Genetic terms (AQA GCSE Biology Combined Science): Revision Notes
Genetic terms
Inside a cell
Gametes are special sex cells that form through a process called meiosis in reproductive organs. These cells are crucial for reproduction because they carry genetic information from one generation to the next.
When fertilisation happens, gametes join together. In animals, this means sperm cells and egg cells fuse together. In flowering plants, pollen cells and egg cells combine to create new offspring.
The fusion of gametes during fertilisation combines genetic material from two parents, creating genetic diversity in offspring. This is why children share characteristics with both parents but are never identical to either one.
Genes are tiny sections of DNA found on chromosomes. Think of each gene as a set of instructions that tells your body how to make a specific protein. For example, one gene might contain the instructions for making the protein that determines your eye colour.
Chromosomes are structures inside the cell's nucleus that contain your genetic material. Most body cells have two copies of each chromosome, which means you have two copies of each gene. These chromosome pairs carry the same genes in the same order, but they might have slightly different versions of those genes.
Understanding that we have two copies of each gene is fundamental to genetics. This is why you inherit one copy from your mother and one from your father, and why genetic inheritance follows predictable patterns.
Understanding alleles
Alleles are different versions of the same gene. For instance, the gene for eye colour might have a brown allele or a blue allele. These different versions create the variety we see in characteristics like eye colour, hair colour, or height.
Genotype refers to the actual alleles that an organism has for a particular gene. This is the genetic makeup that you inherit from your parents, written using letters like BB, Bb, or bb.
Phenotype is what you can actually observe - the physical characteristics that result from those alleles working together. This is what you see when you look at someone, like their actual eye colour or hair texture.
Example: Genotype vs Phenotype
Consider eye colour:
- Genotype: BB, Bb, or bb (the genetic code you inherit)
- Phenotype: Brown eyes, blue eyes, or hazel eyes (what you actually see)
A person with genotype Bb might have brown eyes (phenotype) because the brown allele is dominant over the blue allele.
Most characteristics result from multiple genes working together, rather than just one single gene controlling everything. This is why genetic inheritance can seem quite complex.
When describing genotypes, we use important terms:
Key Genetic Terms:
- Homozygous means having two identical alleles for a gene (like BB or bb)
- Heterozygous means having two different alleles for a gene (like Bb)
Remember: "Homo" = same, "Hetero" = different
Dominant and recessive alleles
Dominant alleles are always expressed in the phenotype, even when only one copy is present. We usually represent dominant alleles with capital letters (like B).
Recessive alleles are only expressed when two copies are present and no dominant allele is around. We represent recessive alleles with lowercase letters (like b).
Worked Example: Mouse Fur Colour
Let's examine how dominant and recessive alleles work:
Given information:
- B allele (dominant) = black fur
- b allele (recessive) = brown fur
Possible genotype combinations:
- BB genotype → black fur (homozygous dominant)
- Bb genotype → black fur (heterozygous - dominant allele shows)
- bb genotype → brown fur (homozygous recessive)
Key insight: Even though the Bb mouse carries the brown fur allele (b), it still has black fur because the dominant B allele masks the recessive b allele.
This explains why mice with the Bb genotype still have black fur. The dominant B allele masks the recessive b allele, so you can't see the brown fur characteristic unless both alleles are recessive (bb).
This masking effect of dominant alleles explains many patterns of inheritance. It's why two brown-eyed parents can sometimes have a blue-eyed child - if both parents are heterozygous (Bb), there's a chance their child could inherit two recessive alleles (bb).
Applying your knowledge
Understanding these genetic terms helps explain how characteristics pass from parents to offspring. Whether we're looking at fur colour in mice, flower colour in plants, or any other inherited trait, the same principles apply.
Remember that in real life, most characteristics are much more complex because they involve multiple genes working together. However, understanding these basic concepts gives you the foundation for more advanced genetic studies.
Remember!
Key Points to Remember:
- Gametes are sex cells formed by meiosis that carry genetic information
- Genes are DNA sections that code for specific proteins and create our characteristics
- Alleles are different versions of the same gene that create variety in traits
- Genotype is your genetic makeup (the alleles you have), while phenotype is what you actually look like
- Dominant alleles always show up in appearance, but recessive alleles need two copies to be visible