When specific features of an organism are determined by many genes, the feature is a polygenic trait. Many of the observable characteristics of an organism are influenced by more than one gene, and the corresponding polygenic inheritance becomes complex.
Descendants may inherit dominant or recessive variations of some of the genes, and the inherited genes influence each other in different ways. Some of the genes are expressed more or less strongly, and environmental factors may influence the trait as well.
Typical examples of polygenic traits in humans are height, eye color and skin color. The combined influence of the many genes results in a continuous variation in the characteristic.
For example, eye color can be any shade from dark brown through light blue and some green as each gene contributes a variable bit of color.
Simple Mendelian Inheritance Applies to Single Genes
Simple genetic interactions were first proposed by Austrian monk Gregor Mendel in the 19th century. Mendel worked with pea plants and experimented with the colors of their flowers, the shape of their pods and other observable characteristics.
The traits Mendel studied were mostly produced by a single gene. For example, the gene for a red flower was either present or not present, and the resulting flower would be either red or white. Based on his studies, Mendel constructed his theory for genetic inheritance, and his work remains valid for single gene traits.
Human examples of Mendelian traits caused by a single gene include the following:
- Color blindness.
- Huntington disease.
- Sickle cell anemia.
- Cystic fibrosis.
These traits follow simple rules of inheritance, but most human characteristics are caused by many genes. These polygenic traits are also called continuous traits. The characteristics for which they are responsible vary continuously, and their inheritance is influenced by many factors.
Polygenic Inheritance and Key Genetic Concepts
The influence of different types of genes on polygenic traits is important for understanding how they work. Key genetic concepts for describing the influence of genes on traits in humans include the following:
- Dominant versus recessive genes: Humans receive two sets of genes, one from the mother and one from the father. The two versions of the same gene are called alleles. Having one or two dominant alleles produces the trait for the dominant gene while having two recessive alleles produces the recessive trait.
- Homozygous vs. heterozygous: An individual who has two dominant or two recessive alleles is homozygous for that gene. Individuals with one dominant and one recessive allele are heterozygous.
- Codominance: When two alleles are different but both are dominant, they are both expressed in the individual and traits from both appear.
- Incomplete dominance: When different alleles are neither completely dominant nor completely recessive, both are expressed weakly, and a mixture of the traits appears in the individual.
Polygenetic traits can result from several different alleles or from multiple genes. The type of alleles and the kind of dominance influences gene expression and the resulting polygenic traits.
The Roots of Polygenic Traits Are Difficult to Track Down
When observable traits vary continuously, geneticists know that multiple genes are at the root of the trait. Tracking down all the genes influencing a polygenic trait is more difficult.
One problem is to determine whether a trait is influenced by different genes or by alleles of the same gene. A gene can have more than two alleles, and the pattern of dominance can affect the gene's expression.
Alleles of a single gene are always found at a particular place or locus on a chromosome, but those genes contributing to the polygenic trait could be anywhere. Some genes for a single trait can be closely linked on a chromosome, in different locations on the same chromosome or on different chromosomes. Finding all the influences is challenging.
The Genes of Polygenic Traits Are Expressed as Phenotypes
Phenotypes are all the observable characteristics and behaviors of an organism. Many phenotypes are based on polygenic traits and are continuously variable characteristics. For example, human skin color shows a continuous variation in various tones and colors, pointing to a polygenic origin.
Phenotypes are often influenced by environmental factors as well. In some cases, the polygenic variation takes place in small steps, but the environmental influence evens out the steps to make the variation seem continuous.
In the case of skin color, the already continuous variation is influenced by exposure to sunlight, which darkens skin tones.
Individuals with the Same Genes May Have Different Phenotypes
When two individuals have the same genes with respect to certain traits, many of those characteristics will be the same, but some phenotypes may differ. This is especially true for genes that make an individual likely to develop a particular disease. The genes code for susceptibility, but environmental factors and other genes may play a role in triggering the disease.
Variable expressivity means that the trait encoded in the genes may be expressed weakly or strongly depending on other factors. Incomplete penetrance means that the trait sometimes does not appear at all. In both cases, environmental factors or other genes affect the expression of the gene responsible for the trait.
Traits Can Be Influenced by Many Factors
Polygenic traits can be expressed in varying intensities and can be affected by external factors. When incomplete dominance allows the recessive gene paired with a dominant gene to influence a phenotype, a continuous variation in the observed characteristic is possible.
Examples of human polygenic traits with continuous variation include the following:
- Height: The continuous variation in human height comes from the influence of a large number of genes, incomplete dominance in some genes and environmental factors such as nutrition.
- Eye color: Variation in color and shade is mostly determined by two genes but influenced by a number of other genes.
- Hair color: Continuous variation from light to dark is influenced by many genes but also by environmental factors such as exposure to sunlight.
Polygenic traits in plants exhibit similar continuous variation, but incomplete dominance is possible with single genes as well. For example, the color of wheat kernels is determined by a gene that has a dominant allele for red over a recessive allele for white.
Because heterozygous wheat kernels exhibit incomplete dominance in the color gene, kernels can be various shades of pink as well.
Phenotype Can Be Changed by Environmental Factors
Genes from a genotype are expressed to create certain traits in the organism, but how these traits appear often depends on environmental factors including the behavior of the organism. Genotypes may create a susceptibility to a specific disease, but whether an individual exhibits the disease symptoms is due to other factors.
For example, phenylketonuria or PKU is a genetic disease that results in an individual unable to metabolise the amino acid phenylalanine. The amino acid builds up to toxic levels in the body and causes mental and physical disability.
Treatment includes a diet with limited amounts of phenylalanine. Individuals who observe this diet will not develop the symptoms, and their phenotype does not include outward expression of the disease.
A gene may cause a specific phenotype under certain environmental conditions, but if the conditions are absent, the phenotype will not appear.
For example, the fur color of Siamese cats is dark when skin temperature is cool but white when skin temperature is warm. This leads to the dark-colored extremities of the cats where skin temperature for ears and paws is cooler. In a warm climate, skin temperature overall will be higher, and the cat's fur will be lighter.
Genes of Polygenic Traits Interact to Produce Widely Varying Phenotypes
While Mendel's hypothesis still applies to simple genetics, the wide variety of observable traits can only be explained by the interactions of non-Mendelian inheritance. The complex influences of polygenic traits create the continuous variations of characteristics in advanced organisms.
Together with environmental factors, they are responsible for the wide range of observed phenotypes.