What Is Polygenic Inheritance?

Many traits are more complicated than the single-gene inheritance patterns we have studied so far.

In polygenic inheritance, a characteristic is influenced by:

two or more genes.

The word polygenic can be broken into:

poly = many

genic = genes.

Therefore:

polygenic = involving many genes.

The combined effects of several genes can produce a wide range of:

phenotypes.

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From Simple to Complex Inheritance

Earlier, we considered traits controlled mainly by:

one gene.

For example, a simplified genetic model might involve:

A and a.

Possible genotypes could be:

AA

Aa

aa.

This produces a relatively small number of genotype combinations.

Polygenic traits are different because:

several genes contribute to the same characteristic.

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Multiple Genes, One Trait

Imagine that three genes contribute to a characteristic:

Gene A

Gene B

Gene C.

Each gene has different alleles.

An individual's phenotype depends on the combined effects of alleles at:

all three genes.

Therefore:

Gene A + Gene B + Gene C → Phenotype

The genes work together to influence the observable:

characteristic.

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A Simplified Polygenic Model

Suppose three genes influence the amount of pigment produced:

A/a

B/b

C/c.

For this simplified example, imagine each uppercase allele contributes one unit of:

pigment.

Then:

aabbcc

might produce very little pigment.

While:

AABBCC

might produce much more pigment.

Many other allele combinations could produce intermediate:

amounts.

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Many Genetic Combinations

Consider these hypothetical genotypes:

aabbcc

Aabbcc

AaBbcc

AABbcc

AaBbCc

AABBCc

AABBCC.

These contain different numbers and combinations of pigment-contributing alleles.

As a result, they can produce a range of:

phenotypes.

This is one reason polygenic traits often do not fall into just two simple categories.

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Polygenic Traits Often Show Continuous Variation

Continuous variation occurs when a characteristic can take many values across a:

range.

Instead of individuals fitting into a few distinct categories, many intermediate phenotypes are possible.

For example, height does not normally occur only as:

short or tall.

People can have many different heights between these:

extremes.

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Continuous vs Discontinuous Variation

Variation can often be described as:

continuous

or:

discontinuous.

Continuous variation

There are many possible values between two extremes.

Examples include:

• height
• body mass
• skin pigmentation

Discontinuous variation

Individuals fall into distinct categories.

A good example is:

ABO blood group.

Possible categories are:

A, B, AB, or O.

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Why Do Polygenic Traits Show So Much Variation?

Each contributing gene can have different:

alleles.

When several genes influence one trait, many combinations of alleles become:

possible.

Different combinations can produce slightly different:

phenotypes.

The greater the number of genes contributing to a trait, the greater the potential number of genetic combinations.

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A Simple Comparison

Suppose one gene has two alleles.

The basic diploid genotypes are:

AA

Aa

aa.

Now imagine three genes contribute to a trait:

A/a

B/b

C/c.

Instead of considering only one locus, we must consider combinations across:

three loci.

This greatly increases the number of possible:

genotypes.

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Polygenic Inheritance Is Not the Same as Multiple Alleles

These two ideas are easy to confuse.

Multiple alleles means:

One gene has more than two possible alleles in the population.

Example:

Iᴬ, Iᴮ, and i

in the ABO blood group system.

Polygenic inheritance means:

Several different genes influence the same trait.

Therefore:

Multiple alleles = many versions of one gene

Polygenic = many genes affecting one characteristic.

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Example: Human Height

Human height is a classic example of a:

polygenic trait.

Many genes influence processes involved in:

• bone growth
• skeletal development
• hormone signaling
• growth regulation
• metabolism

The combined effects of these genes contribute to an individual's:

height.

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Genes Are Not the Whole Story

Height also demonstrates another important idea.

The phenotype is influenced by:

genes AND environment.

Environmental factors affecting growth can include:

• nutrition
• childhood health
• disease
• access to resources
• developmental conditions

Therefore, a person's phenotype is not always determined by genotype:

alone.

