Chromosome pairs | Inheritance and variation | Middle school biology | Khan Academy

• [Instructor] - Hi, everyone. Today, we're gonna talk about chromosome pairs. But first, I've got a question for you. Between a black mulberry plant, a Guinea pig, and a human being, which organism do you think has the most chromosomes?

The mulberry plant has green leaves and delicious berries. The Guinea pigs are adorable, tiny mammals with multiple body systems, and humans have incredibly powerful brains and the ability to form complex social relationships.

Before I tell you the answer, let's learn a little more about chromosomes. Chromosomes are structures that contain an organism's genes. And genes are the units of heredity passed from parent to offspring that affect an organism's traits.

Chromosomes, and the genes they contain, are located inside an organism's cells. In multicellular organisms, the chromosomes are found within the cell's nucleus. Many species have chromosomes that come in matched pairs. For example, here's an image taken with a microscope of chromosomes from a typical human body cell.

The chromosomes are laid out in an organized way, allowing us to see that humans have 46 chromosomes, which can be organized into 23 pairs. Each of these is a pair of chromosomes. This is one chromosome, and this is the second chromosome. These paired chromosomes are called homologous chromosomes.

Homologous chromosomes have the same size and shape. They also contain the same genes in the same locations. Remember, an organism's genes affect its traits. Genes often come in different variations, called alleles. Different alleles of a gene can cause different versions of a trait.

In an organism, a pair of homologous chromosomes may have the same allele for a gene, or they may have different alleles for a gene. To help us understand this, let's take a look at an illustrative example using pea plants. Pea plants can have pink or purple flowers. These colors are determined by two different variations, or alleles, of a single flower color gene.

Pea plants have a copy of this flower color gene on each of two chromosomes in a homologous pair. A single pea plant may have two copies of the pink allele, two copies of the purple allele, or one copy of each. The color of flowers that the pea plants end up with depends on how the alleles interact. But we'll cover that in another video.

An image like this one, of an individual's chromosomes, lined up in an organized way, is called a karyotype. Scientists and medical professionals can examine an individual's karyotype under the microscope to see if they might have a genetic condition. This could show up as size differences between chromosomes within each homologous pair or the presence of an extra chromosome.

Now that you're a little more familiar with chromosomes, let's return to the question I asked at the beginning of this video. Between black mulberry plants, Guinea pigs, and humans, which organism has the most chromosomes? Any guesses?

The black mulberry plants do! These plants have a whopping 308 chromosomes. Guinea pigs come in second with 64 chromosomes. And like we said earlier, us humans only have 46 chromosomes. So we come in third. Are you surprised?

Believe it or not, humans, with their big brains and complex social structures, actually have far fewer chromosomes than many seemingly less complex organisms. Are black mulberry plants more complicated than we perceive? Perhaps. And perhaps there is more to understanding the role chromosomes play in determining traits.

That's it for today, thanks for watching.