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Unlock This CourseMost of the traits we see around us aren't controlled by just one gene. Think about height - some people are tall, some are short, but most people fall somewhere in between. This is because height is controlled by many different genes working together. This type of inheritance is called polygenic inheritance and it's responsible for much of the variation we see in living things.
Key Definitions:
Imagine you're baking a cake and the final taste depends on multiple ingredients. Each gene is like an ingredient - some add sweetness, others add richness. The more 'sweet genes' you have, the sweeter the final result. Similarly, in polygenic inheritance, each gene contributes a small amount to the final characteristic and they all add up together.
Unlike simple inheritance where you get clear-cut categories (like blood type A, B, AB, or O), polygenic traits show continuous variation. This means there's a smooth range of phenotypes from one extreme to another, with most people clustering around the average.
When you plot polygenic traits on a graph, you typically get a bell-shaped curve called a normal distribution. This happens because most people have a mix of genes that produce an average phenotype, while fewer people have combinations that produce extreme phenotypes.
Have mostly genes for 'short' - result in very short height
Have a mix of 'short' and 'tall' genes - result in average height
Have mostly genes for 'tall' - result in very tall height
Human height is controlled by over 700 different genes! Each gene contributes a small amount - maybe 1-2mm to your final height. This is why children's heights usually fall between their parents' heights, but can sometimes be taller or shorter than both parents if they inherit different combinations of height genes.
Polygenic inheritance is everywhere in nature. Let's look at some fascinating examples that show how multiple genes work together to create the amazing diversity we see around us.
Human skin colour is determined by at least 6 different genes that control melanin production. Each gene comes in different versions (alleles) that either increase or decrease melanin production. The more melanin-producing alleles you inherit, the darker your skin colour will be.
Melanin is the pigment that gives colour to our skin, hair and eyes. Different genes control different aspects: some control how much melanin is made, others control what type of melanin and others control where it's distributed in the skin cells.
Intelligence is one of the most complex polygenic traits. Scientists estimate that thousands of genes contribute to cognitive abilities, each having a tiny effect. This is why intelligence shows such continuous variation in populations and why it's influenced by both genetics and environment.
This classic example helped scientists first understand polygenic inheritance. Wheat kernel colour is controlled by 3 genes, each with 2 alleles. Red alleles add colour, white alleles don't. With 6 alleles total, you can get kernels ranging from pure white (6 white alleles) to deep red (6 red alleles), with 5 different shades in between!
Here's where it gets really interesting - the environment can significantly influence how polygenic traits are expressed. Your genes set the potential range, but environmental factors determine where within that range you'll end up.
Consider height again. Your genes might give you the potential to be anywhere from 5'6" to 6'2", but factors like nutrition, exercise, sleep and even stress during childhood will determine your actual final height within that range.
Poor nutrition during growth can prevent you from reaching your genetic potential for height
Regular physical activity can help maximise growth and development
Chronic stress can interfere with growth hormones and affect final height
Scientists study polygenic traits by collecting data from large populations and creating graphs. These graphs reveal important patterns that help us understand how these traits are inherited and distributed.
When you see a bell curve graph of a polygenic trait, you can learn a lot from its shape. A narrow, tall curve means most people are very similar for that trait. A wide, flat curve means there's lots of variation in the population.
Human birth weight shows a perfect example of polygenic inheritance with environmental influence. Multiple genes control foetal growth rate, but maternal nutrition, smoking and health during pregnancy significantly affect the final birth weight. The result is a bell curve with most babies weighing 3-4kg, but with a range from about 2-5kg.
Understanding polygenic inheritance has huge practical applications. In medicine, it helps us understand disease susceptibility and drug responses. In agriculture, it's crucial for breeding programmes.
Many diseases like diabetes, heart disease and mental health conditions are polygenic. Understanding this helps doctors provide better prevention advice and personalised treatments based on genetic risk scores.
Farmers and plant breeders use polygenic inheritance principles to develop crops with desired traits like higher yield, disease resistance, or better nutritional content. By selecting parents with the best combinations of genes, they can gradually improve crop varieties over generations.
Many people misunderstand polygenic inheritance. Here are some key points to remember:
Elite athletic performance is highly polygenic, involving genes for muscle fibre type, oxygen processing, injury resistance and many other factors. However, training, nutrition and mental preparation are equally important. This is why genetic testing alone can't predict who will become a champion athlete!