range of heights and weights. Furthermore, people have a spectrum of skin tones and eye colors. Your eyes probably don't look exactly the same shade as your mother's or siblings' eyes even if they are the same color. Height, weight, hair color, eye color, and skin color are examples of polygenic traits. The appearance of each of these traits is due to interactions between two or more genes.
Alleles for a gene can be dominant, recessive, codominant, or show incomplete dominance. The interaction between two alleles of a gene is easy to see when there is only one gene that causes the trait, such as for sickle cell anemia, cystic fibrosis, or ectrodactyly (fused fingers or toes). Either you have the trait or you don't. It is more difficult to see dominance when there is more than one gene involved in making a trait because you have to examine the interaction between multiple alleles.
Mendelian Traits vs. Polygenic Traits
Polygenic traits are controlled by the interaction of multiple genes, so these traits are also referred to as non-mendelian traits. In contrast, mendelian traits are controlled by a single pair of genes, and these traits exhibit complete dominance, whereas polygenic traits will exhibit incomplete dominance. Complete dominance is where the dominant allele will completely mask the effect of the recessive allele, which then hides the other allele in a heterozygous condition.
Polygenic traits are found in most organisms, whereas mendelian traits are rare. Examples of mendelian traits are the presence of freckles and dimples, and blood type. These traits are passed down by dominant and recessive alleles of one gene. These traits are not controlled by the environment. In contrast, one common example of polygenic traits is height. Human height is controlled by multiple genes, and environmental factors such as diet and health can influence human height. Polygenic traits demonstrate continuous distribution whereas mendelian traits are discrete and do not show continuous variation.
Mendelian Inheritance vs Polygenic Inheritance
Many important human characteristics, such as height, skin color, eye color, and risk of diseases like diabetes, are controlled by many factors which may be genetic, environmental or both. Polygenic inheritance is controlled by a number of different genes. So, a certain trait will be controlled by these various genes and have an equal impact on these genes. An additive effect can be seen on the allele for these genes as well.
A type of incomplete dominance can be known as polygenic inheritance, and the resulting phenotype will be a combination of traits. It is difficult to foresee the pattern of polygenic inheritance as it is complicated. Using human height as an example, most individuals in the population fall in the middle of the bell curve and are the mean height. The individuals on one end of the curve are short and individuals on the other end are tall.
Another example is skin tone. Skin color is controlled by three genes and determined by the amount of the color pigment melanin in the skin. The three genes A, B, and C control dark pigmentation due to the presence of melanin production. The recessive alleles a, b, and c control light color pigmentation. Individuals who inherit no dark alleles will have very light skin color and those with dark alleles will have very dark skin color. With different combinations of light and dark alleles, the phenotype is of varying skin shades. Those with an even number of dark and light alleles will have a medium skin color.
In contrast, Mendelian inheritance is controlled by a single gene. Mendelian traits follow the law of inheritance which tells us that if each parent is heterozygous for a particular trait the crossover results in progeny with {eq}\frac{3}{4} {/eq} carrying the dominant version of the trait whereas {eq}\frac{1}{4} {/eq} carries the recessive version of the trait. If the parents are homozygous, in which case they have two identical copies of the gene and so one parent is homozygous for the dominant version of the gene while the other parent is homozygous for the recessive form, all their offspring would be heterozygous.
A scientist named Gregor Mendel conducted experiments on pea plants and formulated certain laws to understand inheritance and known as Mendel's laws of inheritance. Mendel's laws of inheritance include the law of dominance, the law of segregation, and the law of independent assortment. The law of segregation explains that each individual has two alleles and one of these alleles will be adopted by their offspring. The law of independent assortment explains that the inheritance of a pair of genes are independent to one another.
Polygenic Inheritance Examples
There are many examples of polygenic inheritance in humans such as height, hair color, skin color, blood type, and intelligence. Examples in plants include kernel color in wheat and color in flowers.
