To understand Mendel’s Law of Segregation and Law of Independent Assortment, one must first take a glimpse into the setting and experiment that preceded these laws. Darwin’s Origin of species ushered forth a new age of science. The concept of transference of traits among organisms through time shattered previously held beliefs and impacted all branches of biological science, from anatomy, physiology to systematics and taxonomy. His thesis that traits are selected by nature with the most fit organism being most able to pass on its traits to its offspring also put forth the question of “how?” How are traits passed on from parents to offspring? This sparked the study of heredity.
Heredity deals with how a trait is passed on from parent to offspring. A trait is an expression or a particular form of an anatomic or physiological feature, like blue eyes, cleft chins, blond hair and baldness. The monk Gregor Mendel delved into this study of inheritance during the 19th century.
Gregor Mendel utilized results from several experiments using garden peas (Pisum sativum), observing and crossing several generations. His 7-year work (1856-1863) lead to the publication of “Experiments on Plant Hybridization” which he published in 1866. The paper utilized 7 discrete plant characters: seed form and color, pod form and color, stem place and size, and flower color. He first obtained pure lines for each trait to act as the parent generation. Pure lines were obtained by breeding a particular trait repeatedly until several generations have passed without any deviation (for example a pure green pod pea plant would be a product of several generations that when interbred, would not show any other color in the offspring). Mendel then interbred pure lines to produce an F1 generation and interbred those to form the F2 generation. He also bred plants between generations. He then documented the results of his experiments and found a mathematical pattern to the breeding results.
Mendel found that breeding between plants containing one purebred trait with another plant containing another resulted in only one trait being found in the F1 generation with no blending of the traits. For example, the breeding of a purebred plant containing white flowers with one containing purple flowers would produce no white flowers, instead producing offspring only having purple flowers. He then self-crossed the F1 generation, which yielded a ratio of three purple flowers for every one with white. This led Mendel to conceive of a hereditary factor that contains a representation of a trait (later to be defined as a gene having alleles or traits). He postulated that a parent can only pass down one factor to the offspring, with the other coming from the other parent. He further postulated that one trait was dominant to the other, in this case having purple flowers being dominant to having white ones. The law of segregation Mendel proposed was that each parent contains a pair of factors and an offspring randomly inherits one of those via sex cells from each parent. With the discovery of the gene and alleles it was modernized to “As the parent produces gametes, each gamete gets a copy of the gene (an allele). Gametes between two individuals randomly unite during sexual fertilization forming an offspring with a gene copy from each parent. Thus the 3:1 ratio he obtained could be obtained by having two purebred parents (PP x pp) yielding all purple flowers (dominant P) in the F1 generation (Pp) and when self crossed would get a 3:1 ratio in the F2 generation (PP, 2Pp, pp).
The law of independent assortment on the other hand states that genes and traits are passed on independent of other genes and traits. This further means that during gamete formation, the copy of a gene for one trait (flower color) is assigned or assorted independently of the gene copy for another trait (seed texture). This means that a dihybrid cross for two traits would yield a ratio of 9:3:3:1 (9 occurrences or expressions of both dominant traits in a plant, 3 of a plant having a dominant expression of one trait and a recessive in the other, 3 for the inverse, and one that is purely recessive for both traits) but when each traits is singled out, would still yield 3:1. This has been proven true by observing the process of meiosis, but only for traits not found within linked genes.
The Laws of Segregation and Independent Assortment laid the foundation of heredity and genetics as we see it today. These Mendelian laws of inheritance allowed scientists to find numeric patterns in organism reproduction, trace their ancestry, predict diseases and breed for particular traits. Mendel’s work gave a satisfactory answer to Darwin’s “how”, and is still a mainstay even in today’s study of modern genetics.