Curious Kids: Why do people grow to certain sizes?

This is an article from Curious Kids, a series for children. The Conversation is asking kids to send in questions they’d like an expert to answer. All questions are welcome: find out how to enter at the bottom. You might also like the podcast Imagine This, a co-production between ABC KIDS listen and The Conversation, based on Curious Kids.

Why do people grow in certain sizes? - Audrey, age 6, Brisbane.

About 150 years ago, there was a very curious English person who asked the same question. His name was Sir Francis Galton.

One day, Sir Francis Galton looked at his friends. He saw that most of his taller friends had taller parents and most of his shorter friends had shorter parents. Francis Galton was one of the first scientists to do some tests to work out why this was the case. He published his findings in a book. Not all of his ideas were correct, though. Some of his ideas were actually very wrong. But he made a start on what we now call “genetics” or the science of genes. I’ll explain what that means.

Children and parents

Children with brown hair often have parents with brown hair. Children who are good runners, often have parents who are very good runners. Children who are a bit shy often have parents who can be a bit shy.

Like parents and children, brothers are sisters are quite alike too.

Do you look like your brother or sister? Have you had grown-ups saying to you: “Oh, you look just like your mum (or dad)!”

The reason behind all this is a thing called DNA. That stands for “deoxyribonucleic acid”, but don’t worry, everyone just calls it DNA.

Humans have a special code, and it’s called DNA

Every human carries an instruction booklet with a very special code. Actually, we carry trillions of instruction booklets. In each booklet, the same special code is written. The code, called DNA, is made out of only four letters, A, C, T, and G. This looks simple, but it is very cleverly set up.

Our eyes can’t read the code, but our bodies can. The code tells our body what to do and how to look. For example, it tells our hair to grow curly or straight, or to make our eyes brown or blue. But also, how much to grow and when to stop growing. Some people have instruction booklets with a code that tells their bodies to grow tall. Other people have a code that tells their body to grow to a smaller size.

Did you know that DNA code is unique for every person? That means there is nobody in the entire world with the same code as you – unless you have an identical twin brother or sister.

Read more: Curious Kids: How do we get allergic to food?

Your code is very similar to your biological parents’ code. This is because they pass on their code to you. You share half of your DNA code with your mother and half of your code with your father. If you’re adopted – or your parents used a donor egg or donor sperm – then you share half your DNA code with the person whose egg and person whose sperm were used to make you.

So your code that tells your body what size to grow in is similar to your biological parents’ code on what size to grow in.

Even though our DNA code is very similar to our biological parents’ DNA code, we all still turn out a little bit different. This is because you have your own experiences.

Experiences are a part of being human

Every human being has experiences. An experience is something we do, or something that happens to us. Eating is an experience.

Some experiences we share, others we experience on our own.

For our body to follow the code in our instructions booklets, it needs energy. Energy comes from eating food, and more importantly, eating healthy food. If we don’t eat, we won’t grow. Even if the code in our instruction booklets is telling our body to grow tall. Some children get to eat lots of food that makes them grow. Other children may not get enough food or don’t eat healthy food.

Getting sick is also an experience. Some diseases may make you grow less. These days, we are getting sick less than humans did in the past and have more healthy food than the people who lived a long time ago. That’s why we are all a bit taller than the people who lived a long time ago.

So, both your DNA code and your experiences make you grow to a certain size.

Read more: Curious Kids: Why do we have bones?

Hello, curious kids! Have you got a question you’d like an expert to answer? Ask an adult to send your question to us. You can:

* Email your question to curiouskids@theconversation.edu.au * Tell us on Twitter by tagging @ConversationEDU with the hashtag #curiouskids, or * Tell us on Facebook