Mendelian Inheritance Laws

Mendelian inheritance originated from a set of laws that are related to the transmission of characters which are found to be hereditary from parents to offspring. It is studied with the help of genetics. They were found to be controversial at first but soon they became the core of genetics. These laws were given by Gregor Johann Mendel and were published around 1865. He grew pea plants and generalised two laws Mendel’s law of hereditary and Mendel’s law of inheritance. Several laws explaining heredity were formed by Mendel with the help of his 7 years long experiment on pea plants. 

Mendel’s law: 

He discovered that by crossing true breeding white flowers and true-breeding purple flowers, a hybrid offspring formed. He expected a mixture of two colours but instead of that, he received only purple flowers. He discovered a heredity unit which he called “Factors” which were later called genes. He said that genes occur in pairs in an ordinary cell. They segregate at the time of gamete formation. He exclaimed that the dominant gene like in the above experiment was purple and would have the properties of recessive just as in white. Purple hides the characteristics of white. After self fertilising F1 generations and obtaining F2 generation, he found the results to be in a ratio of 3:1. He concluded from this experiment that genes occur in three pairs (AA, Aa, aa).

Mendel’s experiments:

He conducted his experiments from 1856-to 1863. He chose 7 contrasting pairs. He conducted his experiments in such a way that he used to observe a pair of contrasting characters. He used pure breed lines for contrasting characters. He crossed two pure lines, their results were called F1 generations. 

The appearance of the organism is called the phenotype. Whereas genetic makeup or constitution of an organism meant genotype.

Law of Inheritance:

• Law of dominance

• Law of segregation
• Law of independent assortment

Law of Dominance:

The law states that in a heterozygous condition (different allele) one allele is expressed more over the other allele. The allele expressed more is the Dominant allele. Dominant allele represses recessive allele. The recessive characters are seen only during F2 generations. The term dominant and recessive were referred to the genotypic interaction of constitutive alleles. It states that one trait repressed the presence of another trait. Mendel did this experiment only with the pea plant. The Law of dominance does not always hold. Dominant alleles can be expressed in heterozygous conditions, recessive alleles can be expressed in homozygous conditions. If both the alleles are recessive.

Law of Segregation:

Segregation’s law states that in diploid organisms, a selected allele of traits is passed to the offspring. In this condition, offspring receive one allele from two different parents. Heterozygosity can arise in two different ways. Heterozygous dominant and homozygous dominant are phenotypically similar. It was observed that true plant breeding varieties with contrasting traits always gave rise to the F1 generation with all expressing dominant characters. In F2 generations, both dominant and recessive traits are expressed in the ratio 3:1. In the process of meiosis, each gamete gets one of the two alleles as chromosome segregation and separates from each other during the process of division. This role which is played by meiotic segregation was not accepted by research workers at that particular time within Mendel’s lifespan.

Law of Independent Assortment:

With the help of this law, one can calculate the phenotypic and genotypic ratios based on the genes’ combinations. This law states that genes are not influenced by others in terms of allele sorting into the gamete. Every possible combination is to occur. Basically, in easy words, we can say separate genes for separate traits are independently passed from one generation to another. This law can be demonstrated with the help of a dihybrid cross. This cross will be between two true-breeding varieties that express different traits. Suppose we take green wrinkled seeds (yyrr) crossed with round yellow (YYRR), In F1 will get all offsprings to be YyRr. In F2 this law requires each either r or R and on the other side either Y or y. There is a possibility of the formation of four types of gametes YR, Yr, yR, yr. This cross on Punnett square selfing gives us 16 genotypes possible. 

CONCLUSION:

Discoveries of Mendel have helped this modern life to develop with ease. These laws helped in determining the new combinations in the progeny and their frequency was easily predictable. Plants and animal breeders’ works have been made a lot easier after the discovery of these laws. These laws have helped them in producing better breeds. A new type of plant varieties with desired characteristics. 

Some important terms for genetic inheritance

 Factors:  Factor is the unit of heredity which is responsible for the inheritance and appearance of characters.

  Allele: It is an alternative form of a gene that is located in the same position (loci) on the homologous chromosome.

  Homozygous:  A zygote formed by the fusion of two gametes having identical factors is called homozygote and an organism developed from this zygote is called homozygous.

Ex. TT, RR, tt.

 Heterozygous: A zygote is formed by fusion of two different types of gamete carrying different factors is called heterozygote (Tt, Rr) and an individual developed from such a zygote is called heterozygous.