Difference Between Mendel’s First and Second Law

The main difference between Mendel’s first and second law is that Mendel’s first law (law of segregation) describes the separation of allele pairs from each other during gamete formation and their pairing during fertilization whereas Mendel’s second law (law of independent assortment) describes how alleles of different genes independently segregate from each other during the formation of gametes. 

Mendel’s first and second laws describe the behavior of the ‘factors’ that determine the phenotype of the offspring during the formation and fusion of gametes. Gregor Mendel first described the patterns of inheritance of traits using pea plants.

Key Areas Covered

1. What is Mendel’s First Law
     – Definition, Features, Role
2. What is Mendel’s Second Law
     – Definition, Features, Role
3. What are the Similarities Between Mendel’s First and Second Law
     – Outline of Common Features
4. What is the Difference Between Mendel’s First and Second Law 
     – Comparison of Key Differences

Key Terms: Alleles, Genes, Independent Assortment, Mendel’s First Law, Mendel’s Second Law, Segregation, Phenotype

What is Mendel’s First Law

The first law of Mendel is the law of segregation that describes the separation of the two copies of each hereditary factor or gene during the formation of gametes. Each gene exists in two copies called the alleles within a diploid genome. Each allele comes from each parent. During the formation of gametes, the allele pair separates from each other in such a way that each gamete receives one allele from the pair. Therefore, the offspring acquires one copy from each parent. During the fusion of gametes, it acquires two alleles from each parental gamete.

Here, alleles can be either homozygous or heterozygous. One allele in the heterozygous pair is dominant while the other is recessive. The expression of the dominant allele to produce the phenotype is called complete dominance. Shown in figure 1 is the punnett square describing Mendel’s first law by a monohybrid cross.

Figure 1: Mendel’s First Law

The phenotypic ratio between the offspring in Mendel’s first law is 3:1.

What is Mendel’s Second Law

The second law of Mendel is the law of independent assortment. This law describes how alleles of different genes independently segregate from each other during the formation of gametes. However, it is applicable only when two or more factors are inherited together. According to Mendel’s second law, the transmission of different alleles of different genes into the gametes is not affected by each other. Mendel describes the second law with a dihybrid cross. The phenotypic ratio of a dihybrid cross is 9:3:3:1. Figure 2 below shows the behavior of the characters; short tail (S), long tail (s) brown coat (B), and white coat (b) in a dihybrid cross.

Figure 2: Mendel’s Second Law

Independent assortment occurs during prophase 1 of meiosis 1 in the formation of gametes. The random orientation of the bivalent chromosomes on the equatorial plate of the cell during the metaphase 1 is the physical basis of the independent assortment. Genetic linkage violates Mendel’s second law.

Similarities Between Mendel’s First and Second Law

• Mendel’s first and second laws describe the Mendelian inheritance patterns by means of alleles.
• Segregation and independent assortment of alleles are useful in increasing genetic diversity among individuals within a population.

Difference Between Mendel’s First and Second Law

Definition

Mendel’s First Law: A principle that describes the separation of the two copies of each hereditary factor during the production of gametes

Mendel’s Second Law: A principle that describes the independent assortment of alleles of different genes during the formation of gametes

Also Called

Mendel’s First Law: The law of segregation

Mendel’s Second Law: The law of independent assortment

Type of crosses

Mendel’s First Law: Monohybrid cross

Mendel’s Second Law: Dihybrid cross

Ratio of the offspring

Mendel’s First Law: 3:1.

Mendel’s Second Law: 9:3:3:1

Conclusion

Mendel’s first law describes the segregation of the two copies of alleles of a particular gene into the gametes. Mendel’s second law describes the independent assortment of alleles of different genes from each other during the formation of gametes. Both Mendel’s first and second law describe the behavior of alleles during sexual reproduction. The main difference between Mendel’s first and second law is the number of hereditary factors involved in the cross.

Reference:

1. Bailey, Regina. “Genes, Traits and Mendel’s Law of Segregation.” ThoughtCo, ThoughtCo, Available Here 
2. “The Law of Independent Assortment.” Khan Academy, Khan Academy, Available Here

Image Courtesy:

1. “Punnett square mendel flowers” By Madprime – Own work (CC BY-SA 3.0) via Commons Wikimedia 
2. “Dihybrid cross” By Tocharianne (PNG version), WhiteTimberwolf (SVG version) – PNG version (Public Domain) via Commons Wikimedia