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Genotype + Environment

For many complex traits:

Phenotype = genetic influences + environmental influences + their interactions

This means two individuals with similar genetic potential may develop somewhat different phenotypes if they experience different:

environments.

Likewise, people growing in similar environments can differ because of genetic:

variation.

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Example: Skin Pigmentation

Human skin pigmentation is influenced by many genes involved in:

• melanin production
• melanin type
• pigment distribution
• regulation of pigment-producing cells

Different combinations of alleles contribute to a broad range of:

skin pigmentation.

Environmental exposure, particularly ultraviolet radiation, can also alter pigmentation through:

tanning.

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Example: Eye Colour

Human eye colour is sometimes incorrectly presented as a simple:

brown-dominant-over-blue trait.

In reality, eye colour is influenced by:

multiple genes.

These genes affect the amount and distribution of pigment in the:

iris.

This helps produce a range of eye colours and shades rather than only two simple:

categories.

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Example: Hair Colour

Hair colour is another complex characteristic influenced by:

multiple genes.

Genes affect:

• pigment production
• pigment type
• pigment concentration
• pigment distribution

This contributes to the wide variety of natural hair colours seen in human:

populations.

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Other Polygenic Traits

Polygenic inheritance is common in living organisms.

Examples can include:

• human height
• skin pigmentation
• eye colour
• hair colour
• body size
• crop yield
• fruit size
• milk production in cattle
• growth rate in animals

Many economically important agricultural characteristics are:

polygenic.

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Polygenic Traits in Plants

Imagine a crop plant in which several genes influence:

seed size.

One gene might affect:

cell division.

Another might affect:

nutrient storage.

Another might influence:

growth hormones.

Another might affect:

seed development.

Together, these genes contribute to the final:

seed size.

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Polygenic Traits in Agriculture

Farmers and plant breeders often select organisms based on traits such as:

• crop yield
• fruit size
• growth rate
• drought tolerance
• disease resistance

Many of these traits have complex genetic bases involving:

multiple genes.

This makes selective breeding more complicated than following one simple dominant or recessive:

allele.

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Additive Effects

In simplified models of polygenic inheritance, alleles may have:

additive effects.

This means each contributing allele adds a small amount to the:

phenotype.

For example, imagine:

A, B, and C

each increase pigment production.

An individual with more contributing alleles may produce more pigment than an individual with fewer.

This is a useful model for understanding continuous variation, although real gene interactions can be more:

complex.

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A Simplified Pigment Example

Suppose each uppercase allele contributes one pigment unit.

Then:

aabbcc = 0 units

Aabbcc = 1 unit

AaBbcc = 2 units

AaBbCc = 3 units

AABbCc = 4 units

AABBCc = 5 units

AABBCC = 6 units.

This creates:

seven possible levels

in this simplified model.

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Why Intermediate Phenotypes Are Common

There are often more genetic combinations that produce intermediate values than combinations producing extreme:

values.

For example, there may be only a few combinations that produce extremely high or extremely low values.

Many more combinations can produce values near the:

middle.

As a result, polygenic traits often show a distribution in which intermediate phenotypes are more common.

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Bell-Shaped Distributions

When a polygenic trait is measured in a large population, the data may sometimes form an approximately:

bell-shaped distribution.

In such a distribution:

• extreme values are relatively uncommon
• intermediate values are more common
• individuals are spread across a continuous range

Human height often provides a useful example.

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Reading a Continuous Variation Graph

A graph of a continuous characteristic might have:

x-axis = value of the characteristic

and:

y-axis = frequency or number of individuals.

For height:

x-axis → height

y-axis → number of people.

The highest part of the graph represents the range containing the greatest number of:

individuals.

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Extreme Phenotypes

Extreme phenotypes often require less common combinations of contributing genetic and environmental:

factors.

For this reason, individuals at either extreme of a continuous distribution may be less common than individuals near the:

middle.

However, real trait distributions depend on both biology and the population being studied.

