The Genome, Genes and Alleles

Your body is built and run using instructions stored as DNA. The genome is the word for all of that DNA together — the entire DNA of an organism.

Think of the genome as the complete instruction manual for building and running a living thing. Every organism (a human, a dog, a buttercup) has its own genome.

• The genome contains all the genetic information an organism needs.
• It is made of the chemical DNA.
• A small part of the genome at a time is read to make the proteins the organism needs.

Exam tip. The exact marking phrase for spec 3.14 is "the genome is the entire DNA of an organism". Saying "all the genes" is close but the precise definition is all the DNA.

A gene is a section (length) of a molecule of DNA that codes for a specific protein. In other words, a gene is a short part of the DNA that carries the instructions to build one particular protein.

Why do proteins matter? Proteins do most of the work in your body. They include:

• enzymes (which speed up the chemical reactions in cells),
• structural proteins (such as those in muscle), and
• many other molecules that control your features.

Because each gene codes for a specific protein, and proteins control your characteristics, your genes ultimately decide many of your inherited features.

A useful way to picture it:

DNA  →  (a gene = one section of the DNA)  →  codes for  →  one specific protein

Exam tip. Use the full phrase: a gene is "a section of a molecule of DNA that codes for a specific protein". Two marking points are often hidden here — section of DNA and codes for a (specific) protein.

Genes are not floating loose inside a cell. They are located on chromosomes, and chromosomes are found in the nucleus of the cell.

A chromosome is a long, coiled-up molecule of DNA. Because a chromosome is made of DNA, and a gene is a section of DNA, you can think of a chromosome as a string carrying many genes along its length.

So the whole structure nests like this, from the largest to the smallest part:

Exam tip. A frequent 1–2 mark question asks "where are genes found?" The full answer is "on chromosomes, in the nucleus". Naming both the chromosome and the nucleus secures the marks.

A single gene can exist in slightly different versions. These alternative forms of a gene are called alleles.

For example, there is a gene that affects eye colour. One allele of that gene may lead to brown eyes, while a different allele may lead to blue eyes. The gene is the same "address" on the chromosome, but the allele (the version present) can differ from person to person.

Because alleles are slightly different versions of the instructions, they give rise to differences in inherited characteristics:

• Different alleles → slightly different proteins (or amounts of protein).
• This leads to variation in features such as eye colour, between individuals.
• Alleles are passed from parents to offspring, which is why characteristics are inherited.

You usually inherit two alleles of each gene — one from each parent — though the full detail of how they interact (dominant/recessive) belongs to later inheritance topics.

Exam tip. Keep the words straight: a gene is the instruction for a feature; an allele is a version of that gene. The spec phrase is "alleles… give rise to differences in inherited characteristics".

These four terms are easy to muddle, so it helps to see how they fit together — from the biggest idea to the smallest:

Term	What it is	Key point
Genome	The entire DNA of an organism	All the genetic information
Chromosome	A long, coiled molecule of DNA in the nucleus	Carries many genes; found in the nucleus
Gene	A section of DNA	Codes for a specific protein
Allele	An alternative form of a gene	Gives differences in inherited characteristics

A one-line way to remember the order of size:

Genome (all the DNA) → Chromosomes (in the nucleus) → Genes (sections of DNA) → Alleles (versions of a gene).

And the cause-and-effect chain for variation:

1. A gene codes for a protein.
2. Different alleles make slightly different proteins.
3. Different proteins give differences in inherited characteristics (variation).

Exam tip. Many marks are lost by swapping gene and allele. Lock in: a gene is the instruction; an allele is a version of that gene.