Dictionary > Law of Independent Assortment

Law of Independent Assortment

Law of Independent Assortment
n.,
[lɔː ɒv ‚in·də′pen·dənt ə′sȯrt·mənt]
Definition: Mendelian law stating that the process of random segregation and assortment of pairs of alleles during gamete formation

Law of Independent Assortment Definition

The Mendelian inheritance principles are derived from and named after the monk, Gregor Johann Mendel in the nineteenth century. The Mendelian principles were formulated based on certain experiments conducted by Mendel with pea plants in the monastery’s garden.

Mendel’s inheritance laws are the following:

  • Law of Dominance and Uniformity states that alleles are either dominant or recessive and where a living organism with one or more dominant alleles will show the influence of this allele.
  • Law of Segregation, where each gene alleles segregate from each other during the formation of the gamete to allow the gamete to carry one allele only for each gene.
  • Law of Independent Assortment is related to the law of segregation where it states that segregation of different genes carrying different traits occurs independently during gamete formation.

During gamete production, the normal diploid number of chromosomes is reduced to half the chromosome number during meiotic division to produce haploid gametes containing 23 chromosomes only. The normal human diploid cell contains 46 chromosomes; half the number of chromosomes is derived maternally, i.e., from the female gamete (the egg cell or ovum) and the other half is derived paternally, i.e., from the male gamete (the sperm cell). During sexual reproduction, female and male gametes fuse together to produce a new organism or a diploid zygote.

When does independent assortment occur? The principle of independent assortment states that: “different alleles and genes are independently inherited during the meiosis of organisms that reproduce sexually”. The independent assortment of chromosomes is a result of the independent division of chromosomes into separate gametes. Then, crossing over takes place where genes on each chromosome are rearranged.

The Law of Independent Assortment discusses the random genetic inheritance from both parents. As stated in the Law of Segregation, the two homologous chromosomes separate from each other during meiotic division. Therefore, the chromosomes of both maternal and paternal gametes are assorted independently; in other words, chromosomes found in one gamete do not necessarily end up in the same source after division. As a result, one gamete may eventually have all chromosomes from the maternal source while another gamete can end up having different mixtures of chromosomes from both maternal and paternal sources.


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Even though genes that are found on the same chromosome or linked genes are not randomly assorted, crossing over that takes place during meiosis allows such genes to rearrange. During this process, the exchange of homologous parts of chromosomes occurs in both maternal and paternal chromosomes to ensure the independent assortment of linked genes as well. As a result, different gene combinations create great variety among individuals due to the production of gene mixtures that were not previously found.

Mendel’s law of independent assortment states that the resulting chromosomes are sorted randomly by mixing the maternal and paternal chromosomes. In the end, the zygote has a mix of chromosomes and not a defined set of specific traits from each parent. That’s why chromosomes are considered to be independently assorted so the zygote will eventually have a combination of different maternal and paternal chromosomes. Since the number of chromosomes in each gamete is 23 and the number of gametes is 2; thus, the number of possible combinations is 223, or 8,388,608. This number of possibilities allows for great variability in progeny genes. This gene variation has a great effect on evolution and evolutionary processes.

Law of Independent Assortment (biology definition): (genetics) One of the Mendelian Laws of Inheritance, stating that the process of random segregation and assortment of pairs of alleles during gamete formation results in the production of gametes with all possible combinations of alleles in equal numbers. Gregor Mendel discovered the principles of inheritance in his work with garden peas. In his paper describing this work, he described several features of inheritance, which after the rediscovery of Mendel’s work were formalized as the Mendelian Laws. Two of these laws are the Law of Segregation, and the Law of Independent Assortment. The Law of Independent Assortment is expressed in the inheritance of two or more different traits associated with different genes.

Principle of Independent Assortment

What does independent assortment mean? The law of independent assortment means that separate traits of different alleles are inherited by the zygote independently from each other. Where the random selection of one allele for a certain trait is not connected by any means to the selection of another allele for a different trait.

What is an independent assortment? Independent assortment states that the inheritance of various genes occurs independently of each other. In the law of independent assortment, the combination of genes and their probability is calculated and assumed by multiplying the probabilities of each gene. Moreover, the probability of having one gene does not influence the probability of having the other.

What stage of meiosis does independent assortment occur? Independent assortment in meiosis takes place in eukaryotes during metaphase I of meiotic division. It produces a gamete carrying mixed chromosomes. Gametes contain half the number of regular chromosomes in a diploid somatic cell. Thus, gametes are haploid cells that can undergo sexual reproduction at which two haploid gametes are fused together forming a diploid zygote having the complete set of chromosomes. The physical basis is the random distribution of chromosomes during the metaphase in relation to other chromosomes.

Why is independent assortment important? Independent assortment is responsible for the production of new genetic combinations in the organism along with crossing over. Thus, it contributes to genetic diversity among eukaryotes.

To define independent assortment, you should understand the law of segregation first. The law of segregation states that in meiosis, different gamete cells get two different independently assorted genes. On the other hand, the two maternal and paternal DNA are randomly separated allowing for more diversity in genes. The law of independent assortment is apparent during the random division of the maternal and paternal DNA sources. Due to random assortment, the gamete may get maternal genes, paternal genes, or a mixture of both. The genetic distribution is based on the initial stage of meiosis where these chromosomes are lined up randomly.

