# Genetic Linkage: Problems, Mapping & Studies

Instructor: Stephanie Gorski

Steph has a PhD in Entomology and teaches college biology and ecology.

In this lesson, we'll discuss what happens when genes don't assort randomly. We'll then discuss how to calculate this non-randomness. It's easier than you might think!

## How Genes Get Reshuffled Each Generation

Are you a ginger - someone with pale skin, red hair, and freckles - or do you know a ginger? Why is it that gingers usually have blue or green eyes? The answer has to do with genetic linkage.

You know that genes get shaken up as they are passed down from generation to generation. That's why you probably look a little bit like your biological mom and dad, but not exactly like either one of them. Your genetic information is packaged into 23 sets of chromosomes. Half of your dad's chromosomes went into each one of his gametes, or sex cells, and half of your mom's chromosomes went into each one of her gametes. Each gamete has only half as much genetic material as other cells; otherwise, you'd end up with too many genes! You are the result of the fusion between one of your dad's gametes and one of your mom's.

Say you are looking at two different genetic traits, one of which is on Chromosome 7 and one of which is on Chromosome 8. Your dad has two copies of the gene for Trait A and two copies of the gene for Trait B. He might have given you the copy of the gene for Trait A that he got from his mom, and the copy of the gene for Trait B that he got from his dad. Or the reverse. Or he could have given you the copy of the genes for both Trait A and Trait B that he got from his mom, or from his dad. It's anyone's guess.

But what if two different traits are located on the same chromosome? Say the genes for Trait C and Trait D are both located on Chromosome 9. Now, because those genes are packaged on the same chromosome, the rules of chance work differently. It's more likely that they will travel together. In other words, if your dad gave you the copy of the gene for Trait C that he got from his mom, it's more likely you'll get the gene for Trait D from his mom as well. If your dad gave you the copy of the gene for Trait C that he got from his dad, it's more likely that you'll get the gene for Trait D from his dad. This increased chance of inheriting two traits that are close together on a chromosome is called genetic linkage.

## Crossing Over

That doesn't mean that the genes for Trait C and Trait D always travel together. There is a process called crossing over that occurs when gametes are being formed during meiosis. During crossing over, chromosomes twist around each other and swap genetic information. Crossing over is important to keeping our gene pool diverse!

If Trait C and Trait D are far apart from each other on the chromosome, there is a good chance that crossing over will happen between them. If they're very close together, it's unlikely that crossing over will happen between them.

Traits determining skin and eye color are complex and they occur in multiple locations, but most of them occur on Chromosome 15 and Chromosome 19. Because they're fairly close together, the trait that produces the pigments for hair color and the trait that produces the pigments for eye color are often genetically linked - they will pass on to the next generation together. The traits for red hair and green eyes probably both first evolved in the British Isles. Since they are located in about the same place on the genome and arose from the same population, they often travel together.

## Calculating Distance

Now let's try an example. Since real-life inheritance of red hair and blue eyes is complex and covers multiple chromosomes, let's imagine inheritance of purple hair and glow-in-the-dark eyes for our example. Suppose you want to map the difference between the genes for purple hair and glow-in-the-dark eyes.

Let's represent the purple hair gene with P and the glow-in-the-dark eye gene with G. Each gene has two alleles (P and p, G and g) that represent different variations on the same gene.

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