Introduction to Mendel's Law of Independent Assortment

This image shows the results of a dihybrid cross in plants that are true-breeding for two different traits - seed shape and seed color.
Adapted from work in Wikimedia Commons/CC BY-SA 3.0

Independent assortment is a basic principle of genetics developed by a monk named Gregor Mendel in the 1860s. Mendel formulated this principle after discovering another principle known as Mendel's law of segregation, both of which govern heredity.

The law of independent assortment states that the alleles for a trait separate when gametes are formed. These allele pairs are then randomly united at fertilization. Mendel arrived at this conclusion by performing monohybrid crosses. These cross-pollination experiments were performed with pea plants that differed in one trait, such as the color of the pod.

Mendel began to wonder what would happen if he studied plants that were different with respect to two traits. Would both traits be transmitted to the offspring together or would one trait be transmitted independently of the other? It is from these questions and Mendel's experiments that he developed the law of independent assortment.

Mendel's Law of Segregation

Foundational to the law of independent assortment is the law of segregation. It was during earlier experiments that Mendel formulated this genetics principle.

The law of segregation is based on four main concepts:

  • Genes exist in more than one form or allele.
  • Organisms inherit two alleles (one from each parent) during sexual reproduction.
  • These alleles separate during meiosis, leaving each gamete with one allele for a single trait.
  • Heterozygous alleles exhibit complete dominance as one allele is dominant and the other recessive.

Mendel's Independent Assortment Experiment

Mendel performed dihybrid crosses in plants that were true-breeding for two traits. For example, a plant that had round seeds and yellow seed color was cross-pollinated with a plant that had wrinkled seeds and green seed color.

In this cross, the traits for round seed shape (RR) and yellow seed color (YY) are dominant. Wrinkled seed shape (rr) and green seed color (yy) are recessive.

The resulting offspring (or F1 generation) were all heterozygous for round seed shape and yellow seeds (RrYy). This means that the dominant traits of round seed shape and yellow color completely masked the recessive traits in the F1 generation.

Discovering the Law of Independent Assortment

This image shows results of the self-fertilization of F1 plants resulting from the dihybrid cross of a true-breeding plant with round, yellow seeds and a true-breeding plant with wrinkled, green seeds.
Wikimedia Commons/CC BY-SA 3.0

The F2 Generation: After observing the results of the dihybrid cross, Mendel allowed all of the F1 plants to self-pollinate. He referred to these offspring as the F2 generation.

Mendel noticed a 9:3:3:1 ratio in the phenotypes. About 9/16 of the F2 plants had round, yellow seeds; 3/16 had round, green seeds; 3/16 had wrinkled, yellow seeds; and 1/16 had wrinkled, green seeds.

Mendel's Law of Independent Assortment: Mendel performed similar experiments focusing on several other traits such as pod color and seed shape; pod color and seed color; and flower position and stem length. He noticed the same ratios in each case.

From these experiments, Mendel formulated what is now known as Mendel's law of independent assortment. This law states that allele pairs separate independently during the formation of gametes. Therefore, traits are transmitted to offspring independently of one another.

How Traits Are Inherited

Genotypes and Phenotypes in F2 Generation
Adapted from work in Wikimedia Commons/CC BY-SA 3.0

How Genes and Alleles Determine Traits

Genes are segments of DNA that determine distinct traits. Each gene is located on a chromosome and can exist in more than one form. These different forms are called alleles, which are positioned at specific locations on specific chromosomes.

Alleles are transmitted from parents to offspring by sexual reproduction. They are separated during meiosis (process for the production of sex cells) and united at random during fertilization

Diploid organisms inherit two alleles per trait, one from each parent. Inherited allele combinations determine an organisms genotype (gene composition) and phenotype (expressed traits).

Genotype and Phenotype

In Mendel's experiment with seed shape and color, the genotype of the F1 plants was RrYy. Genotype determines which traits are expressed in the phenotype.

The phenotypes (observable physical traits) in the F1 plants were the dominant traits of round seed shape and yellow seed color. Self-pollination in the F1 plants resulted in a different phenotypic ratio in the F2 plants.

The F2 generation pea plants expressed either round or wrinkled seed shape with either yellow or green seed color. The phenotypic ratio in the F2 plants was 9:3:3:1. There were nine different genotypes in the F2 plants resulting from the dihybrid cross.

The specific combination of alleles that comprise the genotype determines which phenotype is observed. For example, plants with the genotype of (rryy) expressed the phenotype of wrinkled, green seeds.

Non-Mendelian Inheritance

Some patterns of inheritance do not exhibit regular Mendelian segregation patterns. In incomplete dominance, one allele does not completely dominate the other. This results in a third phenotype that is a mixture of the phenotypes observed in the parent alleles. For example, a red snapdragon plant that is cross-pollinated with a white snapdragon plant produces pink snapdragon offspring.

In co-dominance, both alleles are fully expressed. This results in a third phenotype that displays distinct characteristics of both alleles. For example, when red tulips are crossed with white tulips, the resulting offspring can have flowers that are both red and white. 

While most genes contain two allele forms, some have multiple alleles for a trait. A common example of this in humans is ABO blood type. ABO blood types exist as three alleles, which are represented as (IA, IB, IO).

Further, some traits are polygenic, meaning that they are controlled by more than one gene. These genes may have two or more alleles for a specific trait. Polygenic traits have many possible phenotypes and examples include traits such as skin and eye color.