Independent assortment diagram
Figure 1: All possible arrangements are obtained according to the law of independent assortment. Credit: Christinelmiller, CC BY SA 4.0.

Independent Assortment Examples

Gregor Mandel carried out several experiments on pea plants. as a result, he was able to identify the way by which the units of heredity work, which are now known as genes after the discovery of DNA and genetic information.

How does independent assortment occur? Independent assortment occurs spontaneously when alleles of at least two genes are assorted independently into gametes. Consequently, the allele inherited by one gamete does not affect the allele inherited by other gametes.

Mendel noted that the transmission of different genes appeared to be independent events. In independent events, the probability of a particular combination of traits can be predicted by multiplying the individual probabilities of each trait. In independent events, the inheritance pattern of one trait will not affect the inheritance pattern of another.

For example, when Mendel crossed plants with round yellow peas to plants with wrinkled green peas, all of the F1 peas expressed the dominant traits round and yellow. In the F2, along with round yellow and wrinkled green peas, he observed round green and wrinkled yellow peas.

Each of the dominant traits was present in ¾ of the progeny and each of the recessive traits was present in ¼ of the progeny.

The four possible combinations of color and shape appeared in the ratio of 9:3:3:1, which represents the independent assortment of the genes for the two pairs of traits into the gametes.

If you have ¾ yellow and ¾ round then independent events predict that ¾ x ¾ = 9/16 will be both yellow and round. The proportions of the other three combinations can be similarly calculated.

Mendel observed 9 yellow round: 3 yellow wrinkled: 3 green round: 1 green wrinkled peas.

round yellow x wrinkled green peas Punnett square
Figure 2: Punnett Square for a cross between round yellow trait x wrinkled green trait.

Later, after the discovery of chromosomes, and of their behavior in meiosis, it was possible to explain independent assortment as a consequence of the independent movement of each pair of homologous chromosomes during meiosis. An independent assortment of genes is important to produce new genetic combinations that increase genetic variations within a population.

independent assortment of alleles in pea plants - diagram
Figure 3: Independent assortment of different alleles in pea plant according to shape and color. Credit: LadyofHats, public domain.
The development of the law of independent assortment was based on Mendel’s breeding of pea plants having different characteristics: garden pea plants that produced wrinkled green peas and another garden pea plant that produced rounded yellow peas. Yellow and round characters were more dominant; therefore, all offspring of the first generation were yellow and rounded peas. However, the second generation showed marked variation after breeding the first generation with each other.  The experiment proved the independent inheritance of homologous traits on different alleles in yellow and green peas as the produced offspring were not only yellow and round or green and wrinkled as their parents.

 

What is independent assortment as explained with a suitable example? Let’s take for example a random population of cats and track two traits: eye color (brown or green) and fur color (white or grey). The dominant allele for the eye color, for example is brown eyes (B) and the recessive allele, green eye color (b). As for the color of the fur, let’s say that the white fur (W) allele is dominant over the gray fur allele (w). Heterozygous cats with dominant traits, brown eyes and white fur, will produce gametes at sexual maturity. During gamete production, the alleles for eye color will be sorted independent of the alleles for the fur color, if we are to base it on the law of segregation. The resulting gamete after meiosis will contain random alleles such that when two heterozygous cats are crossed, their offspring will likely have mixed traits. One of the kitten, for example, could have brown eye color (BB or Bb) and grey fur color (ww). Another kitten might have green eyes (bb) and grey fur (ww). Others still could have brown eyes and white fur (thus, possible genotypes could be BBWW, BBWw, BbWW, BbWw). Now, this is just an illustrative example. In nature, the eye and fur color traits are polygenic, meaning several alleles are involved in determining the phenotype of the offspring.

The independent assortment is now explained according to the behavior of chromosomes during meiosis and the random movement of each homologous pair of chromosomes during meiosis. Independent assortment is an important process for the production of new genetic combinations that contribute to the genetic diversity among individuals that reproduce sexually.


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Further Reading


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References

  • Libretexts. (2020, December 1). 12.3D: Mendel’s Law of Independent Assortment. Biology LibreTexts. https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/12%3A_Mendel’s_Experiments_and_Heredity/12.3%3A_Laws_of_Inheritance/12.3D%3A_Mendels_Law_of_Independent_Assortment.
  • Monaghan, F., & Corcos, A. (1984). On the origins of the Mendelian laws. Journal of Heredity, 75(1), 67-69.
  • Ruch, D. G. (1998). A cookie model for the development of the concept of independent assortment. The American Biology Teacher, 60(9), 696-698.
  • Semagn, K., Bjørnstad, Å., & Ndjiondjop, M. N. (2006). Principles, requirements and prospects of genetic mapping in plants. African Journal of Biotechnology, 5(25).

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Quiz

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1. Which of the following is NOT true about Law of Independent Assortment?
2. When does independent assortment occur?
3. Which of these processes depicts the Law of Segregation?
4. Each gene segregates from each other during gamete formation
5. Why is independent assortment important?

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