Law of Independent Assortment Definition
The Law of Independent Assortment is a law that describes how different genes and their alleles are inherited within sexually reproducing organisms. Unless genes are linked on the same chromosome, the traits that they carry will be randomly assorted into different gametes during meiosis. This means that the combination of traits seen in the parental generation will not necessarily reflect the various combinations seen in the offspring.
When Does Independent Assortment Occur?
Independent assortment occurs during the process of meiosis. Meiosis is similar to mitosis, only the final product is gamete cells. Gamete cells have half the DNA of regular, diploid cells and are considered haploid. This is a necessary part of sexual reproduction which allows two gamete cells to then fuse together to create a diploid zygote, containing all the DNA necessary to create a new organism.
To understand when independent assortment occurs, you must also understand the Law of Segregation. This law states that during meiosis, the two different copies of every gene are sorted into different gamete cells. The law of independent assortment, on the other hand, deals with the maternal and paternal sources of DNA being separated at random. To see these concepts in action, look at the image below:
As you can see, the law of independent assortment takes place as maternal and paternal sources of DNA are randomly divided. Sometimes, the gamete inherits the maternal version of a gene, and sometimes it inherits the paternal version. Which version it gets is completely random, based on the order that these chromosomes lined up in during the first stage of meiosis.
Law of Independent Assortment Examples
Independent Assortment in Meiosis
As a basic example, let us consider a hypothetical population of bunny rabbits that only have two visible traits: fur color (black or white), and eye color (green or red). The black fur allele (B) is dominant over the white (b), while the green eye allele (G) is dominant over red (g).
In this hypothetical example, two hybrid rabbits are mixed. What this means is that both rabbits look black with green eyes, but are really they have a heterozygous genotype. Both rabbits have the genotype BbGg. In this population of 2 rabbits, all the individuals have the same mixture of characteristics. In other words, they are all black with green eyes.
Before breeding, each rabbit will have to produce gametes. During this process, not only are the alleles separated (law of segregation), but each copy of each chromosome is randomly assigned to a different gamete. This means regardless of the parental phenotype (black with green eyes), the babies can inherit different combinations of these traits. For instance, one baby could receive the bbgg genotype, giving it white fur and red eyes. Alternatively, a baby rabbit could also receive the genotype Bbgg, giving it black fur and red eyes. This is the law of independent assortment.
Independent Assortment in Mendel’s Experiments
Gregor Mendel performed many experiments involving breeding pea plants. In doing so, he gleaned information about how “units of heredity” work, which would later on become known as genes after DNA was discovered and determined to be the material that encodes genetic information.
Mendel developed the Law of Independent Assortment after breeding two different pea plants with two different characteristics; he bred plants with yellow, round peas with plants that had wrinkled, green peas. Since yellow and round were dominant over wrinkled and green, all the offspring had yellow, round peas.
But, when this first generation was crossbred with each other in a dihybrid cross, there was a lot of variation in the second generation. Peas were no longer either just yellow and round or green and wrinkled; some were green and round, while some were yellow and wrinkled. Furthermore, the offspring showed their characteristics in a ratio of 9:3:3:1. Nine were round and yellow, three were round and green, three were wrinkled and yellow, and one was wrinkled and green. This ratio stayed the same even when hundreds of dihybrids were crossed.
This occurred because each of the parent plants only gave their offspring one allele and because yellow and round were dominant traits and masked the green and/or wrinkled traits in certain individual plants. The diagram below depicts Mendel’s dihybrid cross.
Mendel’s experiment showed that the alleles for round or wrinkled peas were inherited independently from the alleles for yellow or green peas since the plants were not just round and yellow or green and wrinkled. We now know that they exist on different chromosomes, which allows them to be mixed up during the process of meiosis.
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