Mendel's Law of Independent Assortment: Dominance, Segregation

The Law of Independent Assortment is one of Mendel’s Laws of Inheritance. It states that “during gamete formation, the alleles of different genes assort independently of each other”. The law is important for understanding the genetic diversity and predicting the characteristics of offspring, particularly in the dihybrid crosses.

This Story also Contains

1. What is the Law of Independent Assortment?
2. Mendel's Experiments
3. Mendel's Law of Independent Assortment
4. Mechanism behind Independent Assortment
5. Contribution to Genetic Diversity
6. Recommended Video for Law of Independent Assortment
7. MCQs on Law of Independent Assortment

This points to the fact that the inheritance of one pair is not influenced by the inheritance of another trait. This happens when the genes are located on different chromosomes or are far apart on the same chromosome. The Law of Independent Assortment is a topic of the chapter Principles of Inheritance and Variation in Biology.

What is the Law of Independent Assortment?

Mendel is credited for making significant studies with the help of pea plants in mid - the mid-19th century that suggest the contemporary idea of heredity. His systematic and detailed observations and mathematical computations gave rise to fundamental principles, which are known as Mendel’s laws of inheritance.

The principle that came from Mendel’s pea plant experiment was the Law of Independent Assortment, which showed that alleles relating to different traits do not influence each other during the formation of gametes. This fundamental knowledge regarding genetic variation and inheritance was a breakthrough.

This realisation is important in present-day genetics to the initiation and calculation of genetic relations, and promotion of other sectors such as farming and medicine through selection and biotechnology.

Mendel's Experiments

Gregor Mendel’s studies with the help of pea plants in the middle of the nineteenth century can be considered the beginning of genetics studies. Mendel meticulously studied seven distinct traits in pea plants: seed shape (round or wrinkled), seed colour (yellow or green), pod shape (inflated or constricted), pod colour (green or yellow), flower colour (purple or white), the position of the flower (axial or terminal) stem height length (tall or short ).

To understand the patterns of inheritance, Mendel used dihybrid cross–breeding in which two different traits are followed at the same time. Hence, the different crosses included by Mendel include: in one experiment, he only crossbred plants of peas with both round seeds and yellow seeds, with the other being wrinkled green seeds. He also performed the monohybrid cross, which took into consideration only one pair of contrasting characters.

Thus, the ratios of phenotypes in the offspring, known as the F2 generation, exposed the Law of Independent Assortment according to Mendel. This law was formulated to explain that loci for different genes behave independently of each other, indicating that genes controlling different characteristics are passed on independently, although it is possible to experience ‘linkage’ if genes are located on the same chromosome.

Mendel’s consistent and accurate experimentations and subsequent analyses laid the basis of what came to be known as Mendelian genetics, which indeed had the most significant impact on the field of biology and basic knowledge of genetic transmission in species.

Mendel's Law of Independent Assortment

The Law of Independent Assortment is that alleles for different genes, thus the traits they control, are sorted independently of each other in the formation of gametes. What this means is that one character is not inherited together with another character if the genes for these characters are on different chromosomes or sufficiently far apart on the same chromosome so that they assort independently.

How does it differ from The Law of Segregation?

The Law of segregation, which was proposed by Mendel earlier, suggest that the two alleles of a trait separate during the gamete formation and are randomly distributed between each of the gametes. This law concerns only one gene and shows that the offspring receives one allele of a gene from each parent for that particular trait.

Illustration of the Concept using a Dihybrid Cross (Eg, Seed Shape and Seed Colour)

Let us also use the Law of Independent Assortment to explain the following dihybrid cross: between the pea plant with round yellow seeds RRYY and the pea plant with wrinkled green seeds. According to Mendel's experiments:

The F1 generation of this cross will consist of all F1 hybrids which will be heterozygous for both traits, RrYy.

If these plants are self-crossed (F2 generation), the factors for seed shape (Rr), as well as seed colour (Yy), and dims will segregate independently during gamete formation.

This results in a phenotypic ratio of 9:3:3:1 phenotypic ratio in the offspring, where:

• 9 plants exhibit both dominant traits (round yellow seeds). Since r is not capitalised, the possible combinations are RRYY, RRYy, RrYY, and RrYy.

• 3 plants exhibit one dominant and one recessive trait (round green seeds: Rryy, rrYY).

• 3 plants exhibit the other dominant and recessive traits (wrinkled yellow seeds: rrYy, Rryy).