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Single-Gene Inheritance

In single-gene inheritance, one gene has a major influence on the trait being:

studied.

Examples studied earlier include simplified:

• dominant and recessive traits
• some genetic disorders
• ABO blood-group inheritance at the ABO locus

Single-gene problems can often be represented conveniently using:

Punnett squares.

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Polygenic Inheritance

In polygenic inheritance:

many genes contribute to one characteristic.

Because many genes are involved:

• many genotype combinations are possible
• phenotypes often occur across a range
• environmental influences may also be important
• inheritance cannot usually be represented by one simple 2 × 2 Punnett square

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Single-Gene vs Polygenic Inheritance

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Discontinuous Variation

A single-gene trait may produce distinct:

categories.

Consider ABO blood group.

An individual belongs to:

A

B

AB

or:

O.

There are no intermediate categories such as:

half A and half O.

This is an example of:

discontinuous variation.

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Continuous Variation

A polygenic characteristic such as height can take many:

values.

For example:

150 cm

151 cm

152 cm

153 cm

and many values between measurements depending on precision.

There is no natural dividing line separating everyone into only:

short or tall.

This is continuous variation.

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Continuous Variation Is Often Measured

Continuous characteristics are usually:

measured.

Examples include:

• height in centimetres
• mass in kilograms
• fruit mass in grams
• crop yield in kilograms
• leaf length in millimetres

Because these characteristics can take many values, numerical measurements are useful for describing them.

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Discontinuous Variation Is Often Counted or Categorized

Discontinuous characteristics often place individuals into:

categories.

Examples include:

ABO blood group

or certain clearly defined genetic phenotypes.

These categories can often be:

counted.

For example:

25 Type A

18 Type B

7 Type AB

20 Type O.

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Polygenic Does Not Mean Environmental

A trait can be polygenic because:

multiple genes influence it.

Environmental influences are a separate:

concept.

Many polygenic traits are also strongly influenced by the environment, but the word polygenic specifically refers to:

the number of genes involved.

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Environmental Variation

Some variation can occur because organisms experience different:

environments.

For plants, environmental factors might include:

• water availability
• mineral nutrients
• temperature
• sunlight
• soil conditions

Two genetically similar plants can therefore grow to different heights if they experience different:

conditions.

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Genetic and Environmental Interaction

Consider plant height.

Genetic factors may influence:

growth potential.

Environmental factors such as water and mineral availability influence whether that potential is:

reached.

Therefore:

genotype influences potential

while:

environment influences expression.

In reality, genes and environment often interact in complex ways.

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Example: Identical Genotype, Different Environment

Imagine genetically identical plants.

Plant A receives:

• sufficient water
• strong light
• appropriate minerals

Plant B receives:

• little water
• poor light
• limited minerals

The plants may grow to different:

heights.

The difference cannot be explained by genetic variation between the plants because their genotypes are:

identical.

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Example: Different Genotypes, Same Environment

Now imagine several genetically different plants grown under very similar:

conditions.

They may still develop different:

heights.

This provides evidence that genetic differences contribute to the:

variation.

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Polygenic Inheritance and Punnett Squares

Could we use Punnett squares for polygenic inheritance?

In principle, individual genes can still be followed using inheritance:

rules.

However, a simple 2 × 2 Punnett square becomes impractical when many genes contribute to the same:

trait.

The number of possible combinations increases rapidly.

Scientists therefore often use statistical methods to study complex:

traits.

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Why Predictions Become More Difficult

Suppose one gene affects a trait.

We may need to track:

one pair of alleles.

Now suppose 20, 50, or hundreds of genetic variants influence the characteristic.

We must consider:

• many allele combinations
• interactions between genes
• environmental influences
• random inheritance
• developmental effects

The phenotype becomes much harder to predict precisely.

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Polygenic Inheritance and Families

Parents can pass many different combinations of alleles to their:

children.

Therefore, siblings can inherit different combinations of alleles involved in a polygenic trait.

