This chapter introduces central topics in the principles and applications of Mendelian genetics in plants. It will also cover genetic variation in plants, plant breeding approaches, and DNA technology tools such as genetic engineering and genome editing. Mendelian genetics studies how traits are passed through generations via dominant or recessive alleles. Genetic variation, arising from mutations, recombination, and polyploidy, drives diversity in plant populations and helps their adaptation to changing environments. Utilizing this variation is central to selective plant breeding, where traditional methods such as crossbreeding and selection are complemented by advanced molecular tools. Genetic engineering enables the introduction of desirable traits from different species, creating genetically modified organisms (GMOs) with enhanced yield, pest resistance, or stress tolerance. Genome editing tools like CRISPR/Cas systems allow precise modification of plant genomes, accelerating trait development. Quantitative trait loci (QTL) mapping links genetic regions to complex traits and guides targeted breeding strategies. Bioinformatics analyzes genomic data to identify genes, predict functions, and accelerate breeding programs. By integrating traditional knowledge with modern techniques, plant science continues to unlock the potential of genetic resources for crop development and sustainability.