• 1 plant exhibits both recessive traits (wrinkled green seeds). Of these, the four-letter phrase ‘rryy’ was extracted.

Mechanism behind Independent Assortment

The law of independent assortment explains how, during gamete formation, the transmission of one trait is completely independent of the transmission of another trait. The mechanism is explained below-

Meiosis and Genetic Variation:

Meiosis consists of two stages: meiosis I and meiosis II. In prophase I, homologous chromosomes form pairs and undergo crossing over. This process leads to the formation of different alleles on chromosomes, thus increasing the genetic variation of the young ones.

Role of Homologous Chromosomes and Allele Behaviour:

Homologous chromosomes are two chromosomes that are inherited from the first allele and two that are inherited from the second allele, both having the same genes. During the stages of meiosis, these chromosomes separate into different gametes during anaphase I, and this is why alleles for different traits are independent unless they are linked on the same chromosome.

Contribution to Genetic Diversity

Independent assortment is also important in increasing genetic variation since it produces distinct allele combinations within offspring. This variability helps in fitness, whereby organisms can change in response to different circumstances and helps in evolutionary mechanisms by enhancing the genetic pool’s flexibility.

Recommended Video for Law of Independent Assortment

MCQs on Law of Independent Assortment

Q1. How many genetically different gametes can be made by an individual of genotype AaBbccDDEe, assuming they are independently assorting?

Option 1: 3

Option 2: 5

Option 3: 8

Option 4: 32

Correct answer: (3) 8

Explanation:

To determine the number of genetically different gametes that can be produced by an individual of genotype AaBbccDDEe, we need to consider the independent assortment of alleles.

Independent assortment means that alleles for different genes segregate independently during gamete formation. In this case, we have five different genes (A, B, C, D, and E) with two alleles each (capital and lowercase letters represent different alleles).

For each gene, the individual can produce two different gametes, one carrying the capital allele and the other carrying the lowercase allele. Since we have five independently assorting genes, we can multiply the number of gametes for each gene to determine the total number of genetically different gametes.

Therefore, the total number of genetically different gametes that can be made by an individual of genotype AaBbccDDEe is:

2 (alleles for gene A) x 1 (allele for gene B) x 2 (alleles for gene C) x 2 (alleles for gene D) x 1 (allele for gene E) = 8

So, the individual can produce 8 different genetically unique gametes.

Hence, the correct answer is option (3) 8

Q2. Which one of the following statements is incorrect?

Option 1: Quantitative inheritance results in a range of measurable phenotypes for a polygenic trait.

Option 2: Polygenic traits often demonstrate continuous variation.

Option 3: Certain alleles of quantitative trait loci (QTL) have an additive effect on the character/trait.

Option 4: Alleles governing quantitative traits do not segregate and assort independently.

Correct answer: (4) alleles governing quantitative traits do not segregate and assort independently is incorrect.

Explanation:

Quantitative traits, which are complex traits influenced by multiple genes and environmental factors, can indeed segregate and assort independently. However, the inheritance patterns for quantitative traits are more complex compared to simple Mendelian traits.

In quantitative genetics, the contribution of each allele to the phenotype is typically assessed through statistical analyses and heritability estimates. While the inheritance patterns of quantitative traits may involve interactions among multiple genes and environmental factors, the alleles themselves can still segregate and assort independently during sexual reproduction.

Therefore, the correct answer is Option (4) alleles governing quantitative traits do not segregate and assort independently.

Hence, the correct answer is option (4) Alleles governing quantitative traits do not segregate and assort independently.

Q3. Mendel’s Law of independent assortment holds good for genes situated on the:

Option 1: non-homologous chromosomes

Option 2: homologous chromosomes

Option 3: extranuclear genetic element

Option 4: same chromosome

Correct answer: (1) Non-homologous chromosomes

Explanation: Mendel's Law of Independent Assortment states that genes located on non-homologous chromosomes (i.e., chromosomes that are not paired or do not have corresponding counterparts) assort independently during gamete formation. This means that the inheritance of one gene does not affect the inheritance of another gene located on a different chromosome. This law applies to genes that are far apart on the same chromosome or to genes on different chromosomes, as they segregate independently during meiosis, resulting in a variety of genetic combinations in the offspring.

Hence, the correct answer is option (1) Non-homologous chromosomes

Also Read:

• MCQ Practice on Mendel's Law of Independent Assortment

• Principles of Inheritance and Variation

• Molecular Basis of Inheritance