This helps explain why siblings can differ in characteristics such as:

height or pigmentation

even though they share the same biological parents.

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Why Siblings Are Different

Except for identical twins, siblings do not normally inherit exactly the same combination of:

alleles.

Meiosis produces genetically varied gametes.

Random fertilization creates further variation.

When many genes influence a characteristic, different allele combinations can produce different:

phenotypes.

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Polygenic Traits and Populations

Polygenic variation is important because populations contain many different:

alleles.

These create genetic diversity.

Variation within populations can influence:

• natural selection
• adaptation
• evolution
• selective breeding

Therefore, polygenic inheritance has important consequences beyond individual families.

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Natural Selection and Polygenic Traits

Suppose a population varies continuously in:

body size.

If environmental conditions favour a particular range of body sizes, individuals within that range may have greater reproductive:

success.

Over many generations, allele frequencies may change.

Therefore, natural selection can act on:

polygenic variation.

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Worked Example 1

A characteristic has only two clear phenotypes and is strongly controlled by one gene.

Is this likely to be a simple polygenic trait?

No.

The evidence is more consistent with:

single-gene inheritance.

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Worked Example 2

A characteristic ranges continuously from very small to very large and is influenced by many genes.

What type of inheritance is involved?

Polygenic inheritance.

The continuous range is consistent with the combined effects of:

multiple genes.

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Worked Example 3

Researchers measure the heights of 5,000 plants.

They find many different heights, with most plants near the middle and fewer at the:

extremes.

This pattern suggests:

continuous variation.

One possible explanation is:

polygenic inheritance, potentially combined with environmental effects.

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Worked Example 4

A gene has three alleles:

A¹

A²

A³.

Is this automatically polygenic inheritance?

No.

This is:

multiple alleles.

There is still only one gene involved.

Polygenic inheritance requires:

multiple genes.

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Worked Example 5

A characteristic is influenced by five genes.

Each gene contributes a small amount to the final phenotype.

What inheritance pattern is this?

Polygenic inheritance.

The combined effects of the genes produce the:

phenotype.

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Comparing Concepts

Complete dominance

One allele masks another in a heterozygote.

Incomplete dominance

The heterozygote has an intermediate phenotype.

Codominance

Both alleles are expressed.

Multiple alleles

More than two alleles of one gene exist in the population.

Polygenic inheritance

Multiple genes influence one trait.

These concepts describe different aspects of:

genetics.

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Common Mistake: Polygenic Means Many Alleles

Polygenic means:

many genes.

It does not simply mean:

many alleles.

A trait influenced by five genes is polygenic.

A single gene with five possible alleles demonstrates:

multiple alleles.

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Common Mistake: One Gene Produces the Entire Trait

For a polygenic trait, there is not usually one single gene that completely determines the:

phenotype.

Many genes contribute.

Some may have larger effects than others.

The phenotype results from their combined:

influence.

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Common Mistake: Continuous Variation Is Caused Only by Genes

Environmental factors can also contribute to:

continuous variation.

For example, height has a strong genetic component, but nutrition and health during development can also affect final:

height.

Both genetic and environmental factors should therefore be considered.

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Common Mistake: Everyone Near the Middle Has the Same Genotype

Many different genotype combinations can sometimes produce similar:

phenotypes.

Therefore, two people with similar values for a polygenic trait do not necessarily have the same:

genotype.

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Common Mistake: A Bell Curve Proves Polygenic Inheritance

A continuous or approximately bell-shaped distribution can be consistent with:

polygenic inheritance.

However, the shape of a graph alone does not prove the genetic mechanism.

Environmental and other biological factors can also influence a:

distribution.

Scientists need additional genetic evidence.

