5 Independent Assortment Example: Detailed Explanations

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The Law of Independent Assortment states that alleles for separate traits are passed independently of one another.

The biological evolution of one allele for a particular trait is not related to the biological evolution of one allele for any other trait. Mendel proved this law with the help of his experiments that he performed on the dihybrid cross. That is to say, an allele received by a gamete for a specific gene does not disturb any other allele that other gene receives.

Here are some independent assortment example –

Independent assortment takes place during the procedure of meiosis. Meiosis is the same as the process of mitosis. Their end products are gamete cells. These gamete cells contain half the DNA of the regular diploid cells and thus are considered haploid. This part is important for reproduction between two parents because it permits the two gametes to combine with each other to form a zygote that is diploid in nature and contains all the DNA that is required to produce a new organism.

The law of independent assortment

The law of segregation helps us estimate how a single trait linked to a single gene will be passed down.

We need to know if the two genes are inherited separately or not in order to produce a precise forecast. So, we need to determine whether they “neglect” one another while sorting into gametes or if they “stay together” and are inherited as a group.

Gregor Mendel discovered that several genes acceded to each other independently, and this is what is known as the law of independent assortment. Mendel’s law of independent assortment states that the alleles of two (or more) different genes get sorted into gametes independently of one another. That is, an allele obtained by a gamete for a specific gene does not interfere with any other alleles received by that gene.

The law of assortment explains the random inheritance of the genes that take place from paternal and maternal sources. This law states that various genes and their corresponding alleles are acceded to separately or independently within the organisms that can reproduce sexually. While the procedure of meiosis takes place, chromosomes get separated into various gametes. When there is crossing over, the genes present on a chromosome get interrelated so that they can reorganize themselves. As a result, each gene is transmitted autonomously.

Independent assortment in alleles

In a Mendelian system, alleles that are located at loci on distinct chromosomes must be acquired separately since each homologue will assort individually.

The biological evolution or biological selection of one allele for one trait is not related to the biological evolution of another allele for any trait. With the help of his experiments with the dihybrid cross, Mendel discovered this law. In his experiments where he found monohybrid crosses, he achieved a theoretical result of 3:1 ratio between the phenotypes of dominant and recessive.

Whereas in his other experiments where there were dihybrid crosses, he found the ratio to be 9:3:3:1. This proves or demonstrates that one of the pairs of alleles is acquired separately from the other, with each having a 3:1 phenotypic ratio.

Independent assortment in meiosis

IN RABBITS:

Let us talk about a hypothetical population of the rabbits that are said to have only two traits that are visible: firstly, the fur color, that can either be black or white; and secondly, the eye colors, which can either, be green or red. The black fur allele (denoted as B) acts dominant over the white (denoted as b), whereas on the other side, the green eye allele (denoted as G) acts dominant over the red (denoted as g).

In this hypothetical example, two bunny rabbits that are hybrid in nature are mixed. This means that, both the bunny rabbits will have black colored fur and their eyes will be green in color, but still, their genotype will be heterozygous in nature. Thus, their genotype will be denoted as BbGg. All of the rabbits in this population of two share the same mix of features. To put it another way, they’re all black and have green eyes.

Before breeding starts, each bunny rabbit is required to create gametes. While this process continues, not only that the alleles are divided (the law of segregation), but each copy of each chromosome is assigned to a different gamete at random. This means though the parental phenotypes are black fur with green eye color, the offspring or the babies can acquire various combinations or sequences of these features or traits.

For example, a single baby can receive a genotype of bbgg, where its phenotype will be white fur and red eyes. On the other hand, an offspring can also have Bbgg as its genotype, where its phenotype will be black fur and red eye color. This shows the Law of Independent Assortment.

Independent assortment in chromosomes

While meiosis takes place after the discovery of chromosome, it became easy to explain the independent assortment as a result of the independent movement of every pair of homologous chromosomes and also their mode of action in the process of meiosis.  For every gene independent assortment is necessary so that they can create new genetic sequences and combinations that will help increasing the genetic diversity, also known as genetic variations, within the organisms.