An example of polygenic inheritance is genetic basis of coat color in Labrador. The three recognized colors of this breed of dog are black, chocolate and yellow. The existence of the different coat color in Labradors can be identified by the allele found in the B and E loci (the specific location on a gene). The black coat color (BB) is dominant to chocolate color (bb). A separate gene (E) is necessary for yellow color in dogs. If a dog is chocolate, we know it is (bbE-), where the dash indicates it may have either an (e) allele or a second (E). If it has a yellow parent it must have received an (e) from that parent, and is (bbEe). If it has produced yellow pups, it must have the capability to give them the (e) allele, and again must be (bbEe). A dog with a recessive (b) allele at this locus is unable to produce color and is yellow irrespective of the allele present at locus B. So, the genotypes (BBee), (Bbee), and (bbee) all produce the same yellow phenotype. A cross between heterozygotes for both genes (BbEe x BbEe) would generate offspring with a phenotypic ratio of 9 black: 3 chocolate: 4 yellow.
Polygenic Inheritance and Disorders
An inherited disease or disorder is defined as the consequences of the simultaneous actions or interactions of two or more genes. More than one gene is associated with this type of disease, so it has a complex pattern compared to monogenic diseases. In humans, polygenic diseases or disorders occur more often than monogenic diseases. In addition to the involvement of multiple genes, non-genetic factors may also play a part in expression of these diseases. Some examples of polygenic inheritance disorders include hypertension, coronary heart disease, autoimmune disorders, cancer, obesity, and type 2 diabetes.
Type 2 diabetes can be caused by polygenic genetic factors and non-genetic factors such as lifestyle factors and diet. Environmental factors can play a significant role in the development of diabetes. Environmental factors such as lack of physical activity lead to obesity, which is a leading cause of type 2 diabetes and many other diseases in the US. Type 2 diabetes is classified as a polygenic disease due to the complex interaction between multiple genes and environmental factors. Some of the genes found to be associated with type 2 diabetes also appear to be linked to dyslipidemia, atherosclerosis and cancer.
Another example of how environment effects the phenotype is the hereditary disorder phenylketonuria (PKU). Individuals who are homozygous for the disease alleles of the PKU gene lack activity of an enzyme phenylalanine, which breaks down amino acids. This enzyme rapidly builds up to toxic levels in the body. If PKU is left untreated, the extra phenylalanine can prevent the brain from developing normally, leading to intellectual disability, seizures, and mood disorders.
Lesson Summary
A polygenic trait can be defined as a trait that is controlled by various genes and not just one gene. These traits will display a persistent variation. A common example of this is height in humans, which is hereditary. To help understand polygenic traits, the explanation of various keywords is important. Different version of genes are called alleles. An allele can be either dominant or recessive, where dominant will show its effect even if only one copy of the allele is present, and recessive will show its effect in the presence of two copies of the allele. Phenotype is a visible physical characteristic of an organism which results from the association of its genotype. Genotype is the genetic material that is passed between generations and is a combination of alleles influenced by an organism for a specific gene.
Polygenic inheritance can be defined as the inheritance of traits that are governed by various genes and not just one gene, and can also be known as non-mendelian inheritance. These traits tend to have a bell-shaped distribution when graphed and show various combinations of dominant and recessive alleles. In contrast, traits passed through mendelian inheritance are determined by a single gene. Examples of polygenic inheritance include height, eye color, and hair color; these traits exhibit incomplete dominance. Mendelian traits follow the law of inheritance and exhibit complete dominance. In humans, polygenic diseases or disorders occur more often. Besides the involvement of multiple genes, non-genetic factors may also play a part in expression of these diseases. Some examples of polygenic disorders include hypertension, coronary heart disease, autoimmune disorders, cancer, obesity, and type 2 diabetes.
What Is Polygenic Dominance?
Polygenic dominance describes a relationship between the alleles of a polygenic trait where some alleles are dominant and others are recessive. Eye color is an example of a polygenic trait that demonstrates polygenic dominance. Some alleles for eye color dominate other alleles, where the presence of even one allele dominates all the other alleles. For example, having one allele for brown eyes means that you will most likely have brown eyes. The way this happen is described further in the next section.
How Is Eye Color Determined?
The eye colors blue, brown, and green are determined by two genes. At least for this lesson, there are only two genes. If you want to impress people with random knowledge they are 'gey' and 'bey2'. It is important that you understand that the combination of 'gey' and the combination of 'bey2' alleles work together to determine what eye color you have. The 'bey2' gene determines whether your eyes will be brown or blue. This is the gene that people normally discuss in biology classes; brown (B) is dominant over blue (b).
