- One of two principles of heredity first formulated by Gregor Mendel, founded on his experiments with pea plants and stating that the members of a pair of homologous chromosomes segregate during meiosis and are distributed to different gametes. Also called law of segregation, principle of segregation.
- The second of these two principles, stating that each member of a pair of homologous chromosomes segregates during meiosis independently of the members of other pairs, with the result that alleles carried on different chromosomes are distributed randomly to the gametes. Also called law of independent assortment, principle of independent assortment.
Mendel's First Law: When a plant with two dominant (DD) alleles is crossed with a plant having two recessive (rr) alleles (top row), the first generation of plants (middle row) will all have one dominant and one recessive (Dr) allele. In the second generation (bottom row), on average one of four plants will have two recessive alleles.