Law of Independent Assortment

Objective

To verify the law of independent assortment.  

 

Theory

Law of independent assortment 

The law of independent assortment is one of the fundamental principles of genetics formulated by Gregor Mendel, the father of modern genetics, in 1865. This law describes how different genes segregate or assort independently during the formation of gametes (sex cells). Mendel's work with pea plants led to the formulation of several laws, and the law of independent assortment is one of them. 

The law of independent assortment states that the alleles (gene variants) for different traits segregate, or assort, into gametes independently. In other words, one trait's inheritance does not affect another trait's inheritance. This principle applies to genes located on different chromosomes or far apart on the same chromosome. 

The process of independent assortment occurs during meiosis, the cell division process that produces gametes. During meiosis, homologous chromosomes (chromosomes that carry the same genes but may have different alleles) segregate randomly into different gametes. The random assortment of chromosomes during meiosis leads to a variety of possible combinations of alleles in the offspring.  

The law of independent assortment is a key concept in understanding the inheritance of multiple traits simultaneously and helps explain the diversity observed in the offspring of sexually reproducing organisms. However, it's important to note that this law is not applicable when genes are closely linked on the same chromosome, as they tend to be inherited due to genetic linkage. 

 

Here are examples to illustrate the Law of Independent Assortment: 

1. Mendel's Dihybrid Cross: 

  • Mendel performed a classic dihybrid cross involving pea plant that differed in seed color and shape. 
  • The alleles for seed color (yellow or green) segregate independently of the alleles for seed shape (round or wrinkled). 
  • The F1 generation resulted in round, yellow seeds, demonstrating the independent assortment of these traits. 

  

2. Fruit Fly Eye Color and Wing Length: 

  • In fruit flies (Drosophila), eye color and wing length are controlled by different genes. 
  • The alleles for eye color segregate independently of the alleles for wing length during gamete formation. 
  • This independence allows for various combinations of eye color and wing length in the offspring. 

 

3. Human Genetics - Hair Color and Height: 

  • Consider the genes responsible for hair color and height in humans. 
  • The alleles for hair color segregate independently of the alleles for height when passed from parents to offspring. 
  • A person with black hair may have either a tall or short stature, demonstrating the independent assortment of these traits. 

  

4. Plant Genetics - Flower Color and Leaf Shape: 

  • Imagine a plant species where flower color is determined by one gene and leaf shape by another. 
  • The alleles for flower color and leaf shape segregate independently during the formation of reproductive cells. 
  • This results in diverse combinations of flower colors and leaf shapes in the plant population. 

  

Mendel's Dihybrid Cross involved considering two different traits controlled by two pairs of alleles. The traits he chose were seed color (yellow or green) and seed shape (round or wrinkled) in pea plants. Mendel conducted a series of experiments to understand how these traits were inherited.  

In standard form, Mendel's Dihybrid Cross is represented as 

 

Parental Generation (P):  

  • Mendel started with pea plants that were true breeding for each trait. For instance, one parent could have yellow, round seeds (YYRR) and the other green, wrinkled seeds (yyrr). 

  

First Filial Generation (F1): 

  • He crossed the true-breeding parents, resulting in the first filial generation (F1). 
  • All the F1 offspring had yellow, round seeds (YyRr). 
  • This indicated that the dominant alleles (Y and R) masked the expression of the recessive alleles (y and r). 

 

Dihybrid Cross (F1 × F1): 

  • Mendel then performed a dihybrid cross by mating the F1 individuals with each other. 
  • The possible gametes from each parent were YR, Yr, yR, and yr. 
  • The combinations of these gametes produced a 9:3:3:1 phenotypic ratio in the offspring. 

  

Second Filial Generation (F2): 

  • The F2 generation exhibited various combinations of seed color and shape. 
  • The phenotypic ratio observed in the F2 generation was 9 yellow round: 3 yellow wrinkled: 3 green round: 1 green wrinkled. 

                                                                 

                                                                                 Fig. 1 Punnett square - Law of Independent assortment  
 

  • Each box in Punnett square represents a possible combination of alleles from the respective parents. 
  • The letters on the top and side of the square represent the alleles from one parent, and the combinations in the boxes represent the alleles from the other parent. 
  • For example, the top-left box represents the combination YYRR, the top-right box represents YYRr, and so on. 
  • The resulting genotypes in the F1 generation are YyRr, indicating heterozygosity for both seed color and shape. 

 

Phenotypic Ratio in F2 Generation: 

The phenotypic ratio among the offspring in the F2 generation, resulting from this dihybrid cross, is 9:3:3:1 (yellow round: yellow wrinkled: green round: green wrinkled). This ratio corresponds to the different combinations of alleles resulting from the independent assortment of genes. 

 

Learning Outcomes

Students learn about  

  • The law of independent assortment. 
  • The segregation arrangements.  
  • The Punnett square assortment.