Genetic linkage is a key principle in understanding the inheritance of traits. Why do some characteristics always seem to occur together? Dive into this lesson to see exactly what it means for genes to be linked.
Definition of Genetic Linkage
Genetic linkage is the coupling of two genes’ patterns of inheritance because they are located on the same chromosome.
But what does that actually mean?
The recipe of life is written in a DNA code. Long strings of DNA form genes. Each gene codes for part of the human recipe. Since there are different versions of genes, called alleles, we each have a slightly different recipe, which makes us unique. Our genes are organized into 23 chromosomes. Each chromosome is a long string of many, many genes attached together. Humans have two copies of each of the 23 chromosomes, and thus, they have two copies of each gene.
Genetic information must be passed along to the next generation. Humans reproduce sexually, and the mother and father each contribute one copy of each gene. Thus, the resulting offspring will correctly have two copies, one from each parent. In order to only pass along one copy of each gene, the gametes, which are just egg or sperm cells, undergo a process called meiosis that splits the DNA in half.
In most instances, it is a 50/50 shot determining which copy is included in each gamete. However, since chromosomes are the traveling unit during meiosis, some genes have to travel together if they are on the same chromosome. In other words, they are genetically linked.
Genetic Linkage Example
Remember, genes often come in more than one allele. This means that it matters which of the two copies gets packaged in a gamete, since the two copies might be different alleles.
Imagine that chromosome-1 has a gene for hair color and a gene for eye color. Perhaps one of your copies of chromosome-1 contains alleles for red hair and red eyes, and your other copy has alleles for blue hair and blue eyes.
Since these genes are on the same chromosome, if a gamete gets the allele for red hair, it also gets the allele for red eyes. A gamete can’t get red hair and blue eyes because the genes are not distributed individually. Meiosis distributes one copy of each chromosome to a gamete rather than distributing one copy of each gene.
Genetic Linkage Analogy
Imagine that a chromosome is like a calendar, and the different months are the different genes within that calendar. Each calendar has the same 12 months (genes), but the calendars have different pictures (different alleles). Maybe Calendar A has a snowy farm scene in January and a summer garden in June. Maybe Calendar B has an Arctic fox in January and puppies in June.
If a person (gamete) needs one copy of each month (gene), they are given a full attached calendar unit because it is easier than distributing each month individually. However, this means that the Arctic fox picture is always passed together with the puppies picture. They are linked. Nobody gets the Arctic fox and the summer garden pictures (alleles) because those are on different calendars (chromosomes).
Now, let’s add in chromosome-2 to our original example. Chromosome-2 has a gene for height. One chromosome copy has an allele for tallness, and the other has an allele for shortness.
If a gamete gets the chromosome with alleles for red eyes and red hair, it could get packaged with either version of chromosome-2. It could be packaged with the chromosome with the allele for tallness or the chromosome with the allele for shortness. The height gene and the hair color gene are unlinked, so they are not coupled together when it comes to inheritance.
If we go back to our calendar analogy, chromosome-2 would be like a daily planner instead of a monthly calendar. It wouldn’t matter which monthly calendar you have, you could still get any of the daily planners. One option does not influence the other. Of course, once you’ve picked a daily planner, then the January 1st picture is stuck together with the June 1st picture, since those are linked within the daily planner.
The monkey wrench in this simple lesson is recombination. This is a process early in meiosis where the two versions of a chromosome swap some DNA. In our case, if the red hair gene gets swapped with the blue hair gene, then red eyes will now be packaged on a chromosome with blue hair. Now these alleles are linked instead of the original red alleles. It is truly a game changer.
Genetic linkage is caused when two genes are located on the same chromosome. A chromosome is a long string of many, many genes attached together. The two genes will travel together during meiosis, which is important for separating chromosome pairs. This means that the linked alleles, a different version of genes which are often inherited together, will occur in the same gamete, egg or sperm cells, more often than if the genes were on different chromosomes, which we can figure out by looking at how often they occur together in new offspring. But, as we discovered, there’s a monkey wrench that can be thrown into this whole thing: recombination, which is a process early in meiosis where the two versions of a chromosome swap some DNA. It disrupts typical genetic linkage, but it can really be called a game changer.