Mendelian Genetics — How Traits Pass Down

MENDELIAN GENETICS is the study of how traits pass from parents to offspring through individual genes. It's named after GREGOR MENDEL, a 19th-century monk who experimented with pea plants. By tracking traits like color, height, and seed shape over thousands of pea plants, Mendel uncovered patterns that became the foundation of genetics.

Key concepts. GENE: a section of DNA that codes for a trait. ALLELE: a version of a gene (e.g., the gene for eye color has different alleles for blue, brown, green). DOMINANT allele (e.g., B for brown eyes): expresses its effect with just one copy. RECESSIVE allele (e.g., b for blue): only expresses when both copies are recessive. GENOTYPE: the gene combination (BB, Bb, bb). PHENOTYPE: the visible trait (brown or blue).

PUNNETT SQUARES are a tool to predict offspring outcomes. Draw a 2x2 grid. Put one parent's alleles across the top, the other's down the side. Fill in the boxes with the combination — that's the possible genotypes of children. Each combination has equal probability. Useful for pets (cat colors), plants (flower colors), and humans (some genetic conditions).

Mendel's rules apply only to single-gene traits — most real traits (height, intelligence, skin color) are POLYGENIC (many genes). But the basics he discovered still underlie all of genetics.