Genetic terms (AQA GCSE Biology): Revision Notes
Genetic terms
What's inside a cell?
Every cell in your body contains important genetic material that controls your characteristics. Understanding where this material is stored helps us learn how traits are passed on.
Most cells have a nucleus that contains thread-like structures called chromosomes. These chromosomes carry all the genetic information needed to make you who you are. Humans have 23 pairs of chromosomes in most body cells.
Each chromosome pair means you have two copies of every gene - one inherited from your mother and one from your father. This is why you might have some characteristics that resemble each parent.
Basic genetic vocabulary
Genes
A gene is a small piece of DNA found on a chromosome. Each gene contains instructions for making a specific protein. These proteins control different characteristics like your eye colour or height.
Think of genes like recipes in a cookbook - each recipe (gene) gives instructions for making a different dish (protein).
The Cookbook Analogy
Just like a cookbook contains many different recipes, each chromosome contains many different genes. And just like following a recipe produces a specific dish, following a gene's instructions produces a specific protein that affects your traits.
Chromosomes
Chromosomes are long structures made of DNA that carry many genes. You inherit one chromosome from each parent in every pair. This means you have two copies of each gene in your body cells.
Having two copies of each gene is crucial for genetics - it's why you can have different combinations of traits and why some characteristics might be hidden but still passed on to your children.
Gametes
Gametes are special sex cells made through a process called meiosis. In animals, sperm and egg cells are gametes. In flowering plants, pollen and egg cells are gametes.
During fertilisation, gametes join together. This is how genetic information from both parents combines to create offspring.
Unlike regular body cells that have 23 pairs of chromosomes, gametes only have 23 single chromosomes. When fertilisation occurs, the gametes combine to restore the full 23 pairs in the offspring.
Alleles explained
Alleles are different versions of the same gene. For example, the gene for eye colour might have a brown allele or a blue allele.
Since you get chromosomes from both parents, you have two copies of each gene. This means you have two alleles for every characteristic. These alleles might be:
- The same (called homozygous)
- Different (called heterozygous)
Remember that having two alleles for each gene is what makes genetics interesting - the combination of these alleles determines what characteristics you actually show.
Genotype vs phenotype
These two terms describe different aspects of your genetic makeup:
Genotype refers to the actual alleles you have for a particular gene. You can't see this just by looking at someone - it's the genetic code written in their DNA.
Phenotype refers to the observable characteristics you can actually see. This includes things like hair colour, height, or blood type.
The phenotype is produced when the genotype's instructions are followed at the molecular level.
Key Difference to Remember:
- Genotype = what's written in your genetic code (invisible)
- Phenotype = what you can actually observe (visible)
Your genotype is like a recipe, while your phenotype is like the finished dish!
Dominant and recessive alleles
Not all alleles have the same effect when present in your cells:
Dominant alleles
A dominant allele is always expressed when present, even if you only have one copy. Dominant alleles are often written with capital letters (like B for black fur).
If you have at least one dominant allele, you'll show that characteristic.
Dominant Allele Rule: You only need ONE copy of a dominant allele for it to be expressed. This is why dominant traits are more commonly seen in populations.
Recessive alleles
A recessive allele is only expressed when you have two copies of it. Recessive alleles are often written with lowercase letters (like b for brown fur).
You need both alleles to be recessive to show the recessive characteristic.
Recessive Allele Rule: You need TWO copies of a recessive allele for it to be expressed. This is why recessive traits are less common and can "skip generations."
Simple example
Worked Example: Mouse Fur Colour
Let's look at fur colour in mice:
- B = dominant allele for black fur
- b = recessive allele for brown fur
Possible combinations:
Step 1: Homozygous dominant
- BB = black fur (dominant allele expressed)
Step 2: Heterozygous
- Bb = black fur (dominant B masks recessive b)
Step 3: Homozygous recessive
- bb = brown fur (only expressed when both alleles are recessive)
Key insight: Even though both BB and Bb have different genotypes, they have the same phenotype (black fur) because B is dominant.
Key Points to Remember:
- Genes are sections of DNA that code for proteins
- Alleles are different versions of the same gene
- Genotype = the alleles you have, Phenotype = what you look like
- Dominant alleles show up even with one copy
- Recessive alleles need two copies to be expressed
- Gametes are sex cells that carry genetic information from parents to offspring