what is mendel's law of segregation, how was it derived?

The Mendelian Principle or Law of Segregation, also called Mendel’s First Law, has been stated in the following manner: a hybrid between two parents differing in a set of characters possesses both parental factors which subsequently separate (segregate) in the gametes. This genetic principle, along with complete dominance, modified the early belief that heredity is purely a “blending” process in which the offspring exhibits a character that is intermediate of the two parents.

Originating from Gregor Mendel’s historic research work with garden pea, the law established that the transmission of traits from parents to progeny is carried by elementary units, which he called factor, in uniform, predictable fashion. This “factor” is now called gene or, with reference to those in the same gene pair but carry the blueprint for contrasting characteristics, alleles. This Law of Segregation is one of two laws Mendel formulated providing for the fundamental rules in the transmission of traits from parents to progeny.

Specifically as applied to monohybrid inheritance, Mendel found that the two alternative genes carrying the characters in the true-breeding parents originally crossed are combined in the hybrid (F1). The hybrid possesses both the genes (alleles) coming from the male and female parents in the proportion of 1:1. These genes in turn separate or segregate through meiosis in the production of sexual gametes in equal proportion so that one-half of both the male and female gametes carries the male parental gene while the other half carries the female gene.

But what exactly did Mendel do which led to the Law of Segregation?

For convenience in retracing how Mendel derived the Law of Segregation, the gene for smooth seeds in garden pea is denoted by the letter capital S while that for the wrinkled character is represented by the small letter s. The gene that manifests itself to the complete exclusion of a present but contrasting gene is called dominant while the latter which is hidden is called recessive.

First, crossing the original parents.

He first crossed (hybridized) two variants of garden pea with contrasting characters like smooth x wrinkled seeds. He did this by first emasculating the flower of one plant (for example smooth) and cross-pollinated it by dusting the stigma with pollen from another plant (for example wrinkled). Subsequently the flower developed into a pod which contains seeds. He then harvested the seeds upon maturity and planted them. He found that only one character of either parent, i.e. smooth, was exhibited by the hybrid or F1 plants and none of the wrinkled.

In addition to smooth x wrinkled cross, Mendel conducted identical experiments on six other sets of characters. He performed the following six other monohybrid crosses with the first character found to be dominant: yellow x green cotyledongrey-brown x white seed coats (violet-red x white flowers), inflated x constricted podgreen x yellow unripe podaxial x terminal inflorescence, and long x short stem (also referred to as tall x short).

Second, selfing the F1 hybrids producing the F2 generation.

Next he selfed the F1 plants (allowed the plants to perform natural self pollination). From 253 selfed F1 plants in the smooth x wrinkled cross, he harvested 7,324 seeds and planted them. He discovered that the characters that were not shown in the F1 reappeared in the F2 but in a frequency of only one-fourth (25%) of the total population. Of the 7,324 F2 plants, 5,474 had smooth (round or roundish) seeds and 1,850 had wrinkled seeds. These values are equivalent to the ratio 2.96:1 smooth and wrinkled seeds or 3:1. A similar ratio was observed in the other 6 sets of characters tested.

The above observations indicate that there are two genes that carry the contrasting characters. One, called dominant, is exhibited in the F1 (all individuals show the dominant character) and in the F2 (three-fourths of all individuals show the dominant character). The other, called recessive, is not manifested in the F1 even though it is present because it is masked (hidden) by the dominant gene, but it is exhibited in the F2 (one-fourth of all individuals). The appearance of both parental characters in the F2 suggests strongly that both parental genes are present in the F1 which is an essential element of the Law of Segregation.

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Next, successive selfing of the Fn generation.

To learn more about the patterns of transmission of the parental factors, Mendel continued with the selfing experiments. He selfed the dominant and the recessive F2 plants separately and successively. He found that selfing of the wrinkled-seed plants (recessive plants) yielded F3 plants all of which have wrinkled seeds. This indicates that the wrinkled plants possessed only the recessive gene s but no smooth S gene. In contrast, the smooth-seeded plants (dominant plants) yielded F3 progeny with 3:1 smooth to wrinkled-seed plants.

Out of 565 F3 plants that Mendel grew from the selfed smooth-seed F2 plants, 372 had smooth and wrinkled-seed with a ratio of 3:1. The remaining 193 plants had smooth seeds only and the succeeding selfed progeny of these plants retained the same character. This means that the smooth-seeded F2 plants are of two types: 372/565 (65.84% ≈ 2/3) hybrid smooth and 193/565 (34.16% ≈ 1/3) pure smooth. This is equivalent to the ratio 1.93:1. But considering all 7 characters, the average ratio of hybrid plants to true-breeding dominant plants was equivalent to 2:1 or 2/3:1/3. The hybrid smoothplants are those which produced smooth seeds but upon selfing yielded selfed progeny consisting of smooth and wrinkled-seed plants while pure smooth refers to those in which the progeny consisted of plants exhibiting only the smooth character.

Note: The plant (garden pea) referred to as smooth or smooth-seeded in Mendel's experiments leading to the formulation of the Law of Segregation are those which produced seeds that are all smooth (Mendel used round or roundish) while wrinkled-seed plants are those which produced only wrinkled seeds. Having been grown from a single seed, each individual plant was either smooth or wrinkled. No single plant produced a combination of  smooth and wrinkled seeds.

In sum, each gene carrying any trait is continuously transmitted and is not lost although one may be hidden, that is, a smooth-seeded plant may also possess the s gene but it is not visible. This is obvious in the way one character disappeared in the F1 but reappeared in the F2 and other selfed progeny of the plants exhibiting the dominant character.

The results showed also that the smooth hybrid plants and other selfed progeny exhibiting dominant character, having continuously produced both smooth and wrinkled-seed plants in the succeeding selfed generations, contained both the S and s alleles or with the heterozygous genotype Ss. Similarly, the pure smooth must also have contained two alleles but in identical forms, i.e. SS, and the pure wrinkled plants ss. Having identical alleles in the gene pair, both pure smooth and pure wrinkled plants would have exhibited the same phenotypes in the succeeding selfed generations.

Finally, the distinguishing feature of the Law of Segregation in monohybrid inheritance is the continuous transmission of the dominant and recessive traits in the ratio of 3:1

(Ben G. Bareja February 2012)

More on the Law of Segregation: click here to read)

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