Genetics, but bitesize.

Part 1Every cell has an instruction book

Your body is made of about 30 trillion cells. Inside almost every single one of them, there's a tiny instruction book that tells the cell what to do — what colour to make your eyes, how tall to grow you, what shape your nose should be.

That instruction book is called DNA. It lives in a part of the cell called the nucleus. The DNA is folded up tightly into bundles called chromosomes, and small sections of the DNA — the bits that actually do a job — are called genes.

Part 2Where did your 46 chromosomes come from?

You have 46 chromosomes in (almost) every cell of your body. Half of them came from your biological mum, and the other half came from your biological dad.

It works like this. A sperm cell from dad carries 23 chromosomes. An ovum (egg cell) from mum also carries 23 chromosomes. When they join together — that's called fertilisation — they make a single new cell called a zygote. Add the two halves together: 23 + 23 = 46. Job done.

That's why you might have your dad's hair colour but your mum's eye colour. You inherit a mix.

Part 1Differences are everywhere

Look around your family. Even if you share genes with them, none of you are identical. You might be taller, your sister might have curlier hair, your dad might have a different blood group.

The differences between living things — even in the same species — are called variation. Variation comes from two sources:

1. The genes you inherit from your parents (genetic variation).
2. The environment you grow up in — your diet, your activity, your weather (environmental variation).

Most things about you are a mix of both. Your maximum possible height was set by your genes — but if you don't eat enough as a child, you'll end up shorter than that maximum.

Part 2Two flavours of variation

Variation comes in two types, and you need to be able to tell them apart.

Part 1The twisted ladder

DNA isn't a straight line. It's two strands that twist around each other to form a shape called a double helix — imagine a ladder, then twist it so it spirals. That's DNA.

The "rungs" of the ladder are made of pairs of bases. There are only four types of base in DNA. Their names are long, but you only need to know their letters: A, T, C, G.

Part 2The bases pair up — but only in fixed ways

Here's the clever bit. The four bases (A, T, C, G) only pair up in two specific ways:

• A always pairs with T
• C always pairs with G

That's it. No other combinations. We call this complementary base pairing. So if one strand has the sequence A-G-C-T, the other strand must read T-C-G-A.

Part 3How we found out

Nobody knew what DNA looked like until 1953. Four scientists were involved:

Rosalind Franklin and her assistant Raymond Gosling at King's College London used X-rays to take a famous photo of DNA — known as Photograph 51. It showed the twisted shape clearly.

James Watson and Francis Crick, working in Cambridge, were trying to build a model of DNA. They saw Franklin's photograph (without her knowing) and used it to figure out the double helix structure. They published their model in 1953 and became famous for it.

Franklin died young in 1958 and never received a Nobel Prize for her work. Today, scientists make sure new findings go through peer review — other scientists check the work before it's published — to keep the process fair and accurate.

Part 1Different versions of the same gene

Remember: a gene codes for a feature, like eye colour. But there's more to it. Most genes come in different versions, called alleles.

For example, the gene for eye colour has different alleles. One allele might give you brown eyes. Another might give you blue eyes.

You inherit two copies of every gene — one from your mum, one from your dad. So you have two alleles for each feature.

Part 2Predicting what a child might inherit

Scientists use a tool called a Punnett square to work out what alleles a child might inherit. Don't worry — you don't need to draw one yourself. Just understand what it shows.

Part 1What is a genetic disorder?

A genetic disorder is a health condition caused by a fault in someone's DNA. The fault might affect just one gene, or several. Because DNA is passed from parent to child, genetic disorders can be inherited — they run in families.

Two examples you should know:

Part 2Carriers — people who have it but don't have it

This is where Lesson 4 comes back in handy. Both cystic fibrosis and sickle cell anaemia are caused by recessive alleles. So you only have the disorder if you've inherited two faulty copies — one from each parent.

If you only have one faulty copy, you don't have the disorder yourself — the working allele takes care of business. But you still carry the faulty allele, and you can pass it on to your children. We call this person a carrier.

Two carriers having a baby together is exactly the same situation as the Bb × Bb Punnett square from Lesson 4. There's a 1 in 4 chance any child they have will inherit two faulty copies and therefore have the disorder.

Part 3Should we test for it?

Today, doctors can test someone's DNA to find out if they're a carrier of a genetic disorder. Some couples planning a baby choose to be tested. Embryos can also be tested before being implanted (this is called genetic screening).

This raises ethical questions — there isn't a single right answer:

• Should parents be told if their unborn child has a genetic disorder?
• If a couple find out their baby will have a disorder, what should they do?
• Could screening lead to people only choosing "perfect" babies?

You don't need to memorise an opinion on this. You just need to know that genetic testing exists, and that different people have different views on whether and when it should be used.