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Unlock This CourseEvery living thing has instructions written in its DNA that tell it how to grow, develop and function. These instructions are called genes and they're like recipes in a cookbook - each one tells your body how to make a specific characteristic. But here's where it gets interesting: for each characteristic, you don't just have one recipe - you have two versions and these different versions are called alleles.
Think of it like having two different recipes for chocolate cake. Both are for chocolate cake (the same gene), but one might use dark chocolate whilst the other uses milk chocolate (different alleles). The final cake you get depends on which recipe dominates!
Key Definitions:
Genes are like chapters in your body's instruction manual. Each gene contains the code for making a specific protein, which then creates a particular characteristic. For example, there's a gene for eye colour, one for hair texture and another for blood type. Humans have about 20,000-25,000 genes in total!
Since you inherit DNA from both your mum and dad, you get two copies of every gene - one from each parent. These two copies might be identical, or they might be slightly different versions. These different versions are what we call alleles.
Not all alleles are created equal! Some are bossy and always get their way (dominant), whilst others are more shy and only show up when there's no dominant allele around (recessive). Scientists use letters to represent alleles - capital letters for dominant ones and lowercase letters for recessive ones.
Represented by capital letters (like B for brown eyes). Only need ONE copy to be expressed. They're like the loud person in a group conversation - they dominate!
Represented by lowercase letters (like b for blue eyes). Need TWO copies to be expressed. They're like whispers - you only hear them when it's quiet!
Both alleles are expressed equally. Like having both chocolate AND vanilla in your ice cream - you taste both flavours!
Let's look at eye colour as a simple example. Brown eyes (B) are dominant over blue eyes (b). If you have BB or Bb, you'll have brown eyes. Only bb gives you blue eyes. This is why brown eyes are much more common than blue eyes in most populations. However, real eye colour inheritance is actually more complex, involving multiple genes!
Now we need to distinguish between what genes you have (your genotype) and what you actually look like (your phenotype). It's like the difference between the ingredients in your kitchen cupboard and the meal you actually cook with them.
When we look at the two alleles for any particular gene, they can either be the same or different. This gives us some important terminology that you'll use throughout genetics.
When both alleles are the same. This could be homozygous dominant (BB) or homozygous recessive (bb). Think "homo" = same. These individuals are sometimes called "pure breeding" because they can only pass on one type of allele.
When the two alleles are different (Bb). Think "hetero" = different. These individuals are sometimes called "carriers" if they carry a recessive allele that isn't expressed in their phenotype.
Let's explore some fascinating examples of how genes and alleles work in the real world. These examples will help you understand the concepts better and see how genetics affects everyday life.
The ABO blood group system is a perfect example of multiple alleles and co-dominance. There are three alleles: A, B and O. A and B are co-dominant (both expressed when present together), whilst O is recessive to both.
Genotypes: AA or AO. The A allele codes for A antigens on red blood cells. People with type A blood can donate to A and AB recipients.
Genotypes: BB or BO. The B allele codes for B antigens. Type B individuals can donate to B and AB recipients.
Genotype: AB. Both A and B alleles are expressed (co-dominance). AB individuals are universal plasma donors but can only receive AB blood.
Sickle cell anaemia demonstrates how recessive alleles can persist in populations. The condition is caused by a recessive allele (s) that changes the shape of red blood cells. People with SS have severe anaemia, but those with Ss (carriers) have some protection against malaria. This explains why the sickle cell allele is more common in areas where malaria is prevalent - being a carrier actually provides a survival advantage!
Plants provide some of the clearest examples of genetic inheritance. Gregor Mendel, the father of genetics, used pea plants to discover the basic principles of inheritance that we still use today.
Tall plants (T) are dominant over short plants (t). TT and Tt plants are tall, whilst only tt plants are short. This was one of Mendel's key discoveries about dominant and recessive traits.
In many flowers, colour follows simple dominant-recessive patterns. Purple flowers might be dominant (P) over white flowers (p), so PP and Pp plants have purple flowers, whilst pp plants have white flowers.
There are several common mistakes students make when learning about genes and alleles. Let's clear these up so you can avoid them!
Understanding what genes and alleles are NOT is just as important as understanding what they are.
Genes are small sections of DNA that code for specific traits. Chromosomes are much larger structures that contain thousands of genes. Think of chromosomes as books and genes as individual chapters.
Just because an allele is dominant doesn't mean it's more common in the population. For example, having six fingers is dominant over having five, but most people have five fingers!
Gene: Eye colour gene (controls what colour your eyes can be). Allele: Brown eye allele or blue eye allele (specific versions of the eye colour gene). Genotype: Bb (the specific alleles you have). Phenotype: Brown eyes (what you actually observe).
Understanding genes and alleles isn't just academic - it has real-world applications in medicine, agriculture and conservation. This knowledge helps us understand inherited diseases, develop better crops and protect endangered species.
Genetic counsellors use knowledge of genes and alleles to help families understand the risk of passing on genetic conditions. By knowing whether someone is homozygous or heterozygous for certain alleles, doctors can predict the likelihood of their children inheriting specific conditions.
Plant breeders use understanding of dominant and recessive alleles to develop crops with desirable traits. They might cross plants to combine disease resistance (dominant) with high yield (recessive) to create better varieties.