The study of inheritance of a single pair of alleles or factors of a trait at a time (monohybrid cross) is called one gene inheritance.

Based on his observations on monohybrid crosses, Mendel proposed two general rules in order to consolidate his understanding of inheritance in monohybrid crosses.

These rules are called the principles or laws of inheritance.

They are :

Law of Dominance (First Law) : $(i)$ Characters are controlled by discrete units called factor. $(ii)$ Factors occur in pairs. $(iii)$ In a dissimilar pair of factors, one member of the pair is dominant while the other is recessive.

This law is used to explain the expression of only one of the parental characters in a monohybrid cross in the $F_{1}$ generation and the expression of both in the $F_{2}$ generation. It also explains the proportion of $3: 1$ ratio obtained in the $\mathrm{F}_{2}$ generation.

Law of Segregation (Second Law) : This law states that though the parents contain two alleles during gamete formation the factors or alleles of a pair segregate from each other, such that gamete receives only one of the two factors.

Hence, the alleles do not show any blending and both the characters are recovered as such in the $\mathrm{F}_{2}$ generation, though one of these is not seen in the $\mathrm{F}_{1}$ generation.