• Written by Alison McEwen, Head of Discipline of Genetic Counselling, Graduate School of Health, University of Technology Sydney
Identical twins look the same, are the same sex, share the same birthday and shares the same genes. www.shuttershock.com , CC BY

If you have a question you’d like an expert to answer, send it to curiouskids@theconversation.edu.au.


Why are some twins identical and some not? - Chloe, age 12, Australia

We have spent many years explaining how genes work to people who would like to have children, so we’re happy to answer this excellent question.

There are two types of twins: fraternal and identical.

Fraternal twins may be born on the same day but are not genetically the same. They look different, have different genes and may be of the same sex or the opposite sex.

Identical twins, on the other hand, look the same, share the same birthday and share the same genes. They are the same sex, meaning they will both be girls or they will both be boys.

To understand why, we need to look at what happens at the time a pregnancy starts. We call the start of a pregnancy the time of conception.


Read more: Curious Kids: Why do people grow to certain sizes?


What happens when a woman becomes pregnant?

Most women produce one egg each month. Each time a man and woman have sex, the man produces thousands of sperm. If the man and woman have sex and do not use contraception (for example a condom, IUD or the Pill), there is a chance that a sperm will fertilise the egg and the woman will become pregnant.

Most of the time, a single egg is fertilised, and goes on to develop into a single baby. If the egg is not fertilised, the woman will soon have her period, which is the way a woman’s body prepares itself for a new egg to be fertilised the next month.

We call a fertilised egg a “zygote”. This is a good word to remember, as we use it to help us understand the different ways identical twins develop during pregnancy (and knowing a word like zygote might impress your science teacher one day).

Identical twins have come from a single egg and a single sperm, so they share the same genes as each other. Heather/flickr, CC BY-NC-ND

How do identical twins happen?

As you may know, genes are the instructions that tell our bodies how to develop and grow. They are like a recipe for creating each of us as unique individuals.

We have two copies of each of our genes: one from our biological mother and one from our biological father. That’s why we look like both our mother and our father.

Identical twins happen when a zygote splits into two in the first few days after conception. They have come from a single egg and a single sperm, so they share the same genes as each other. The reason the zygote splits is thought to be inherited, which may be why some families have a few sets of identical twins.

Because identical twins come from a single zygote that splits in two, they have exactly the same genes – exactly the same recipe. They will both have the same coloured eyes and hair, and will look the same. Identical twins are always the same sex too – they will both be girls or they will both be boys.

How do non-identical twins happen?

Identical twins are also called monozygous twins. This just means that they have come from the same, single zygote (mono means “one”). Non-identical twins are sometimes called dizygous twins (di means “two”, so dizygous means two zygotes).

Earlier on, we said that most women produce one egg each month. Occasionally, a women will produce more than one egg in a month. Non-identical twins happen when a woman produces two eggs (in the same month) and both eggs are fertilised by two different sperm.

Unlike identical twins, non-identical twins do not share the same genes as each other. They grow together and share the same birthday, but they are only as related as any other brothers and sisters. Non-identical twins could both be girls, or both be boys, or could be one girl and one boy twin.

Identical twins come from a single zygote that splits in two. Non-identical twins happen when a woman produces two eggs (in the same month) and both eggs are fertilised by two different sperm. Shutterstock

Interestingly, there are more non-identical twins in Australia now than there have been before. The number of twin pregnancies has grown over the past 30 years. This might be partly because women in countries like Australia are having children when they are older and the chance of a twin pregnancy increases as women get older (as they are more likely to produce more than one egg in the same month).

The chance of a twin pregnancy is also higher if a couple uses assisted reproductive technology to help them to become pregnant.


Read more: Curious Kids: how do babies learn to talk?


Hello, curious kids! Have you got a question you’d like an expert to answer? Ask an adult to send your question to curiouskids@theconversation.edu.au

Alison McEwen is the vice president of the Human Genetics Society of Australasia.

Chris Jacobs does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.

Authors: Alison McEwen, Head of Discipline of Genetic Counselling, Graduate School of Health, University of Technology Sydney

Read more http://theconversation.com/curious-kids-why-are-some-twins-identical-and-some-not-121435

Some Of The Best No Cost And Paid Options Of Making Videos For Instagram By Video Editing Tools

We, in our lifetime, have always thought of making an Instagram video. Because videos are more engaging and compelling than other kinds of posts, they serve and deliver a lot more value to the v...

News Company - avatar News Company

For a stress-free move – make the right checklist

It’s often said that success has less to do with the actions you take and much more to do with the quality of your planning and your goals. But great planning is about the right checklist; not...

News Company - avatar News Company

Are your kids using headphones more during the pandemic? Here's how to protect their ears

Shutterstock During the coronavirus pandemic, have your kids been using headphones more than usual? Maybe for remote schooling, video chats with relatives, or for their favourite music and Netflix sh...

Peter Carew, Lecturer, University of Melbourne - avatar Peter Carew, Lecturer, University of Melbourne

how Australia compares to the rest of the world

Molly Glassey/Pexels, CC BYWhen it comes to coronavirus cases, deaths and tests, Australia is performing better than many other countries with comparable populations and geographies, a new COVID-19 da...

Sunanda Creagh, Head of Digital Storytelling - avatar Sunanda Creagh, Head of Digital Storytelling

A Guide On How To Choose The Best Drinking Water Supplier

There’s something nice about having quick and convenient access to fresh crisp water. Here in Perth, there isn’t a shortage of water but most of it is a little on the salty side. That’s wh...

Aussie Natural - avatar Aussie Natural

Installation Guidelines and Standards for Suspended Ceilings

If you are thinking of installing a suspended ceiling in your residential or commercial premises in Perth, you may want to know about the standards that are required so that you can comply with ...

Heron Ceilings - avatar Heron Ceilings

The fascinating history of clinical trials

Wellcome Collection, CC BY-SAClinical trials are under way around the world, including in Australia, testing COVID-19 vaccines and treatments.These clinical trials largely fall into two groups. With o...

Adrian Esterman, Professor of Biostatistics, University of South Australia - avatar Adrian Esterman, Professor of Biostatistics, University of South Australia

Why does crowd noise matter?

Sporting codes are restarting as part of easing restrictions amid the coronavirus pandemic. In Australia, the NRL season has just restarted, the AFL will resume on June 11, and Super Netball will retu...

Alex Russell, Senior Postdoctoral Fellow, CQUniversity Australia - avatar Alex Russell, Senior Postdoctoral Fellow, CQUniversity Australia

Plates, cups and takeaway containers shape what (and how) we eat

ShutterstockHome cooks have been trying out their skills during isolation. But the way food tastes depends on more than your ability to follow a recipe. Our surroundings, the peoplewe share food with ...

Abby Mellick Lopes, Associate Professor, Design Studies, Faculty of Design, Architecture and Building, University of Technology Sydney - avatar Abby Mellick Lopes, Associate Professor, Design Studies, Faculty of Design, Architecture and Building, University of Technology Sydney



News Company Media Core

Content & Technology Connecting Global Audiences

More Information - Less Opinion