The Big Idea
Genes are instructions written in DNA that tell cells how to make proteins. These instructions are passed from parents to offspring through chromosomes, and cells divide in different ways depending on the job: mitosis makes identical copies for growth and repair, while meiosis shuffles the instructions to create sex cells with new combinations of genes.
What You Need to Know
- Chromosomes are DNA packages; genes are instructions on them; alleles are different versions of a gene
- Sex inheritance is determined by chromosomes: females are XX, males are XY (the sperm decides the sex)
- Protein synthesis happens in two steps: transcription (DNA → mRNA) and translation (mRNA → protein at ribosomes)
- Mitosis creates two identical diploid cells; used for growth, repair, and asexual reproduction
- Meiosis creates four different haploid cells; used to make sex cells (gametes)
- Dominant alleles show up with just one copy; recessive alleles need two copies to appear
- Punnett squares predict offspring ratios from known parent genotypes
- Codominance means both alleles show up in the phenotype (e.g. blood group AB)
- Sex-linked traits are on the X chromosome; males are more likely to show recessive ones because they only have one X
Chromosomes, Genes & Proteins
What Is Inheritance?
Inheritance is the passing of genetic information from one generation to the next. It's the reason you look a bit like your parents and why you inherit certain traits—not because of the environment, but because of the DNA instructions they gave you.
The Structure: From Organism to DNA Base
Think of it like nested Russian dolls:
- Cell nucleus → contains chromosomes
- Chromosomes → thread-like structures made of DNA (46 in human body cells; 23 in gametes)
- Genes → short sections of DNA on a chromosome that code for a specific protein
- DNA → a double helix made up of four bases: A, T, G, C
ORGANISM
↓
NUCLEUS (in cell)
↓
CHROMOSOMES (46 in body cells)
↓
GENE (section of chromosome)
↓
DNA DOUBLE HELIX
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BASES (A, T, G, C in triplets = codons)
Key Terms: Genes and Alleles
Gene: A short length of DNA on a chromosome that codes for a particular protein (e.g. the gene for eye colour). It's like an instruction manual for one specific job.
Allele: A variation, or version, of that gene. For example, you have a gene for eye colour, but alleles for that gene might code for brown eyes, blue eyes, or green eyes. Different alleles = different instructions for the same job.
Why does everyone have two alleles for each gene? Because chromosomes come in pairs—you inherit one chromosome (and the genes on it) from your mum and one from your dad. So you get two copies of each gene, and they might be the same allele or different alleles.
Example: Eye Colour
You have a gene for eye colour. One allele (from mum) codes for brown eyes. The other allele (from dad) codes for blue eyes. You inherit both, so you have two alleles for eye colour—one brown, one blue. Whether your eyes are brown or blue depends on which one is dominant (we'll explain that soon).
Diploid vs Haploid
Why Does This Matter?
What to Memorise