Independent Assortment In Meiosis: What, Why, How, Causes, Types And Facts

In this article we will get to know about the independent assortment in meiosis.

Genes on the same chromosome linked to each other are more inclined to move together during meiosis. As a result, connected genes do not assort separately. If the genes are positioned on different chromosomes, they will assort independently. The process of meiosis provides the foundation for independent assortment.

To understand independent assortment in meiosis, you must first understand Mendel’s second law, in which he described independent assortment while experimenting on his garden green peas. According to Mendel’s second law, during meiosis, alleles from two (or more) separate gene pairs assort independently, leading to a random combination of genes from each pair ending up in the gametes. In simple words, as genes that represent different qualities segregate in cells, they will not follow a stable pattern. All dominant alleles don’t need to be assorted together in the cells.

That is why, in the end, we have gametes with a variety of possible combinations. Depending on how the chromosomes arrange on the metaphase plate, there may be possibilities to obtain alternative combinations.

However, Mendel’s second law does not extend to all genes.

Since multiple characters must be examined at once, a dihybrid cross is appropriate to explain this law. Therefore, a dihybrid cross or a higher cross involves more traits than the mono-hybrid cross.

An example that defines the independent assortment in meiosis: 

Imagine a fictitious population of dogs with only two distinguishing characteristics: fur color (black or white) and eye color (amber or hazel). The amber eye allele (A) is dominant over hazel (a), whereas the black fur allele (B) is dominant over white (b).

Two-hybrid dogs are mingled here, which means that both dogs appear to be black with amber eyes, yet they have a heterozygous genotype. The genotype BbAa is shared by both dogs. All dogs in this population of two contribute the same mix of features to one another. To put it another way, they’re completely black and have amber eyes.

Each dog will have to release gametes before the breeding. During this phase, alleles are separated according to Mendel’s law of segregation, but each copy of each chromosome is allocated to a different gamete at random. It indicates that the puppies can inherit diverse combinations of these features independent of the parental phenotype (black with amber eyes). One puppy, for example, could be born with the bbAa genotype, which results in white fur and brown eyes. A baby dog or puppy could also be born with the genotype Bbaa, which results in black fur and brown eyes. Such is the independent assortment law, which is enforced by the meiosis process.

What happens in independent assortment in meiosis?

At the cell equator, homologous chromosomes line up opposite each other in meiosis I. Each homologous pair’s paternal and maternal chromosomes randomly fall on opposite sides of the equator. One of each homologous pair tends to end up in the daughter cell after these pairs are separated.

When does independent assortment occur?

In metaphase I of meiotic division, eukaryotic organisms undergo independent assortment in meiosis. A gamete with mixed chromosomes is the consequence. Gametes in a diploid somatic cell have half the number of normal chromosomes as normal chromosomes.

We know that the chromosomes align themselves on the equatorial plane during cell division, which is metaphase; in other words, it is on the metaphase plate. Similarly, one chromosome will align on one side or align randomly or alternately. There are several methods for this to occur. So this is purely coincidental. When the chromosomes align, there is no set pattern or sequence that they must follow. Now, if the chromosomes are randomly divided during the metaphase of meiosis, it is evident that the genes on them will likewise be randomly separated.

As a result, gametes are haploid cells that can reproduce sexually by combining two haploid gametes to make a diploid zygote with all chromosomes. The random distribution of chromosomes forms the structural basis during metaphase concerning other chromosomes.

Independent assortment in meiosis genetic variations

Meiosis introduces genetic variation through two mechanisms:

The same pair of chromosomes are randomly assigned in anaphase I as cells divide during meiosis, splitting and segregating independently. It is referred to as self-assortment. It results in gametes with unique chromosomal configurations.

It creates many possible chromosomal combinations in the daughter cell produced. The 2ⁿ technique can be used to determine this, with ‘n’ equaling the number of homologous pairings. Let’s go through this calculation in humans(i.e., 2²³). There are about 8,388,608 different combinations of which chromosomes of the homologous pairs would have in the gametes, which is a tremendous number of variants.

There is another form of variation that is introduced by crossing over. When homologous pairs line up facing each other at the equator, pieces of chromatids can become twisted around each other, which happens simultaneously, as in meiosis I. It causes the chromatids to become tense, causing pairs of chromatids to split.

At a stage known as synapsis, the split pieces of one chromatid reunite with those of another chromatid. However, the swapped section of the chromatids leads to a unique mix of alleles on this chromatid and the entire chromosome. As a result, we have new allele variations in the gametes.

