Question

Explain the law of independent assortment with a suitable example.

Answer 1

Hint: Law of independent assortment is a part of Mendel’s law of inheritance which explains the inheritance of genes with two different traits from parents to the offspring. It is the third law of inheritance.

Complete answer:
Gregor Mendel, well known for his study in genetics, was a monk who was interested in farming. He studied most of the Laws of inheritance in garden pea plants (Pisum sativum). Mendel’s law of inheritance states the manner in which traits are acquired from one generation to another. How these traits are transferred, whether they have any effects on other traits or even if they are transferred whether it will be expressed or silenced. The first law of heredity postulated by Mendel was the “Law of Segregation”. Based on this law Mendel then postulated the “Law of Independent Assortment”.
The Law of independent assortment defines the independent inheritance of genes and their alleles irrespective of the other trait genes or their alleles. This was studied by Gregor Mendel in a dihybrid cross (crossing over of two plants with two distinguishable traits/features) in pea plant (traits studied: round and yellow seed). The results showed a different combination of a trait in the offsprings with regards to their parents (figure). The independent assortment of genes occurs during meiosis, wherein each haploid cell contains a mixture of genes from both the parents. The Law of independent assortment also defines the process of recombination. Recombination also occurs during meiosis where DNA undergoes crossing over to form a new combination of genes. Since the DNA from both parents undergo crossing over, the genes are sorted independent of their origin (parental origin) and independent of one another. And thus the offspring can have traits similar to either of the parents or a combination of both the parents.

                      Figure: Dihybrid cross showing Law of independent assortment

Example: Taking the example of a dihybrid cross in a pea plant with two distinguishable features like round and yellow seeds. The round and yellow character of a pod is a dominant trait, presented as RR (round pods) and YY (yellow pods). When this pea plant with round and yellow pods is crossed with a pea plant having green (rr) and wrinkled (yy) pods (RRYY X rryy). All the offspring in the first generation ($F_1$ generation) had yellow and round seeds as phenotype and RrYy as genotypes. This was due to the dominant nature of both yellow and round genes (as explained by the Law of segregation). When these $F_1$ generation heterozygous (RrYy) plants where inbreed they formed $F_2$ generation with a variety of phenotypic and genotypic characters in the offsprings (figure 1). As the phenotypic characters seen in $F_2$ generation had traits of parents from both $F_1$ generation and $F_2$ generation, it was concluded that traits are inherited independently without affecting the other traits. Since the characters of the $F_1$ generation parents were conserved and seen in the $F_2$ generation, it was concluded that the alleles of each trait are independently assorted in the offspring.

Note:
Mendelian work was discovered independently by three scientists-Hugo de Vries, Carl Correns, and Tshermak. Advancements made in microscopy helped them to observe the mechanism of cell division at various stages. This also led to the discovery of chromosomes.