Concepts | Punnet Square | Monohybrid Cross | Dihybrid Cross
Punnett square
Reginald Punnett developed a method to analyze data from genetic crosses that is
particularly useful where small numbers of genes are involved. The results
of any cross can be depicted, and thus predicted, by constructing a Punnett
square.
The Punnett square is a good visual representation of all possible genotypic and phenotypic outcomes for a given cross. When higher order crosses (dihybrid, trihybrid, and tetrahybrid crosses) are done, the dimensions of the Punnett Square increase and this method of analyzing genetic data becomes unwieldy.
Monohybrid Cross
Gregor Mendel chose to study genetic transmission in the common garden pea, Pisum
sativum. Mendel started with true-breeding strains obtained from a
local seed merchant. When planted, the seeds produced progeny plants with
characteristics identical to those of the parent. Mendel chose to study
traits—such as pea color, pod location,
pod shape and pea shape—that had two
distinct forms. Pea shape, for example, could be either round or wrinkled
The phenotypic ratio in the F2 generation is three fourths dominant trait to one fourth recessive trait (3:1 phenotypic ratio). By definition, the genotype of individuals with the recessive phenotype for the trait is homozygous recessive. Individuals with the dominant form of the trait may be homozygous dominant (true-breeding) or heterozygous (monohybrid). The genotypic ratio in a dihybrid cross is 1:2:1.
Dihybrid Cross
Instead of studying a single pair of contrasting traits, it is possible to
design experimental crosses where two pairs of traits are examined
simultaneously. Starting with parents that breed true (i.e. homozygous parents)
the assortment of the traits into the progeny is followed. The resulting
phenotypic ratio for a two factor cross provides insight into the nature of
events that occur during gamete formation and fertilization
In a dihybrid cross two traits are considered simultaneously, but the cross can be described as two single monohybrid crosses. Problems in genetics can always be solved by working with one trait at a time. Mendel’s fourth law (his second postulate) speaks to the fact that the traits in a dihybrid cross can be treated separately. The rule of independent assortment states that during formation of the gametes, segregating pairs of genes assort into gametes independently of one another.
