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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Polygenic Inheritance
Not all traits are as simple as Mendel's pea plants! While some characteristics like blood type or the ability to roll your tongue are controlled by just one gene (monogenic), many of the traits we see around us are influenced by multiple genes working together. This is called polygenic inheritance and it explains why so many characteristics in nature show a range of variation rather than distinct categories.
Key Definitions:
Monogenic traits: Controlled by a single gene with clear patterns of inheritance (like Mendel's peas). Examples include blood type, widow's peak and attached earlobes.
Polygenic traits: Controlled by multiple genes, showing continuous variation. Examples include height, skin colour, eye colour and intelligence.
Polygenic traits typically show a normal distribution (bell curve) in populations. Most individuals have values near the average, with fewer individuals at the extremes.
This is different from monogenic traits, which often show distinct categories in specific ratios (like the 3:1 ratio in Mendel's experiments).
Let's explore how multiple genes can work together to create a range of phenotypes using a simple example: skin colour.
Human skin colour is determined by at least three different gene pairs (and likely more). Each gene contributes to the amount of melanin produced. For simplicity, let's call these genes A, B and C.
Each gene has two alleles - uppercase letters (A, B, C) contribute more melanin, while lowercase letters (a, b, c) contribute less melanin. The more uppercase alleles a person has, the darker their skin tone will be.
Genotype: aabbcc
This combination has no alleles for increased melanin production.
Genotype: AaBbCc
This combination has three alleles for increased melanin production.
Genotype: AABBCC
This combination has six alleles for increased melanin production.
With three gene pairs, there are seven possible levels of melanin production (0-6 uppercase alleles), creating a continuous spectrum of skin colours rather than distinct categories.
When we measure a polygenic trait like height in a large population, we typically see a pattern called a normal distribution or bell curve. This happens because:
For example, with just 3 gene pairs affecting height, there are 7 possible phenotypic categories. With more genes involved (as is actually the case with human height), the distribution becomes even smoother, creating what appears to be continuous variation.
Human height is influenced by at least 700 different genes! Each gene has a small effect, but together they account for about 80% of the variation in height between individuals. The remaining 20% is due to environmental factors like nutrition.
This explains why:
Unlike some monogenic traits that are strictly determined by genetics, polygenic traits are often influenced by environmental factors as well. This interaction between genes and environment is why we see such variation even among individuals with similar genetic backgrounds.
The genetic component sets the potential range for a trait. For example, your genes might give you the potential to grow to between 170-185 cm tall.
Environmental factors determine where within that range you end up. Poor nutrition during childhood might mean you only reach 172 cm, while excellent nutrition might help you reach 183 cm.
Many characteristics we see in humans and other organisms are the result of polygenic inheritance:
Fingerprints are a polygenic trait! The specific pattern of ridges and whorls on your fingertips is influenced by multiple genes as well as environmental factors in the womb. This is why even identical twins (who share the same DNA) have slightly different fingerprints!
Understanding polygenic inheritance has important applications in several fields:
Many common diseases like diabetes, heart disease and cancer have polygenic inheritance patterns. Understanding these helps with risk assessment and personalised medicine.
Crop yield, disease resistance and animal breeding programs rely on understanding polygenic traits to develop better varieties and breeds.
Studying polygenic inheritance helps scientists understand evolution, adaptation and the complex relationship between genes and environment.
In your IGCSE Biology exam, you might be asked to:
Remember to use specific examples and correct terminology in your answers!
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