Before we explore what Mendel experimented with using a dihybrid cross, let's understand the concept with an alternative example.

Imagine four students: Ram, Sham, Ali, and Mohd. Ram and Sham are friends with each other, and Ali and Mohd are friends with each other. They are all college students (See in below figure).

Situation 1:

We always see that Ram and Sham go to college together, and similarly, Ali and Mohd always go to college together. Now, let’s use the word “assortment” instead of “going to college.” We observe that Ram and Sham always assort together, and Ali and Mohd always assort together.

Next, let's replace the word “together” with “dependent.” It becomes clear that Ram and Sham’s assortment is dependent on each other, and Ali and Mohd also show dependent assortment. This means that we can predict that if Ram comes to college, Sham will also come, and if Ali comes to college, Mohd will also come. This is an interesting case of dependent assortment, where the assortment of one is dependent on the other (See in below figure).

Situation 2: Each individual's decision to go to college is independent of the others.

Now, let's consider a second situation of independent assortment. In this case, Ram is going to college, but Sham's probability of coming to college is very low, and similarly, Ali is going to college, but Mohd's likelihood of going to college is also low. This means that Ram’s assortment is independent of Sham's, and Ali's assortment is independent of Mohd's (See in above figure).

This is a good example of independent assortment, where we cannot predict the assortment of either Ram, Sham, Ali, or Mohd. Each individual's decision to go to college is independent of the others.

Understanding Mendel’s Dihybrid Cross Through the Example

The example of Ram, Sham, Ali, and Mohd helps us grasp the concepts of dependent and independent assortment, which are crucial in understanding Mendel's dihybrid cross.

How the Example Relates to Mendel's Dihybrid Cross:

1. Dependent Assortment (Situation 1):

   • In our example, Ram and Sham always go to college together, and Ali and Mohd also always go to college together. This means their actions are dependent on each other.

   • This situation parallels the idea of dependent assortment in genetics, where two traits tend to be inherited together because they are linked. If two genes are located close to each other on the same chromosome, they are likely to be inherited together, just like how our pairs of friends always travel together (The concept will be explored in Linkage).

2. Independent Assortment (Situation 2):

   • In the second scenario, Ram and Sham, and Ali and Mohd, go to college independently of each other. The actions of one person do not affect the actions of the other.

   • This mirrors Mendel’s principle of independent assortment, which states that alleles for different traits segregate independently during the formation of gametes. If two genes are on different chromosomes or far apart on the same chromosome, they assort independently. This means that the inheritance of one trait does not influence the inheritance of another trait, similar to how the decisions of each individual in our example are independent of one another.

Applying the Above Example to Understand the Mendel’s Dihybrid Cross:

• Mendel’s Experiment:

  • Mendel’s dihybrid cross involved two different traits (e.g., seed shape and seed color in pea plants). He crossed plants that were heterozygous for both traits to observe how the traits assorted.

  • If the traits had shown dependent assortment, we would have expected certain combinations of traits to appear more frequently together, similar to how Ram and Sham or Ali and Mohd always go to college together.

• Mendel’s Discovery:

  • Instead, Mendel found that the traits assorted independently, leading to a 9:3:3:1 phenotypic ratio in the F2 generation. This finding illustrated (see in above figure) that the inheritance of one trait (e.g., seed shape) did not affect the inheritance of the other trait (e.g., seed color), much like how in the second situation, the decision of one person to go to college did not affect the decision of the other.

By comparing the independent and dependent assortments in our example with Mendel’s findings, we can better understand how Mendel’s dihybrid cross experiments revealed the principles of independent assortment and the segregation of alleles. This helps us grasp the fundamental mechanisms behind the inheritance of multiple traits, just as our example helps illustrate the concepts of dependence and independence in a simple, relatable way.

Law  of Independent Assortment

The law of independent assortment states that the alleles for different genes segregate, or assort, independently of one another during the formation of gametes. This means the inheritance of an allele for one trait does not affect the inheritance of an allele for another trait. The law applies to genes located on different chromosomes or those that are far apart on the same chromosome. 

Gregor Mendel's discoveries in the mid-19th century laid the groundwork for our understanding of genetics through three key principles, often referred to as Mendel's Laws:

1. Law of Dominance: In a cross between two homozygous parents (one dominant and one recessive allele), the dominant allele will be expressed in the offspring, masking the expression of the recessive allele.

2. Law of Segregation: Each organism inherits two alleles for each trait, one from each parent. These alleles segregate (separate) during gamete formation, so each gamete receives only one allele for each trait.

3. Law of Independent Assortment: Genes for different traits segregate independently of one another during gamete formation, provided they are located on different chromosomes or are far apart on the same chromosome.

Together, these laws explain how traits are passed down from parents to offspring, how different traits are inherited independently, and how dominant and recessive traits manifest in successive generations. Mendel's work with pea plants established the foundation for genetics, demonstrating the principles of inheritance that apply universally across species.