Mendel’s law of independent assortment defines that the resultant chromosomes are classified in a random manner by fusing the chromosomes of maternal and paternal sources. Finally, the zygote forms a mixture of chromosomes rather than a distinct set of features from each parent.

Meiosis is a kind of cell division the decreases the chromosome number of a parent by half of its original number to create four reproductive cells which are known as gametes. In humans, diploid cells contain 46 chromosomes; from these 46 numbers of chromosomes, half of the number of chromosomes is acquired from maternal (mother) source, that is, 23 chromosomes from female gamete. The other half of the number of chromosomes is from paternal (father) source, that is, 23 chromosomes from male gamete. Pairs of these similar chromosomes are known as the homologous pair of chromosomes.

During the process of meiosis, the homologous pairs of chromosomes are divided in half to give rise to haploid cells, and this division or assortment of the homologous pairs of chromosomes is random in nature. This indicates that almost all the chromosomes from a maternal source will not be grouped together in one cell, whereas almost all the chromosomes from a paternal source will be grouped together in another. Alternatively, after the process of meiosis takes place, every haploid cell consists of a mixture of genes from each parent (mother and father) of the organism.

Independent assortment of plants

PEA COLOR AND PEA SHAPE:

Gregor Mendel executed various experiments that involve breeding of pea plants. He studied how “units of heredity” used to function in their own way, which gradually became to be known as the genes after when DNA was discovered and confirmed that they are the material that codes for genetic information.

Mendel discovered the Law of Independent Assortment after he did his experiment of breeding the two various pea plants with two separate characteristics. His experiment was to breed a yellow pea plant that has round peas with that of a wrinkled plant that has green colored peas. As because the yellow plant with round peas acts dominant over the winkled plant with that of green peas, thus all the offspring or the next generation species or peas will be of yellow and round shaped.

When this first generation was crossbred with each other in a dihybrid cross, the second generation had a lot of variation. Peas were no longer merely yellow and round or green and wrinkled; some were green and round and others yellow and wrinkled.

Moreover, in a ratio of 9:3:3:1, the offspring displayed their traits or characteristics. This means, nine peas were round in shape and yellow in color, three peas were round in shape but green in color, other three peas were wrinkled in shape and yellow in color and only one pea was wrinkled in shape but green in color.

This ratio remained constant for every other dihybrid that was crossed afterwards. Mendel observed 9 yellow round: 3 yellow wrinkled: 3 green round: 1 green wrinkled pea.

independent assortment example
Independent Assortment in Peas- Wikipedia

This kind of variation took place because only one allele was passed on to their offspring by each of the parent. Because the yellow plant with round peas has a dominating role over the winkled plant with green peas, all progeny or subsequent generations of peas will be yellow and round.

Mendel’s experiment indicated that the alleles for round or wrinkled peas were inherited independently from the alleles for yellow or green peas because the plants were not only round and yellow or green and wrinkled. They are now known to exist on distinct chromosomes, allowing them to be jumbled up while the process of meiosis takes place.

Independent assortment of animals

IN Trypanosoma brucei:

Trypanosoma brucei is a zoonotic protozoan parasite species complex that are transferred by tsetse flies and categorized into three subspecies-

  1. Trypanosoma brucei gambiense
  2. Trypanosoma brucei rhodesiense
  3. Trypanosoma brucei brucei

In human, the first two subspecies Trypanosoma brucei gambiense and Trypanosoma brucei rhodesiense cause sleeping sickness. On the other hand, the third Trypanosoma brucei brucei, causes cattle illness. Trypanosoma brucei brucei is not infective to humans.

Because no chromosomal condensation has been observed in any life cycle stage and no gamete-like phases have been identified, traditional genetic analysis has been used to determine if this parasite has a reproduction cycle and undergoes meiosis.

