Punnett Square

PUNNETT SQUARE (Punnett), a simple method for calculating gamete combinations in the second generation (F₂) of a polyhybrid cross (see Mendelism). The gametes of one parent are arranged along the horizontal side of a rectangle divided into as many columns as there are gametes, while the gametes of the second parent are arranged along the vertical side of this rectangle, divided into a number of horizontal rows corresponding to their number. The resulting zygotes are recorded in the corresponding cell of the grid. Example: peas with yellow round seeds (AABB) are crossed with peas with green wrinkled seeds (aabb). The F₁ hybrid - AaBb produces gametes AB, Ab, aB, and ab. Gametes of one parent and gametes of the other parent produce cells with combinations such as AABB (yellow, round), AABb (yellow, round), AaBB (yellow, round), AaBb (yellow, round), AABb (yellow, round), AAbb (yellow, wrinkled), AaBb (yellow, round), Aabb (yellow, wrinkled), AaBB (yellow, round), AaBb (yellow, round), aaBB (green, round), aaBb (green, round), AaBb (yellow, round), Aabb (yellow, wrinkled), aaBb (green, round), aabb (green, wrinkled). Not all liquids have the ability to foam, i.e., to form a stable, long-lasting foam. Only with a significant decrease in surface tension does the tendency of gas bubbles to reduce their free surface by joining together decrease sufficiently. The viscosity and strength of the boundary films separating adjacent air bubbles from each other also play an essential role. If almost the entire volume of the system is filled with tiny gas bubbles closely adjacent to each other, a characteristic foamy structure is obtained. Its similarity to the internal structure of many cells served as the basis for Bütschli's theoretical concepts regarding the corresponding structure of living protoplasm (see Cell - cell structure). A count of phenotypes is made from the cells, yielding the result: yellow round - 9, yellow wrinkled - 3, green round - 3, and green wrinkled - 1. The various genotypes in the grid cells are arranged as follows: diheterozygotes are located along the diagonal a/?, digomozygotes along the y& diagonal, and monoheterozygotes in the remaining squares. The rule of independent assortment of traits is also easily derived using the Punnett square. The ratio of all round to all wrinkled = 12 : 4 = 3 : 1; the ratio of all yellow to all green = 12 : 4 = 3 : 1. Thus, each pair of traits in the given example behaves independently, segregating as in a monohybrid cross (in a 3 : 1 ratio).