Question

Define and give an example of each of the following inheritance patterns: polygenic inheritance, multifactorial trait, incomplete dominance, codominance, and multiple alleles. (pages \(479-81)\)

Short answer

Polygenic inheritance is when a trait is controlled by multiple genes, such as human skin color. A multifactorial trait is determined by genetic and environmental factors, like height in humans. Incomplete dominance results in a blend of traits, seen in pink snapdragon flowers (Rr). Codominance has both traits fully expressed, as in the AB blood type. Multiple alleles involve more than two possible alleles for a gene within a population, exemplified by the ABO blood group system.

Step-by-step solution

Polygenic Inheritance

Polygenic inheritance refers to the inheritance pattern in which a particular trait is controlled by multiple genes, each having a small effect on the overall phenotype. In other words, the genetic information for a certain characteristic is determined by more than one gene. Example: Human skin color is a classic example of a polygenic trait. It is influenced by several genes, and the various combinations of these genes determine the wide range of skin colors in humans.

Multifactorial Trait

A multifactorial trait is a characteristic that is determined by multiple genetic factors as well as environmental factors. Such traits result from the combined effect of genes and non-genetic factors. Example: Height in humans is a multifactorial trait, as it is influenced by various genes as well as environmental factors such as nutrition.

Incomplete Dominance

Incomplete dominance, also known as partial dominance, is a type of non-Mendelian inheritance pattern in which the phenotype of the heterozygous individual is intermediate between those of the two homozygous parents. Neither allele is completely dominant over the other, resulting in a blend of both traits. Example: In snapdragon flowers, there are two alleles for flower color: one for red (R) and one for white (r). When these two alleles are combined in a heterozygous individual (Rr), the flower color is pink, which is intermediate between red and white.

Codominance

Codominance refers to the inheritance pattern in which both alleles in a heterozygous individual are fully expressed, and neither is dominant over the other. As a result, the offspring display a mix of both parental phenotypes instead of a blend. Example: The AB blood type in humans is a result of codominance. When a person inherits the A allele from one parent and the B allele from the other, both alleles are fully expressed, resulting in the AB blood type.

Multiple Alleles

Multiple alleles refer to the presence of more than two possible alleles for a specific gene within a population. While any individual can only carry two alleles for a gene, there can be a larger number of alleles present in the total population. Example: The ABO blood group system in humans is an example of multiple alleles. There are three different alleles for the gene that determines blood type: A, B, and O. While any individual can only inherit two of these alleles (one from each parent), all three can be found within the population.

Key concepts

Polygenic Inheritance

Polygenic inheritance is when multiple genes influence a single trait. These genes each have a small effect and together they create variation in the overall expression of that trait.
Human skin color is a perfect example. Several different genes contribute to the final skin color, which is why we see such a wide range of skin tones in humans.
This is also why polygenic traits often show a continuous spectrum of phenotypes rather than distinct categories.

• Multiple genes influence one trait.
• Contributes to diversity in traits like skin color or height.
• Results in a range or spectrum of traits, not just single outcomes.

Multifactorial Trait

A multifactorial trait is shaped by both genetic and environmental factors. It demonstrates how our environment can impact genetic expression.
For example, human height is determined not just by genes, but also by factors like diet and health during developmental years.
These traits illustrate the complexity and interplay between our genetics and our surroundings.

• Genes and environment both play a role.
• Human height is influenced by nutrition and genetics.
• Shows the interaction between genetic predispositions and lived experiences.

Incomplete Dominance

Incomplete dominance is when neither allele is completely dominant, resulting in a blending of traits in the phenotype. It contrasts with the classic dominant-recessive relationship.
A well-known example is snapdragon flowers, where mixing a red allele with a white allele creates pink flowers.
This type of inheritance shows how allele interactions can create new, intermediate phenotypes.

• Results in an intermediate phenotype.
• Seen in snapdragon flower color: red mixing with white gives pink.
• Illustrates non-Mendelian inheritance patterns.

Codominance

Codominance occurs when both alleles in the genotype are expressed equally in the phenotype. Neither allele masks the other, leading to a coexistence of traits.
The AB blood type in humans is a classic example, as both A and B alleles are expressed equally to produce the AB phenotype.
Codominance differs from incomplete dominance because it doesn’t blend traits but showcases both distinctly.

• Both traits are fully expressed in the phenotype.
• Exemplified by the AB blood type in humans.
• Exhibits clear traits of both alleles, not a mix.

Multiple Alleles

Multiple alleles mean that a single gene can have more than two allele forms in a population. Though an individual can only have two alleles (one from each parent), the gene pool itself contains several options.
A good example is the ABO blood group system. Humans can have blood type A, B, or O depending on which two of the three alleles they inherit.
This complexity allows for more variation and adaptability within a population.

• A gene with more than two possible alleles in the population.
• In ABO blood type, three alleles exist but individuals have two.
• Contributes to genetic diversity and adaptability.