Law of Independent Assortment

What is the Law of Independent Assortment?

The law of independent assortment also called Mendel's laws of inheritance, is the foundation for the massive history of human genetics. Developed by Gregor Johann Mendel (1822-1884), he proposed this context based on 2 fundamental principles: 

  1.  Law of dominance

  2.  Law of segregation

  3.  Law of independent assortment

The 1st two principles, the law of dominance and law of segregation, was developed after thorough observation of a monohybrid cross with pea plants. The characteristics were noted upon the pea plants’ colour (yellow and green) and shape (round and wrinkled). Following which he had the question of how different genes inherited independently to one another, hence resulting in the formation of the 3 postulates which is the principle of independent assortment. The law of independent assortment definition from the words of Mendel is:

“The law of independent assortment states that the alleles of different genes are inherited independently within the organisms that reproduce sexually.”

As per this principle, the alleles of 2 varying genes will differentiate itself into unique gametes (female or male haploid to form a zygote). These gametes will remain independent of each other’s characteristics. Which means, a particular trait created will not resemble their parental genes. Hence, allele 1 cannot influence allele 2 on any terms. 

Reasons for Independent Assortment

To understand the reason behind Mendel's law of independent assortment, it is essential to learn about the process of ‘Meiosis’ in brief. 

Meiosis is a biological process, where 1 cell divides 2 times to produce 4 cells called haploids. The 4 resultant cells will possess half its amount of genetic data. These cells are nothing but our sex cells namely eggs in females and sperms in males. 

Now, combining this with Mendel’s theorem, the paternal and maternal genes will divide independently to each other, thus creating random but unique cells. Due to the nature of different ‘mom’ and ‘dad’ traits, the resulting progeny will be independently distributed to each other and to that of their parents.

Mendel’s Experiment on the Law of Independent Assortment

The experimental procedure to study the law of independent assortment took 8 years of Mendels’ life (1856-1863). He achieved in giving a clear and quick view of the principle of independent assortment works with different generations, using his setup with over 10,000 pea plants that he kept track of. 

According to the law of independent assortment, a di-hybrid (2 trait pairs) cross will assort itself into independent genetic characters. The resulting cells are unique to each other and express differently in genetic information. Gregor Mendel proved this principle using a simple experiment with a di-hybrid cross of round-yellow seeds and wrinkled-green seeds of a pure-breeding pea plant. 

  • In the F1 progeny (first generation crossing) he obtained round-yellow seeds ONLY. 

  • In the F2 progeny (second generation crossing), the seeds were self-pollinated, i.e. pollination between the same flower or genetically same plants.

  • There were 4 unique results: Round-yellow (RRYY), round-green (RRyy), wrinkled-yellow (rrYY) and wrinkled-green (rryy). 

  • Every parent, male or female, is homozygous (non-identical but similar alleles). The parental alleles are RR, YY, rr, yy and their resulting gametes that support this principle of assortment are RY, Ry, rY, ry. 

  • The F2 progeny was labelled in the ratio of 9:3:3:1; the phenotypic ratio of 3:1 was inherited and scattered individually. 

To better understand his concept, have a look at a real-time law of independent assortment example as follows.

Example of Law of Independent Assortment

Say a father has red hair (RR) with a straight hair type (SS) and the mother has brown hair (BB) with curls (CC). The son or daughter (progeny) of this couple (parent) can have the following genetic characteristic: 

  1. Straight Red hair (RS)

  2. Straight Brown hair (RB)

  3. Curly Red hair (CR)

  4. Curly Brown hair (CB)

Similarly, real-life examples can be found for various di-hybrid conditions including blood type, child’s eye colour, the skin colour of cows, etc.

FAQ (Frequently Asked Questions)

1. What is the Basic Rule for the Laws of Inheritance?

According to Mendel, the 3 rules of inheritance include the principles of the law of dominance, law of segregation and law of independent assortment. 

2. When Does an Independent Assortment Usually Occur?

Independent assortment is common to sexual reproduction, where 2 gametes fuse to form a diploid embryo (zygote) that comes with the DNA material from the parents, necessary to form a new living species. 

3. What will be the Result of an Independent Assortment?

The independent assortment resulting from the meiosis of a homologous pair will have an independent and unique set of characteristic combinations. 

4. What is the Ratio for an Independent Assortment?

The phenotypic ratio of 9:3:3:1 will be the classic number for any independent assortment that happens between any di-hybrid genetic pairs. 

5. What is Required for an Assortment to Occur Independently?

More than 2 alleles, either different or genetically similar are needed for getting unique chromosomes from the result of an independent assortment in the progeny.