Law of Segregation and Dominance – Mendel’s law
The principles or laws of “Heredity” were discovered by a monk named Gregor Mendel in the 19th century. He conducted his experiments involving hybridization experiments in garden peas. (Pisum Sativum) He cultivated and tested some 29000 pea plants. He measured only binary characteristics such as color, shape, and position of offspring, rather than quantitative characteristics. His method of data analysis and incorporation of huge sample size gave credibility to his data, which was then published in 1865. Mendel’s finding further provided a chance to other scientists to predict the expression of traits on the basis of mathematical probabilities.
- The Law of Segregation: The law states that when any individual produces gametes, the copies of a gene separate so that each gamete receives only one copy. Either of the alleles will be received by the gamete.
- The Law of Dominance: If there are two alleles coding for the same trait and one is dominant it will show up in the organism while the other won’t.
The law of segregation can be defined in a simple way as when the egg and sperm unite during fertilization, each contributes its allele, finally making the paired chromosome in offspring. It also means that allele pair present in egg and sperm separates or segregates during gamete formation, and randomly unite at fertilization. This phenomenon was later on proved when the meiotic cell division process was found. In meiosis, the paternal and maternal chromosomes get separated (according to the law of segregation) and the alleles with the hereditary character are segregated into two different gametes.
This law forms the first law out of the three Mendel’s law, as follows:
- Mendel’s first law (the law of segregation) states that during the formation of reproductive cell (gametes), pairs of hereditary factors (genes) for a specific trait separate; so that offspring receive one factor from each parent.
- Mendel’s second law (the law of independent assortment) states that chance determines which factor for a particular trait is inherited.
- Mendel’s third law (also called the law of dominance) states that one of the factors for a pair of inherited traits will be dominant and the other recessive, unless both factors are recessive.
The law of dominance can be defined in a simple way as, when two homozygous individuals with one or more set different or contrasting characters are crossed, the characters that appear in first generation hybrid are dominant characters and those which do not appear in first generation characters are recessive characters. The dominant and recessive characters’ appearance in phenotype can be explained on the basis of enzymatic functions of the gene. The dominant genes are capable of producing or result in the formation of functional enzymes, whereas the recessive genes are not capable. These functional enzymes result in specific phenotype due to the presence of a dominant gene, whereas recessive genes fail to produce any specific phenotype. In the heterozygous condition also, dominant genes are able to express themselves, such that heterozygous and homozygous individuals have the same phenotype.
Mendel carried out his experiments on garden pea plants. He discovered that by crossing a white flower plant and a purple flower plant, the result was not a hybrid offspring. The offspring was purple-flowered and not the mix of two. He then derived the idea of heredity units, which he called ‘factors’, later on, known as genes. He also planted the theory that the factors are generally present as a pair in ordinary body cells, yet segregate during formation of sex cells. After segregation, the factor which is dominant gets expressed and the other has a recessive characteristic. Thus, during his crossing experiment, as stated above, he obtained purple flowers which hide the recessive genes of white flower. After that Mendel self-fertilized the first generation and obtained 3:1 ratio, this is how he proved that genes can be paired in 3 possible ways: AA, aa, Aa. [A] represents the dominant factor, and [a] represents the recessive factor.
- Combining all the Laws of Mendel and the theories proposed, the science of heredity becomes clearer. Mendel has clearly stated in his results that each individual has two factors for each trait, one from each parent. The factor which is dominant, its trait gets expressed.
- The two factors present in an individual may or may not be the same. If the factors are the same, the individual is called homozygous, and if the factors have different information, then the individual is called heterozygous.
- The alternative form of a factor is called allele. The genotype of an individual is determined due to the type of allele possessed by the individual, whereas the phenotype of an individual is determined by allele as well as by its environment.
- Every individual possesses two alleles from each trait. One allele is given by the female parent and second allele is given by the male parent, which are then passed on in the gametes: egg and/or sperm.
- During the gamete formation, the paired alleles get separated randomly such that each gamete receives a copy of two alleles. This is how the allele for every phenotypic expression is passed or inherited or received by both the gametes, which then participates in fertilization.
- Also, the presence of allele does not promise that the trait will be expressed in the individual who possesses it. In heterozygous individuals, only the dominant allele gets expressed, the recessive allele is present but its expression is hidden.
- It has been confirmed by various cytological studies that whether dominance occurs or no dominance, the law of segregation holds true to all cases. Its far-reaching applicability has made it a rare biological generalization.
- Gametes unite at a random manner and when gametes are numerous all possible combinations can occur, with the result being in the ratio of 3:1. The results are generally represented in the form of Punnet Square.
The Rediscovery of Mendel’s Theory:
In 1900, the Mendelian theories were rediscovered by three European scientists, Hugo de Vries, Carl Correns, and Erich von Tschermak. The rediscovery made Mendel’s theory important and also more controversial that it can be applied to a certain category of species or traits. A scientist, also a promoter in Europe, William Bateson, coined the terms term “genetics”, “gene”, and “allele” to describe many of the findings of the Mendelian theory. The heredity model was criticized by many scientists, as it inferred that heredity was discontinuous, in contrast to the fact that obvious and apparent continuous variations in heredity were observed in most of the traits. Many biologists dismissed the Mendelian theory because of the uncertainty that it would be applicable on all species, and there seemed to be a very few true Mendelian characters in nature. However, later on, works carried out by biologists and statisticians such as R.A. Fisher showed that if multiple Mendelian factors were involved in the expression of an individual trait, they could produce diverse result observed. Furthermore, a scientist called Thomas Hunt Morgan and his team later integrated the theoretical model of Mendel with the chromosome theory of inheritance, in which the chromosome of cells were thought to hold the actual hereditary information created a scientific field, now known as classical genetics, which was extremely successful and cemented Mendel’s place in history.
Critical Appreciation / Importance of Law of Dominance:
To ensure the applicability and robustness of law of dominance many scientists undertook cross-breeding experiments. The experiments were conducted by Correns on peas and maize, by Bateson on a variety of organisms. The general observations of the scientists were that a large number of characters in organisms are related as dominant and recessive. It is only because of the law of dominance that the harmful recessive character gets suppressed and is not expressed by the normal dominant character in the hybrid. In human beings a form of idiocy, diabetes, hemophilia etc. are recessive characters. A person appears healthy if all these characters are suppressed in the hybrid. For generations, these recessive hybrids are present but are not expressed and are passed on silently to various generations.
The exception to the law of dominance is Incomplete dominance. Various cases were recorded by scientists, where the first- generation hybrids exhibited a blending of characters of two parents. This is called incomplete dominance or blending inheritance. It means that the two genes of a pair of alleles are not related either as dominant or recessive, but each of them expresses itself partially.
General Mendelian Deviation:
- Incomplete Dominance: The type of dominance in which both genes are expressed equally, against the law of dominance. For instance, a cross between red coloured flower plant and a white coloured flowered plant, produces a pink coloured flowered plant.
- Codominance: In this case both the alleles in heterozygote express their phenotypes greater than the intermediate one. For instance, the AB blood group expresses both the alleles of IA and IB. In easy words, co-dominant alleles of hetero-zygotes are phenoltypically similar to both parental types. The main difference in case of codominance and incomplete dominance is that both alleles are active in the first one, whereas both alleles blend in the later one.
- Lethal genes: The gene which causes death when in homozygous condition is called a lethal gene. Mendel’s finding was based on equal survival of genotypes. But it was observed that normal segregation ratio 3:1 shifted to 2:1 sometimes. Lethal gene can be recessive, dominant, conditional or synthetic depending on the gene involved and its environment.