Genes

Season 05

Episode 03

Duration 23:11

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Every cell in your body carries a set of instructions that decided everything from the shape of your earlobes to the colour of your eyes. In this episode of Bill Nye the Science Guy, Bill unpacks how DNA, chromosomes and genes pass those instructions from parents to children.

In this episode you will discover:

What DNA is, and why it is shaped like a twisted ladder
Why every human cell holds 46 chromosomes, arranged in 23 pairs
How just four chemicals, A, T, C and G, spell out every gene
Why some traits are dominant and others are recessive
How mixing genes from two parents gives new combinations in every child
How the same genetic code turns up in every living thing on Earth

From a cell to a set of instructions

Bill zooms in from a droplet of blood to a single white blood cell, then to its nucleus, then to the tightly coiled chromosomes inside, and finally to the long, twisted molecule of DNA itself. That one molecule carries all the information to build a person. Every organism on Earth uses the same four building block chemicals, adenine, thymine, cytosine and guanine, arranged in different orders like letters spelling out different words.

Key concept
DNA
A long twisted ladder of chemicals in every cell that stores the instructions to build a living thing.

Key concept
Chromosome
A tightly coiled bundle of thousands of genes; humans have 46 in every cell, 23 from each parent.

Key concept
Inheritance
The passing of gene copies from parents to child, with one copy of each gene coming from each parent.

Bill Nye’s demonstrations

Bill shows a real white blood cell stained under the microscope so the DNA strands are visible, and then his DNA string reel of science unspools to show how enormous the molecule is compared to its width. He uses a lab coat pattern to make the point that just as many paper pieces are needed to sew a coat, many genes are needed to build a body. Later he cues up a two headed turtle at a zoo to show that mutations in genes happen naturally, and he compares a mule with a horse and a donkey to show what happens when you cross two species with different chromosome counts.

Try this at home

The earlobe survey. Look in a mirror and check whether your earlobes hang free or attach directly to the side of your head. Now ask your parents, grandparents, siblings and cousins to do the same. This is one simple trait controlled by genes, and you will usually see the same pattern running through the family. Attached earlobes tend to be recessive and dangling earlobes dominant, though real inheritance is more complicated than a single gene switch.

The tongue roll test. Try to curl the two sides of your tongue up into a tube. Some people can and some cannot, and it partly depends on a dominant gene copy from either parent. Compare across your family and see how the trait travels through generations.

Cool facts

Every cell in your body except red blood cells and mature sex cells carries a full copy of your DNA.
A mule has 63 chromosomes, from a horse parent with 64 and a donkey parent with 62.
Identical twins share the same DNA profile, which is why DNA fingerprints cannot tell them apart.
The four chemical letters of DNA, A, T, C and G, are the same in every living thing on Earth.

Science update: since this episode aired

Bill quotes about 80,000 human genes; work since finished the Human Genome Project settled that number at around 20,687 protein coding genes, though the genome also contains thousands of non coding genes that turn out to matter for health. Wellcome Sanger Institute
In 2022 the Telomere to Telomere consortium published the first truly complete, gapless sequence of a human genome, filling in the last eight percent that the original Human Genome Project could not read and adding almost 200 million bases and 99 new protein coding gene predictions. Nurk et al., Science, 2022
In November 2023 the United Kingdom became the first country in the world to license a medicine that edits DNA, when the MHRA authorised Casgevy, a CRISPR gene edited therapy for sickle cell disease and beta thalassemia. gov.uk, MHRA

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Curriculum focus

Built on NGSS MS-LS3-1 and MS-LS3-2 (how genes on chromosomes carry information and how sexual reproduction produces variation) and HS-LS3-1 (DNA sequences code for the proteins that make traits). Complementing that, it is pitched at KS3 biology (cells, inheritance and variation) and works as an introduction or revision aid for GCSE and IGCSE biology on genetics, DNA and inheritance, across boards such as AQA, Edexcel, OCR and Cambridge IGCSE. The worksheet and critical thinking questions take these ideas further than the episode alone.

NGSS MS-LS3
KS3 biology
GCSE revision
IGCSE revision
Genetics
DNA