Independent assortment of genes and linkage

Definitions & Key takeaways

Genes are randomly assorted during the production of sperm and eggs. This is called independent assortment. This process is responsible for the different combination of genes that children receive from their parents.

If two genes are located close together on a chromosome, they are said to be linked. If these genes are passed on to a child, they will tend to be passed on together.

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Inheritance is possible because of chromosomes. These chromosomes come in pairs - one from mom and one from dad - so they’re called homologous chromosomes.
Each chromosome has genes, which are segments of DNA that carry genetic information for a specific trait. And different versions of the same gene are called alleles.
As an example, brown eye color and blue eye color are both alleles for the eye color gene. And each parent offers one allele of a gene.
Now, these alleles can be either dominant often represented with a capital letter, or recessive, represented with the corresponding lowercase letter, the difference being that it only takes one dominant allele for its traits to be expressed, whereas it takes two recessive alleles for its traits to be expressed.
Human somatic cells - that is, all of the cells aside from the sperm and eggs, which are called gametes - have 23 pairs of chromosomes; 22 somatic pairs and one sexual pair - adding up to 46 chromosomes in total.
These chromosomes, along with the alleles they carry, segregate during meiosis - which is the process of making new gametes.
Gametes only carry half the genetic information of the parent - so 23 chromosomes. Once the male and female gametes merge during fertilization, their alleles combine to make the genotype —or genetic information— of the new organism.
For every gene, alleles can combine to give rise to three possible genotypes, homozygous dominant - or AA, heterozygous - or Aa - and homozygous recessive - or aa.
This determines all of a person’s features —or phenotype— such as eye color, hair color, or even whether or not they’re color blind.
Now, independent assortment means that no matter which alleles an organism inherits for one gene that codes for a trait like eye color, it won’t affect the alleles it inherits for another gene that codes for a different trait, like hair color.
Let’s start with a simple example. Let’s represent the eye color gene with the letter “a” and the hair color gene with the letter “d”.
Now, the dominant allele for eye color - A - stands for brown eyes and the recessive allele - a - stands for blue eyes. On the other side, the dominant allele for hair color - D - stands for dark hair while the recessive allele - d - stands for blond hair.
So let’s say we have a person with a heterozygous genotype —Aa— for eye color and heterozygous genotype —Dd— for hair color.
This person would have the dominant allele features - so brown eyes and dark hair, even though they still carry a recessive allele for blond hair and blue eyes.
Now, at the molecular level we know that the eye color gene is physically located on a pair of homologous chromosomes, and in this case, let’s say that the chromosome from mom carries the dominant allele —A—, and the chromosome from dad carries the recessive allele —a— .
Similarly, let’s say that the hair color gene is actually physically located on another pair of homologous chromosomes. And let’s say that the chromosome from mom carries the dominant allele —D— , and the chromosome from dad carries the recessive allele —d— .
Now in meiosis, different pairs of homologous chromosomes independently separate into different gametes. In other words, how one pair of homologous chromosomes splits into daughter cells does not affect how another pair of homologous chromosomes decides to split into those same daughter cells.
As a result a person that has a heterozygous genotype for both hair color and eye color can produce four different types of gametes: One that carries the two chromosomes from the mother, and thus the dominant alleles A and D.
One that carries the two chromosomes from the father, and thus the recessive alleles a and d. One that carries the first chromosome from the mother and the second from the father, and thus the alleles dominant A and recessive d.
And one that carries the first chromosome from the father and the second from the mother, and thus the alleles recessive a and dominant D.
So far so good, but let’s bring two more genes along! One that determines the skin color, let’s represent it with the letter “b”.
So the dominant allele for skin color - B - stands for dark skin and the recessive allele - b - stands for white skin. And another one that determines the type of earwax someone has let’s represent it with the letter “c”.
So the dominant allele - C - stands for wet earwax, while the recessive allele - c - stands for dry earwax. Now, let’s say these two genes are physically located on the same chromosome as the eye color gene “a”, and that the skin color gene “b” is really close to “a”, while the earwax type gene “c” is far away from “a” - like at the opposite end of the chromosome.