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Eye color is also determined by other genes. The 'gey' gene is the gene for green eyes where Green (G) is dominant and not green (g) is recessive. However, green color may not always show up because Brown (B) dominates green (G). Green is only seen if you have the bb phenotype in the 'bey2' gene. The most important thing to remember is that the allele for brown eyes (B) dominates over the alleles for blue (b) or green eyes (G) even though eye color is polygenic.
We haven't even considered the explanation for how hazel, grey, black, violet, and yellow eyes are inherited, and we won't. It is likely that our exact eye color is the result of multiple genes (or polygenic) where some alleles are dominant, some are codominant, some are additive, and others show other patterns of inheritance, allowing people to have a spectrum of eye colors.
Lesson Summary
Polygenic dominance is an interaction between alleles from a polygenic trait (a trait that involves multiple genes) where one allele dominates the other alleles. Eye color is an example of polygenic dominance where the brown (BB, Bb) allele dominates the alleles for blue (bb) and green (GG, Gg).
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Polygenetic Traits and Alleles
The traits with the most variation are polygenic. This means that more than one gene controls them, and the interaction of all the genes involved causes the observed appearance. People don't just come as short or tall, or heavy or light; they have a range of heights and weights. Furthermore, people have a spectrum of skin tones and eye colors. Your eyes probably don't look exactly the same shade as your mother's or siblings' eyes even if they are the same color. Height, weight, hair color, eye color, and skin color are examples of polygenic traits. The appearance of each of these traits is due to interactions between two or more genes.
Alleles for a gene can be dominant, recessive, codominant, or show incomplete dominance. The interaction between two alleles of a gene is easy to see when there is only one gene that causes the trait, such as for sickle cell anemia, cystic fibrosis, or ectrodactyly (fused fingers or toes). Either you have the trait or you don't. It is more difficult to see dominance when there is more than one gene involved in making a trait because you have to examine the interaction between multiple alleles.
What Is Polygenic Dominance?
Polygenic dominance describes a relationship between the alleles of a polygenic trait where some alleles are dominant and others are recessive. Eye color is an example of a polygenic trait that demonstrates polygenic dominance. Some alleles for eye color dominate other alleles, where the presence of even one allele dominates all the other alleles. For example, having one allele for brown eyes means that you will most likely have brown eyes. The way this happen is described further in the next section.
How Is Eye Color Determined?
The eye colors blue, brown, and green are determined by two genes. At least for this lesson, there are only two genes. If you want to impress people with random knowledge they are 'gey' and 'bey2'. It is important that you understand that the combination of 'gey' and the combination of 'bey2' alleles work together to determine what eye color you have. The 'bey2' gene determines whether your eyes will be brown or blue. This is the gene that people normally discuss in biology classes; brown (B) is dominant over blue (b).
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Eye color is also determined by other genes. The 'gey' gene is the gene for green eyes where Green (G) is dominant and not green (g) is recessive. However, green color may not always show up because Brown (B) dominates green (G). Green is only seen if you have the bb phenotype in the 'bey2' gene. The most important thing to remember is that the allele for brown eyes (B) dominates over the alleles for blue (b) or green eyes (G) even though eye color is polygenic.
We haven't even considered the explanation for how hazel, grey, black, violet, and yellow eyes are inherited, and we won't. It is likely that our exact eye color is the result of multiple genes (or polygenic) where some alleles are dominant, some are codominant, some are additive, and others show other patterns of inheritance, allowing people to have a spectrum of eye colors.
Lesson Summary
Polygenic dominance is an interaction between alleles from a polygenic trait (a trait that involves multiple genes) where one allele dominates the other alleles. Eye color is an example of polygenic dominance where the brown (BB, Bb) allele dominates the alleles for blue (bb) and green (GG, Gg).
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Frequently Asked Questions
What is a simple definition of polygenic traits?
Traits that are manipulated by multiple genes instead of just one gene are known as polygenic traits. They are also called characteristics or phenotypes.
How do you identify a polygenic trait?
Usually traits are polygenic when there is a wide variation in the trait. For example, humans can be of many different skin tones or color.
What is an example of a polygenic trait?
A polygenic trait can be described as a trait that is regulated by numerous genes and not just one gene, and each of these genes will have numerous alleles. For example, height in humans is controlled genetically and will exhibit persistent distribution.
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Polygenic Traits Overview & Examples | What is Polygenic Inheritance?
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