Mendel’s experimental study on the pattern of inheritance was a turning point in the history of genetics. Based on his investigation, Mendel proposed three laws of inheritance. Mendel’s Law of Inheritance is the fundamental principle of inheritance in genetics. Law of Dominance, Law of Segregation and Law of Independent Assortment are collectively known as Mendel’s Laws of Inheritance.
Mendel chose to perform monohybrid cross of a pair of contrasting traits. The observations of monohybrid cross led to the formulation of Law of Segregation and Law of Dominance. Followed by this, Mendel performed dihybrid cross taking two contradicting traits together for crossing.
Here we will discuss the Law of Independent Assortment.
Also Read: Mendel’s Laws Of Inheritance
Law of Independent Assortment
This law states that the alleles of two more genes get sorted into gametes independent of each other. The allele received for one gene does not influence the allele received for another gene.
Mendel’s experiment always portrayed that the combinations of traits of the progeny are always different from their parental traits. Based on this, he formulated the Law of Independent Assortment.
Reasons for Independent Assortment
Independent assortment takes place during the process of meiosis. In this process, the chromosomes are halved and are known as haploid.
To understand the law of independent assortment, it is very important to understand the law of segregation. In this, two different genes are sorted into different gamete cells. On the other hand, law of independent assortment occurs when the maternal and paternal genes are divided randomly.
Mendel’s Experiment on Law of Independent Assortment
The Law of Independent Assortment states that during a dihybrid cross (crossing of two pairs of traits), an assortment of each pair of traits is independent of the other. In other words, during gamete formation, one pair of trait segregates from another pair of traits independently. This gives each pair of characters a chance of expression.
In the dihybrid cross, he chose round-yellow seed and wrinkled green seed and crossed them. He obtained only round yellow seeds in the F1 generation. Later, self-pollination of F1 progeny gave four different combinations of seeds in the F2 generation. He obtained round-yellow, wrinkled-yellow, round green and wrinkled green seeds in the phenotypic ratio 9:3:3:1.
The phenotypic ratio 3:1 of yellow: green colour and the ratio 3:1 of the round: wrinkled seed shape during monohybrid cross was retained in the dihybrid cross as well. Thus, he concluded that characters are distributed independently and inherited independently. Based on this observation, he developed his third law – Law of Independent Assortment.
The dihybrid crosses between the parental genotype RRYY (round yellow seeds) and rryy (green wrinkled seeds) explains the law. Here the chances of formation of gametes with the gene R and the gene r are 50:50. Also, the chances of formation of gametes with the gene Y and the gene y are 50:50. Thus, each gamete should have either R or r and Y or y.
The Law of Independent Assortment states that the segregation of R and r is independent of the segregation of Y and y. This results in four types of gametes RY, Ry, rY, and ry. These combinations of alleles are different from their parental combination (RR, YY, rr and yy).
Example of Law of Independent Assortment
Let us consider an example of rabbits with two visible traits:
- fur colour (black or white)
- eye colour (green or red)
Two-hybrid rabbits are crossed. Both the rabbits have a genotype BbGg. Before breeding each rabbit produced gametes. During this, the alleles are separated and the copy of each chromosome is assigned to different gamete. That means, regardless of the parental phenotype, the baby rabbits inherit different combinations of the traits. Alternatively, a baby rabbit can have a genotype Bbgg.
Also Read: Genetics
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