Mendel’s Laws Illustrated with Punnett Squares
Gregor Mendel used pea plants. He tracked clear traits like seed color and shape. Punnett squares show his laws visually. They predict offspring genotypes and phenotypes easily.
Law of Segregation – Monohybrid Cross Alleles separate during gamete formation. Each parent gives one allele to offspring.
Take flower color. Purple (P) dominates white (p). Cross two heterozygous parents (Pp × Pp).
Here is the Punnett square:
Offspring ratios: 25% PP (purple), 50% Pp (purple), and 25% pp (white). The phenotype ratio is 3 purple to 1 white. Thus, recessive traits reappear.
Law of Independent Assortment – Dihybrid Cross Genes for different traits separate independently. They mix freely.
Use seed shape and color. Round (R) dominates wrinkled (r). Yellow (Y) dominates green (y). Cross two double heterozygotes (RrYy × RrYy).
The Punnett square expands to 16 boxes:
Phenotype ratio: 9 round yellow 3 round green 3 wrinkled yellow 1 wrinkled green This 9:3:3:1 ratio proves independent assortment.
Law of Dominance Dominant alleles mask recessive ones in heterozygotes.
The same monohybrid square above shows it. Pp looks purple (dominant). Only pp shows white. Dominance hides the recessive allele.
These simple squares explain Mendel’s rules perfectly. Practice them yourself. You will master inheritance fast.
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