Law of Independent Assortment — Definition & Role

What Is the Law of Independent Assortment?

The Law of Independent Assortment is one of the Principles of Mendelian Inheritance. (This is alongside the Law of Dominance and the Law of Segregation.) It says that during gamete production, genes for different traits segregate independently of each other. This means that offspring can have combinations of traits that are different from either of their parents'. Independent assortment gives a chance for new trait combinations to show up that could be advantageous (more fit).

Gregor Mendel developed this law after doing dihybrid crosses. This is where the two parents being crossed have different combinations of two traits. He noticed that when F1 (first generation) offspring were back-crossed with each other, new combinations of traits emerged. A monohybrid cross wouldn't give this kind of information, since it looks at just one trait.

Here's a dihybrid cross as an example of how independent assortment works:

Consider these two little broccoli people who are madly in love. In this species, purple ($P$) is a dominant trait, and pink ($p$) is a recessive trait. A more lumpy head is the result of a dominant allele ($L$), and a less lumpy head is the result of recessive alleles ($l$). They are both homozygotes: The pink one on the left would have genotype $ppLL$, and the purple one on the right would be $PPll$.

Now the happy couple gets married, starts a family, and has a couple of kids.

See how all of their kids have a new, never-before-seen phenotype? They are all purple with five lumps, which is a new combination of traits not seen in either parent. This shows independent assortment.

If the genes for color and head shape did not assort independently, then only combinations of traits already seen in the parents (so pink and more lumpy, or purple and less lumpy) would be possible. The Punnett square that we see here of the possible offspring would also look totally different.

Because the parents are homozygous for their respective traits (also called purebred or true breeding), we know that all the kids are heterozygotes with the genotype $PpLl$. None of them have the recessive phenotype for either trait.

The F1 generation $PpLl$ kids now grow up and have kids with other $PpLl$ individuals (this is called back-crossing):

Why Does the Law of Independent Assortment Work?

Although Mendel didn't know anything about DNA or chromosomes, his law is true because of how meiosis works at the molecular level. During meiosis when gametes (sex cells with half the normal amount of chromosomes) are forming, the homologous chromosomes randomly line up and are split off into different daughter cells.

Linked Genes: They Break the Law

However, the law doesn't work for traits that are encoded on the same chromosome. These traits are called linked genes because they are linked on the same chromosome. A common example of linked genes is sex-linked traits.

Genes and Math: Dihybrid Crosses and Offspring Ratios

In Mendel's dihybrid cross experiments, heterozygous parents always produced the same ratio of offspring phenotypes. That ratio is 9:3:3:1. That means nine dominant for both traits, three dominant for just one trait, three dominant for just the other trait, and one with the recessive phenotype for both traits. This happened because the traits Mendel looked at did sort independently.

But what about monohybrid crosses? And what about linked genes? Well, a monohybrid cross won't tell you anything about whether genes are linked or not. Monohybrid crosses look at the alleles of just one gene at a time. And if you're doing a dihybrid cross and two traits are linked, you won't see that 9:3:3:1 ratio. That's because linked genes behave differently. They move together like a pair of best friends.

Combinations of Traits, Thanks to Independent Assortment

(Video) Mendel's Law of Independent Assortment

by Biology Professor

Independent assortment is a rule in genetics which states that genes split up independently of one another during meiosis. It is a mechanism present in eukaryotes to help promote genetic diversity in offspring. In this video, Biology Professor explains the Law of Independent Assortment.

Since genes move independently, they don't influence a person's other traits. For example, a plant that has genes which give it a long stem is no more likely to have white flowers than a short plant is. This means that the genes for stem length and flower color are independent.

What's Independent Assortment?

Another principle outlined by Gregor Mendel, the Law of Independent Assortment, states that alleles of different traits are inherited separately during the process of meiosis.

His famous experiments with pea plants showed why this is true. In his studies, he often looked at how different traits, like plant height, seed color, and shape, were passed on from parent to offspring.

During the formation of gametes in meiosis, genes for different traits are segregated, or divided. This means that the two different alleles for a specific gene are sorted so that each gamete inherits only one allele.

An allele that a gamete inherits does not impact the other alleles it inherits. The alleles are independently assorted. We can see an example of this in the dihybrid cross diagram above.

A Closer Look at Mendel's Experiments

Mendel the monk's pea plant experiments paved the way for his Law of Independent Assortment. So, what exactly were his experiments? And how did they show that some traits get inherited separately?

Mendel raised and propagated thousands of pea plants. At the time he was working with his plants, prominent scientists believed that children inherited blends of their parents' traits. They thought that, for example, if one pea plant somehow had red peas and another pea plant somehow had blue peas, a baby pea plant with both of those plants' genes would have violet peas. Mendel didn't believe that. It didn't fit with his experiments.

Instead, Mendel said that each parent must be providing some small, physical thing to the offspring. We know these things today as alleles. He said this because he saw in his experiments that baby pea plants didn't have greenish-yellow color or wrinkly-ish seeds. Instead, some offspring had yellow color and round seeds, some had green color and wrinkly seeds, and so on. Those results don't fit at all with the idea of blending.

Now, what about Mendel's three laws? How do they relate to independent assortment?

1. The Law of Dominance: After crossing two pea plants with opposite traits and getting offspring that all looked identical to one another and to one parent, Mendel came up with a new question. Might those offspring still have the ability to pass on the other parent's traits? He crossed two of the offspring together and—aha—he found that some of these new offspring did look like the other grandparent. This information paved the way for our understanding of dominant alleles and recessive alleles today.

2. The Law of Segregation: When Mendel saw that those second-generation pea plants could still pass on the traits of their grandparents, he wondered how. He hypothesized that the things (alleles) that carry traits end up separating from each other when they're being passed on. In other words, Mendel said that alleles are separated in the parent to be passed on to the offspring. That way each kid gets one allele from each parent.

3. The Law of Independent Assortment: This is where Mendel worked on his dihybrid crosses. Mendel did some math and statistics here and found that a specific phenotypic ratio was present when you crossed pea plants and looked at two traits at once. Because of the math, he was able to hypothesize that if those genes didn't sort independently, you wouldn't see that same ratio.

Mendel's Law of Independent Assortment wouldn't make sense without his other two laws. And none of his laws would make sense without his experiments.

$$\color{#8925AE}{\text{Vocabulary Review:}}$$

$\color{#8925AE}{\text{Independent Assortment}}$ : Alleles of different traits are inherited separately and do not influence each other

$\color{#8925AE}{\text{Dominant}}$ : The stronger of two expressed genes

$\color{#8925AE}{\text{Recessive}}$ : The weaker of two expressed genes. Only shown if a gene is homozygous recessive

$\color{#8925AE}{\text{Heterozygous}}$ : Two different kinds of allele, one dominant and one recessive

$\color{#8925AE}{\text{Homozygous}}$ : Two of the same type of allele, both dominant or both recessive