Now, since the three genes are on the same chromosome, one may think that they will be inherited together, but it turns out that this is not always the case.
The reason is that a funny thing happens in meiosis - called crossing over, which is where homologous chromosomes exchange equivalent parts of themselves which carry the same types of genes, and then after they’ve swapped - the two chromosomes part ways.
For example, let’s say the chromosome that comes from mom carries all dominant alleles for these genes - so A, B and C - and the chromosome that comes from dad carries all recessive alleles - so a, b and c.
Now, if crossing over occurs between the two chromosomes at the ends of the two chromosomes, then the dominant allele C that came from mom would end up on the chromosome that came from dad, and vice versa.
So the chromosome that came from mom would end up carrying the original dominant alleles - A and B - for eye color and skin color, and a brand new recessive allele - c - for earwax type.
Whilst the chromosome that came from dad would end up carrying the original recessive alleles - a and b - for eye color and skin color, and the dominant allele - C - for earwax type.
This is how, following crossing-over, two genes initially located on the same chromosome can end up either as part of the same gamete - like genes “a” and “b” - or they can end up in different gametes - like “a” and “c”, or “b” and “c”.
Now, crossing over can happen really at any point along the chromosome, so let’s say it occurred between gene “a” and gene “b”.
In that scenario, gene “b” and gene “c” would swap over together to the other chromosome while only gene “a” would remain on the original chromosome.
Now although, this can happen, it’s pretty uncommon. That’s because crossing over has an equal chance of occurring at any point along a chromosome, and since there’s much more of the chromosome between “b” and “c” then there is between “a” and “b” - that’s the most likely place for crossing over to occur.
Another way to think about it is to assign every possible spot where crossing over can occur a number - and for simplicity let’s say that there are 100 potential spots evenly distributed along the chromosome.
And let’s say that “a” is a gene near number 5 and “b” is a gene near number 10, and “c” is a gene near number “60”. Then if we were to select a number at random for where the crossing over will occur - then there’s a really high chance that we’d pick a number between 10 and 60 - 50%, and a really low chance that we’d pick a number between 5 and 10 - only 5%!
So, the bottom line is that the chance that two genes are inherited together really depends on the distance separating them.
This phenomenon is called genetic linkage, and this is a common exception to the law of independent assortment. Now, any two genes on completely different chromosomes always have a 50% chance to go through crossing over during meiosis and show up in the same gamete.
The same is true for genes that are really far away from one another on the same chromosome, like genes “a” and “c” for example.
This is because there might be more than one crossing over event during meiosis - and an odd number of crossing overs would separate them while an even number would bring them back to the same chromosome.
In that sense, ending up on the same gamete or not really just depends on whether there are an odd or even number of crossing over events - which results in a 50% chance.
So for two genes to be linked, the chance that they end up in different gametes has to be less than 50%. Now of course, even linked genes that are close together, might have a small chance for crossing over - if there’s a cut right in between them, but of course that would be uncommon and why the chance that they end up in different gametes falls below 50%.
And of course the closer they are physically together, the lower the chance that they would be separated. In terms of terminology, when linked genes are inherited together the resulting gametes are called parental gametes - since they carry the same alleles as the original chromosomes.
But when crossing over gets between these linked genes the resulting gametes are called recombinant gametes, because even though the genes were linked, crossing over separated them.
For example, if genes “a” and “b” gave rise to recombinant gametes in 45% of cases, this would mean that they are only slightly linked - so they are close to each other but not so much.
However, if this happened in 5% of cases, then that means they are really close together - so just really few times crossing over manage to separate them.
With genes “a” and “c” on the other hand, they would separate 50% of the time based on the number of crossover events, so they are not genetically linked.
Alright, as a quick recap, the law of independent assortment states that no matter which alleles an organism inherits for one specific gene it won’t affect the alleles it inherits for another gene that codes for a different trait.
In nature however, there are cases where genes that are close to each other on a chromosome sometimes are inherited together.
This is called genetic linkage, and is an exception to Mendel’s second law.