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Check Your Understanding

1. Define polygenic inheritance.

2. What does the prefix "poly-" mean?

3. How many genes are involved in a polygenic trait?

4. Explain how multiple genes can contribute to one phenotype.

5. What is continuous variation?

6. What is discontinuous variation?

7. Give three examples of polygenic traits.

8. Why is human height considered polygenic?

9. Why does height show continuous variation?

10. How can environmental conditions influence height?

11. Explain why skin pigmentation is considered a complex polygenic trait.

12. Why is the simple brown-eye/blue-eye model of eye colour incomplete?

13. Explain how polygenic inheritance can produce many different phenotypes.

14. Why are intermediate phenotypes often common in polygenic traits?

15. What might a graph of continuous variation look like?

16. What does the x-axis represent on a height-frequency graph?

17. What does the y-axis represent?

18. Why are extreme values often less common than intermediate values?

19. Compare single-gene inheritance with polygenic inheritance.

20. Why are simple Punnett squares less useful for highly polygenic traits?

21. Explain the difference between multiple alleles and polygenic inheritance.

22. Is the ABO blood group system polygenic simply because it has three alleles? Explain.

23. Give an example of discontinuous variation.

24. Give an example of continuous variation.

25. Explain how genes and environment can both influence phenotype.

26. Why can genetically identical plants develop different heights?

27. Why can siblings differ in polygenic traits?

28. Explain why polygenic variation is important for natural selection.

29. A trait is controlled by seven genes and occurs across a wide range of values. What inheritance pattern does this suggest?

30. Explain why knowing that a trait is polygenic makes precise phenotype prediction more difficult.

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Key Terms

• Polygenic inheritance: Inheritance in which multiple genes contribute to a single characteristic.
• Polygenic trait: Characteristic influenced by two or more genes.
• Continuous variation: Variation in which phenotypes occur across a range with many intermediate values.
• Discontinuous variation: Variation in which individuals fall into distinct categories.
• Single-gene inheritance: Inheritance in which one gene has a major influence on the characteristic being studied.
• Multiple alleles: More than two alternative forms of one gene existing in a population.
• Genotype: Genetic makeup of an organism.
• Phenotype: Observable characteristics resulting from genetic and environmental influences.
• Additive effect: Situation in which genetic variants contribute cumulatively to a characteristic.
• Environmental variation: Differences in phenotype caused partly or entirely by environmental conditions.
• Distribution: Pattern showing how values of a characteristic occur within a population.
• Frequency: Number of individuals with a particular value or within a particular range.
• Genetic variation: Differences in genetic information among individuals in a population.

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Key Takeaways

• Polygenic inheritance occurs when multiple genes influence a single characteristic.
• "Polygenic" means many genes.
• Each contributing gene may have several possible alleles within a population.
• The combined effects of these genes influence the final phenotype.
• Polygenic inheritance can create many possible genotype combinations.
• This often produces a wide range of phenotypes.
• Polygenic traits frequently show continuous variation.
• Continuous variation includes many intermediate values between extremes.
• Human height is a classic example of a polygenic trait.
• Skin pigmentation is influenced by multiple genes.
• Eye colour is genetically more complex than a simple dominant-recessive model.
• Hair colour also involves multiple genetic influences.
• Many agricultural traits, including crop yield and body size, are polygenic.
• In simplified models, different alleles may have additive effects on a phenotype.
• Intermediate phenotypes are often more common than extreme phenotypes.
• Polygenic traits may produce approximately bell-shaped distributions in large populations.
• Single-gene inheritance often produces fewer phenotype categories.
• Polygenic inheritance usually produces much greater variation.
• Polygenic inheritance is different from multiple alleles.
• Multiple alleles = several versions of one gene.
• Polygenic inheritance = several genes influencing one trait.
• Many polygenic traits are also influenced by environmental conditions.
• Phenotype can result from interactions between genotype and environment.
• Similar genotypes do not always produce identical phenotypes when environments differ.
• Similar phenotypes do not necessarily mean identical genotypes.
• Simple Punnett squares become less practical when many genes contribute to a trait.
• Polygenic variation contributes to diversity within populations.
• This variation provides material upon which natural selection can act.
• Understanding polygenic inheritance helps explain why many real biological characteristics cannot be described using simple dominant-versus-recessive inheritance.