What is Inheritance?

In science, the word “Inheritance” is used to mean the information that is passed from a parent to its child. This information is important. It determines how the child will look, how the child behaves and how healthy it might be. Each parent gives every child they create a set of instructions. This set of instructions is unique for each child. The child will still be similar to the parent, but not identical.

Scientists can compare the differences between the instructions from all types of living things. They can use this information to work out where we came from and how it happened.

Imagine your friend’s cat is about to have kittens and you have been told you can have one. The Mum cat is a beautiful colour with massive eyes and a lovely fluffy tail. You are looking forward to the babies arriving. When they are born you rush round to pick out the one you want. But they don’t look like their Mum. They all have different coloured coats.  Most of them don’t have fluffy tails either and only some have large eyes.

That’s when you see a picture of the Dad cat. He is a completely different colour to the Mum with a flat tail and small eyes. The kittens are a mix of both parents (but still adorable anyway).

So no expensive house, money or art collection?
No! This is “genetic inheritance” and it is all about the stuff that is used to make you in the first place.


So what do we get again?
Two sets of instructions. One from each parent.        


Is that all?
They are very important sets of instructions……


How important?
Vital. You could not exist without them.


How do they work?

Every single living thing needs at least one set of instructions in order to remain alive. Humans have two (scientists call this diploid). Your instructions are for really essential things such as breathing and keeping your heart beating. Less essential things such as how tall you are or what colour your hair is. Medical things such as whether you need glasses or how you might react to certain food or chemicals. Emotional things like how optimistic you might be. They also include everything else needed to make you.

Parents produce babies that will grow up to be similar to them. So a cow will have calves. A hen will have chicks. A chestnut tree will produce another chestnut tree and your parents will make you.

So you need this information to live?

Not only to live, but to be born at all. The body is made up of trillions of small parts called cells. Lots of these cells work together to make bigger parts of you. For example, skin cells make sure you are waterproof. Blood cells make sure you get enough oxygen. (They are also quite useful at letting you know when you have cut yourself!) Liver cells join together to make a weird triangular shape and take all the unhealthy bits out of your food and drink.

Each of these cells needs to know what to do. They also need to know when and how to do it. That is what the instructions are for. They tell the cells how to make you in the first place and how to keep you alive.

Inheritance_Diploid Instructions

When do we get these instructions?

You get one set of instructions from your Mum and one set from your Dad. Each set contains a mixture of the instructions they got from their parents. Mums have a cell called an egg. It contains a single set of instructions, including information on how to be a girl.

Dads have cells known as sperm. Each sperm cell also has a single set of instructions. There are two different types of sperm cell.  Some sperm cells contain one set of instructions, including information on how to be a boy. Other sperm cells contain one set of instructions including information on how to be a girl.

One sperm and one egg have to get together to produce a baby. Whether that baby is a boy or a girl will depend on which type of sperm cell was used.

When the egg and the sperm meet, they fuse together to make one single cell.  The single cell now has two sets of instructions – both of which are different from the parent they came from. These are now your instructions (also known as DNA) and unique to you.


But I’m not a single cell!
No you are not.  The original cell copies itself and your precious instructions. It then splits into two. These two cells then copy themselves (and their instructions) and also split into two. Four cells now do the same. This carries on over and over again until eventually there are trillions of cells each doing different jobs. When there are enough, a baby  – that’s you – will be born.


With instructions from both parents telling me what to do?

Well, telling your cells what to do. BUT not all instructions are equal. Some of the instructions from your Mum are more important than those from your Dad. Other instructions from your Dad will be more important than those from your Mum.

The more important instruction is called dominant. You will almost certainly see the results of this one!

The less important instruction is called recessive. You might not see the results of this one.

Depending on which type of instructions you have, you may look more like one parent than another. You may look like a mix of both parents or you might not look like either parent.


Mum's Instructions


How does the cell know which is which?

It doesn’t. A cell can only do what the instructions tell it to do. If both instructions tell it to make something, it will. If both instructions tell it not to make something, it won’t. If one instruction tells it to make something, and the other instruction tells it not to, it will do both.

We can see whether a cell has made something or not in our bodies. This is called a trait.

There are dominant traits. These can be seen when only one instruction tells the cell to do something.

There are recessive traits. These can only be seen when both instructions tell the cell to do the same thing.


Dad's Instructions


What does a trait look like?

Each trait is different. Traits are characteristics and they all combine in different ways to make you who you are. Dimples are a slight depression in the cheek when a person smiles.  If even one instruction tells the cell to make dimples it will. We can then see those dimples when that person smiles. Dimples are a dominant trait.

Straight hair is also a trait. Both sets of instructions are needed to tell the cell to make straight hair before we see it. Straight hair is a recessive trait.

Your hair, eyes or skin colour are all traits. As are your height and length of your limbs. Whether you are allergic to something or how fast you grow.


Sarah's Instructions
Sarah's Instructions


So why do I look like Aunt Nellie?

You and Aunt Nellie show recessive traits. These traits don’t show up in your Mum because she has dominant traits that hide the recessive ones. BUT your Mum (and your grandparents) have passed recessive instructions for some traits to you (and Aunt Nellie). Your Dad also passed on the same recessive traits (that he got from one of his parents). Because both of your parents have passed on the same recessive instructions, the traits show up.

Sarah is off to the disco. Practising her best moves in front of the mirror, her straight red hair flies in all directions. Curly hair, like her parents’, wouldn’t move like that. She has used make-up and hopes her Mum won’t find out. Sarah is the only one to have green eyes. She smiles at her reflection and her button nose. She just wouldn’t look the same if she had a big nose like her parents.

Sarah’s parents show dominant traits and so need instructions from just one of their parents (remember everyone has instructions from both parents) to make them visible. Sarah shows recessive traits. The same instructions from both her Mum and her Dad are needed to make them appear.

Is inheritance just for humans?

No. Every living thing passes on their instructions to the babies they produce.

(Not everything needs a Mum and a Dad though.)


Why is it important?

If no instructions were passed on, it would be impossible to have many living things at all. They would have to start from scratch every single time. If the instructions were identical each time, every living thing would be the same. It is important that we are all different.

These differences are known as variation.  We look slightly different. We act slightly differently. We all have slightly different skills and we can share these to help each other.

More importantly, however, the environment – the habitat we live in – is always changing, even though we may not notice. Some variations may work better in a different environment. These variations may become very important. It may become impossible to live in a certain place without them.

Using variation as a way of living in a specific environment is known as adaptation. It is a fundamental part of evolution.