A Punnett square is a grid that predicts the probability of offspring inheriting specific alleles. Each parent contributes one allele per gene to each offspring (Mendel's Law of Segregation).
With complete dominance, a dominant allele (A) masks the recessive (a); only aa shows the recessive phenotype. Aa × Aa gives a 3:1 phenotype ratio.
With incomplete dominance, heterozygotes show a blended phenotype (1:2:1). With codominance, both alleles are expressed fully at once.