Chromosomes and genes | Inheritance and variation | Middle school biology | Khan Academy

This is a super cute puppy. He has a pink tongue, black fur, and a very friendly personality. We know that when this puppy grows up, he will have a healthy weight of about 70 pounds. He will love to play fetch and enjoy snuggles with his human family.

We know this because these traits are typical of his breed: Labradors. But where do these traits come from? In this video, we're gonna find out. But first, let's start at the beginning.

What exactly are traits? Traits are specific observable characteristics. They can be physical, like our puppy's eye color. They can be behavioral, such as a dog being more or less aggressive towards strangers. Or they can be health-related, such as a dog's risk of developing a certain disease.

Traits are passed from parents to offspring. This is why our puppy looks so similar to his parents, who were also Labradors. But how does this occur? Well, traits are passed on from one generation to the next through the inheritance of genes, which are pieces of hereditary material that are passed from parents to offspring.

Genes are found inside of an organism's cells as part of structures called chromosomes. In multicellular organisms, chromosomes are found inside the cell nucleus. Chromosomes are often shown as x-shaped structures like this one. Each chromosome is a structure that contains one long molecule of DNA, which stands for deoxyribonucleic acid.

All right, sing it with me: deoxyribonucleic acid, or DNA for short. Here's a model showing a small stretch of a DNA molecule. You can see that it's made up of lots of different atoms represented by these colored balls. The DNA molecule in a chromosome is like this, but much, much longer.

A chromosome's DNA molecule is coiled up in an organized way. This diagram shows what the DNA would look like if it were stretched out. This allows us to see where a chromosome's genes are located. Genes are specific stretches of a chromosome's DNA molecule. Each chromosome's DNA can contain hundreds or even thousands of genes.

In multicellular organisms, each cell contains multiple chromosomes. It's the collection of genes and chromosomes that contain the information about all of an organism's inherited traits. Like our puppy here, his chromosomes contain thousands of genes to help him determine his coat color, eye color, and all the other traits he inherited from his parents.

So our puppy must have one or more genes that cause him to have black fur. If he had been born with different versions of the fur color genes, he may have had yellow or brown fur. Heck, maybe one day someone will figure out how to change a puppy's DNA so that he has pink fur.

And who knows? Maybe you'll be the geneticist to make it happen. That's it for today. Thanks for watching, and I hope to be seeing some pink puppies one day soon.