Meiosis generates even more variation in that the resulting gametes will fuse in the fertilization process: fusing sperm and eggs. As a result, there is more variety.

Independent assortment in the meiosis phase

During F2 generation, independent assortment occurs, which means that unique non-parental pairings emerge. During Anaphase I of meiosis, it is most noticeable when non-homologous chromosomes are randomly distributed as sister chromatids are connected. Meiosis I assures unique gametes by separate genes that are present on other chromosomes or, in other words, genes that carry other features. Hence, Metaphase I is the third phase of the meiosis phase in which it was involved in the random independent assortment. 

What is the effect of an independent assortment in meiosis?

The effects of independent assortment in meiosis result in creating a significant amount of variation compared with previously unknown combinations of genes. It only happens when two genes are connected or when two genes are on the same chromosome. Also, the distribution of maternal and paternal homologous chromosomes to gametes is unpredictable.

In some cases, and particularly in humans, this occurs due to evolutionary traits. The concept of Independent Assortment describes how individual genes separate from one another independently when reproductive cells mature, regardless of any boundaries.

There are 2ⁿ possible chromosomal combinations in gametes, whereas, in humans, there are 2²³. However, when considering random fertilization, we have (2ⁿ)² potential chromosome combinations when we receive a random egg and a random sperm at the end fusing.

In humans, for example, (2²³)² = 7.04×10¹³, which suggests there is a vast amount of variation or different chromosome combinations in the resulting organism. Because of this effect of variation in humans: skin tone, facial appearance (including nose, lips, and eye shape), hair color and shape, eye color, tallness, dwarfism, and many other features all differ from one another. That is why humans are genetically identical unless they are identical twins. As a result of meiosis, genetic variation occurs. Hence, it helps eukaryotes maintain genetic variety.

Causes of Independent Assortment in Meiosis

Meiosis I results in an independent assortment of genes due to the random positioning of pairs of homologous chromosomes. As a result of the independent assortment, the meiosis process causes genetic variation. As a result, there are four primary forms of meiosis that result in genetic variation. During meiosis, it involves 1) mutation, 2) random fertilization, 3) random mating among organisms, and 4) crossing over between homologous chromosomes with chromatids. Non-sister chromatids may split and reunite with their homologous partner during crossing over when in synapsis during the meiosis process. The swapping of DNA material among non-sister homologous chromatids is known as crossing over.

Crossing over results in unique allele combinations on the haploid cells’ chromosomes. At these swap points, referred to as chiasmata, non-sister chromatids remain physically attached. Chiasmata development between non-sister chromatids might, however, result in allele exchange.

Till anaphase I, chiasmata bind homologous chromosomes together as a bivalent.

Genetic diversity is caused by changes in gene number or position, rapid reproduction, the generation of novel alleles, and sexual reproduction.

Types of Independent Assortment in Meiosis

One pair of phenotypes segregates from another pair of phenotypes independently, just as it does during gamete production. It allows each pair of characters to represent themselves independently without obstacles.

Mendel allotted a round yellow seed and a wrinkled green seed for such a dihybrid cross. Only round yellow seeds grew from the F1 generation. The F2 generation produced four unique seed combinations as a part of the self-pollination of F1 progeny. Round-yellow, wrinkled-yellow, round green, and wrinkled green seeds were obtained in the phenotypic ratio i.e., 9:3:3:1. The phenotypic ratios of yellow:green color and round:wrinkled seed shape in the monohybrid cross were also present in the dihybrid cross. As a result, he concluded that characteristics are transmitted and inherited separately.

Based on this finding, he developed his third law, the Law of Independent Assortment. dihybrid crosses of the paternal genotype RRYY as round yellow seeds and rryy as green wrinkled seeds, respectively present the law as an example of this. The probability of gametes forming with the gene R and the gene r are evenly divided in this case. Furthermore, genes Y and y have an equimolar chance of forming gametes. In conclusion, each gamete should present R or r, and Y or y should be present in each.

The separation of R and r is independent of the separation of Y and y, which is based on this law. Hence, there are four unique gamete types: RY, Ry, rY, and ry.

The allele combinations that result are unique from their parents’ (RR, YY, rr, and yy).

Facts about Independent assortment in meiosis

To summarize the above, all maternal chromosomes will not be separated into a single cell, but all paternal chromosomes will be separated into a separate cell. If two genes did not follow a separate assortment in the extreme instance, the character genes, including color and shape, could have always been transmitted as a pair. For instance, as a result, the color and shape of alleles may have always been together, and the features of alleles may have always been the same.

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