Genes/alleles, genotype/phenotype, dominance - Biology IGCSE Study Notes
Overview
Have you ever wondered why you might have your mum's nose or your dad's eye colour? Or why some people have curly hair and others have straight hair? It's all thanks to something called **inheritance**, which is how features (like hair colour or height) are passed down from parents to their children. This topic is super important because it helps us understand how all living things, from tiny bacteria to huge elephants, get their unique characteristics. It's like a secret instruction manual passed down through generations, making each of us a little bit like our family, but also uniquely ourselves. Understanding these basic ideas about genes and how they work is the first step to understanding bigger topics like genetic diseases, selective breeding (how we get different dog breeds!), and even how evolution happens over millions of years.
What Is This? (The Simple Version)
Imagine you're building a LEGO model. The instructions for building that model are like your body's DNA. Inside those instructions, there are specific pages or sections that tell you how to build just one part, like a wheel or a roof. These specific sections are called genes.
- A gene is a tiny piece of your DNA that carries the instructions for a specific feature, like eye colour or hair texture. Think of it as a recipe for one particular trait.
Now, imagine for the 'wheel' part of your LEGO model, the instructions say you can use either a red wheel or a blue wheel. Both are wheels, but they are different versions. These different versions of a gene are called alleles.
- An allele is a different version or flavour of a gene. For example, for the 'eye colour' gene, you might have an allele for blue eyes and another allele for brown eyes.
So, you get one allele from your mum and one from your dad for each gene. What happens if you get a blue eye allele from your mum and a brown eye allele from your dad? This is where dominance comes in! Sometimes, one allele is stronger and 'wins out' over the other, showing its trait. The 'winning' allele is called dominant, and the 'losing' one is recessive.
Real-World Example
Let's think about pea plants, which are famous in biology because a scientist named Gregor Mendel studied them a lot. We'll look at the trait of pea colour.
- The Gene: There's a gene that determines the colour of the peas. Let's call it the 'pea colour gene'.
- The Alleles: This gene has two main versions (alleles):
- An allele for yellow peas (let's use 'Y' for this).
- An allele for green peas (let's use 'y' for this).
- Dominance: In pea plants, the allele for yellow peas (Y) is dominant over the allele for green peas (y). This means if a plant has at least one 'Y' allele, its peas will be yellow. The 'y' allele (for green peas) is recessive; it only shows up if there are two 'y' alleles.
So, if a pea plant gets a 'Y' from one parent and a 'y' from the other, its peas will be yellow because 'Y' is dominant. For the peas to be green, the plant must get a 'y' from both parents.
How It Works (Step by Step)
Let's follow the journey of these genetic instructions from parents to offspring. 1. **Parents' Contribution:** Each parent has two alleles for every gene (one from their own mum, one from their own dad). 2. **Making Sex Cells (Gametes):** When parents make sex cells (sperm or egg), they only pas...
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Key Concepts
- Gene: A small section of DNA that carries instructions for a specific characteristic or trait.
- Allele: A different version or form of a gene, like different flavours of an ice cream.
- Dominant allele: An allele that always shows its trait (phenotype) if it is present, even if there's only one copy.
- Recessive allele: An allele that only shows its trait (phenotype) if two copies are present (no dominant allele to hide it).
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Exam Tips
- →Practice using Punnett squares to predict offspring genotypes and phenotypes; they make complex crosses much simpler.
- →Always use consistent notation: capital letters for dominant alleles and lowercase for recessive alleles of the *same gene* (e.g., T and t, not T and s).
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