T.brucei is diploid in nature and the offspring of the cross would be expected to be heterozygous in nature for markers that are homozygous in nature and the difference between the parents, but would only acquire one allele from each locus where the parents are heterozygous. In a Mendelian system, every heterozygous locus’s two traits are transmitted in a 1:1 ratio, whereas those on separate chromosomes are acquired separately.

What phase is independent assortment?

Independent assortment takes place during the procedure of meiosis. Mitosis and meiosis are similar processes. Gamete cells are the end result of their work. These haploid gamete cells contain half the DNA of typical diploid cells and are therefore classified as haploid.

In eukaryotes , the independent assortment occurs during meiosis in the metaphase l phase of meiotic division. It gives rise to a gamete that carries a mixture of chromosomes. Gametes consist of half the number of chromosomes that are present in a regular diploid somatic cell.

During the production of gamete, the regular number of chromosomes that is diploid in number gets reduced to half of its original number in the course of meiotic division to form gametes that contain only 23 chromosomes thus considered haploid in nature. While there is sexual reproduction, the female gamete and male gamete combine with each other to bring out a newly formed organism or a formation of diploid zygote. The independent assortment occurs in eukaryotes during the metaphase l phase of meiotic division during meiosis.

The Law of Independent Assortment studies the genetic inheritance that is passed randomly between both parents. We know that the normal count for human diploid cells is 46 chromosomes. From these 46 numbers of chromosomes, half of the number of chromosomes is acquired from maternal source, that is, 23 chromosomes from female gamete which is also specified as the ovum or the egg cell.

The other half of the number of chromosomes is from paternal source, that is, 23 chromosomes from male gamete, specifically known as the sperm cell. As mentioned in the Law of Segregation, during the meiotic division two of the homologous chromosomes part from one another.

During crossing over, the linked genes that could not be assorted into a random order permit those genes to reorganize themselves when meiosis takes place. During this procedure, the interchange of the chromosomes that are homologous in nature takes place both in the chromosomes of maternal and paternal sources to confirm that the independent assortment of the linked genes has taken place successfully.

The resultant chromosomes are ordered independently by fusing the chromosomes of maternal and paternal sources, according to Mendel’s law of independent assortment. Finally, the zygote forms a mixture of chromosomes rather than a distinct set of features inherited each parent. As a result, chromosomes are regarded to be independently arranged, resulting in a zygote with a mix of maternal and paternal chromosomes.

Is independent assortment the same as crossing over?

Independent assortment occurs only when the genes are placed on various chromosomes. If the two genes are located far from each other on the same chromosome, then crossing over necessarily separate the genes and thus the genes are now able to assort independently.   

The second law of Mendel is not applicable for all the genes. Genes are “linked” when they are close enough with each other on the same chromosome and travel methodically while meiosis occurs. Thus, independent assortment does not happen with linked genes. Only when the genes are spread across several chromosomes does independent assortment occur.

Independent assortment helps in the production of new combinations or new sequences of alleles. While meiosis l takes place, crossing over occurs during the process of prophase and independent assortment occurs during the process of anaphase. While these happen, new sequences of alleles occur for the sets of chromosomes. Random fertilization of the gametes produced during meiosis also introduces genetic diversity (genetic variation). There are various marks while genetic diversity can get increased during sexual reproduction.

Crossing over happens during prophase and independent assortment happens during anaphase during meiosis l. New allele sequences for the chromosomal sets emerge as a result of these events. Genetic variation is also introduced through random fertilization of gametes produced during meiosis. Any of the genetically unique sperm generated by a male may fertilize the genetically unique egg produced by a female.

While metaphase l takes place, the pairs of chromosomes that are homologous in nature are arranged along the plate of metaphase. The alignments of the pairs that are homologous are random in nature and vary for every cell that passes through the process of meiosis. One maternal and one paternal pair of sister chromatids are seen in each tetrad.

The tetrad corresponding to human chromosome 1 in one cell, for example, may align so that the paternal sister chromatids face one pole and the maternal sister chromatids face the other. In another cell, there is a 50% probability that the opposite orientation will occur during metaphase I.

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