Law of Segregation: Introduction
Johann Mendel was the pioneer in classical genetics, who on doing experiment’s with pea plants and on undergoing plant hybridizations found many characteristics of genes and how it undergoes evolution and also concluded many laws and evolution related to genetics.
We all know that Mendel’s contribution is very important for the field of genetics till now.
His contributions and theories are used in both scientific and medical field until date and law of dominance is one such important theory to be taken note of.
Before knowing about the law of dominance it is very much essential to know about the Mendel and his crossing techniques.
Law of Segregation and Mendel’s Crossing Techniques
Mendel experimented an edible pea plant which is scientifically known as Pisum sativum, which gave him tremendous results and thus sativum is considered as the best hybridization material for experiments.
The seven contrasting characters of pea plants include size as tall and dwarf, color of the cotyledon as green or yellow, seeds may be round or wrinkled, seed coat is colored or not, whether the flowers position is axial or terminal, the flower color is red or white.
The plants with these contrasting characters are led to cross pollinate and the characters of upcoming generations are noted.
The table given below shows the seven pairs of contrasting characters in a Mendel’s pea plant.
| Characters | Dominant Traits | Recessive Traits |
| Seed Shape | Round | Wrinkled |
| Seed Color | Yellow | Green |
| Flower Color | Violet | White |
| Pod Shape | Full | Constricted |
| Pod Color | Green | Yellow |
| Flower Position | Axial | Terminal |
| Stem Height | Tall | Dwarf |
What is Law of Segregation?
This law states that when two heterozygous character or allele in Filial one generation is taken into consideration; they show both recessive and dominant characters.
These two characters do not mix up or separate even at the time of gametogenesis so that the allele expresses only one type of character either dominant or recessive.
Mendel’s law of segregation can be understood by considering the monohybrid cross.
When a homozygous colored red and white color plants are crossed together the resultant F1 generation is expressed as pink color flower plants.
When these plants from F1 generation are let to self-fertilize the resultant alleles will be in red, purple and in white color, flowers occurs.
It is to be noted that white color reappears as same in the second filial generation.
Law of Segregation Mechanism
To know about the mechanism of segregation let us consider the monohybrid cross between homozygous red and white color flowered pea plants, considering RR for homozygous red color plant and rr for homozygous white colored plant.
The plant with homozygous red condition i.e., RR produces R allele and the plant with homozygous white condition i.e., produces allele.
Later when these plants are united to form as a hybrid plant they form as a pink or purple colour flowered plant with the alleles Rr.
Due the incomplete dominance of these alleles the character of both the colour plants mix-up forming a pink or purple character in the first generation.
Again, when these plants are led to undergo self-fertilisation three possible combinations including all the character of an allele occurs in the filial 2 generation including red, pink or purple and white coloured flowers.
Thus, from this it’s clear that the alleles due not mix up during gametogenesis they are just be a part of their gene to express themselves.
Hence Mendel denoted law of segregation as “Law of purity of genes”.
Law of Segregation in Drosophila melanogaster
The law of segregation can be applied widely in many organisms including plants and animals.
Sorting an example in animals it is important to know about T.H. Morgan and his work in Drosophila.
When Morgan crossed the drosophila considering two homozygous condition such as one having a long wing and other with a short wing the traits of the filial 1 generation appears to be long winged and also with versatile wing in the ratio of 3:1 respectively.
When these are left to self-breed and on considering in Filial 2 generation both homozygous and heterozygous characters tend to appear in this generation which means that both the alleles which are recessive and dominant being together for a longer time didn’t contaminate with each other and remains pure.
Law of Segregation in Human
Albinism is considered as an inherited disorder of melanin metabolism in our body due to the absence of a pigment known as melanin.
The Patient affected with this syndrome has a characteristic appearance of milk white coloured skin and marked photophobia.
It is generally known as the inborn error of metabolism as the genes belonging to this trait do not produce a particular enzyme.
This type of lacking of pigment production in the human body is due to the abnormal recessive trait present in a particular gene.
Let us consider an allele A to represent the normal melanin production and a to represent an albino trait i.e., no melanin production.
| Genotypes | Phenotypes |
| AA (Homozygous dominant) | Normal pigment |
| Aa (Heterozygous dominant) | Normal pigment |
| aa (Homozygous recessive) | Albino (No pigment) |
The individual who is having homozygous recessive trait i.e., aa allele they are not capable of producing tyrosine enzyme which is essential for the black pigment synthesis.
An albino patient has a melanocyte producing cell but it is not in an activated form. This also provides a good example for law of segregation.
Law of Segregation Citations
- Statistics of Mendelian segregation-A mixture model. J Anim Breed Genet . 2019 Sep;136(5):341-350.
- Cheaters sometimes prosper: distortion of mendelian segregation by meiotic drive. Trends Genet . 1993 Jun;9(6):205-10.
- A rigorous measure of genome-wide genetic shuffling that takes into account crossover positions and Mendel’s second law. Proc Natl Acad Sci U S A . 2019 Jan 29;116(5):1659-1668.