In genetics, monohybrid and dihybrid crosses are fundamental concepts used to study the inheritance patterns of traits from parents to offspring. These terms were first introduced by Gregor Mendel in the 19th century through his work with pea plants.
Monohybrid Cross:
A monohybrid cross examines the inheritance of a single trait. In this type of cross, both parents are heterozygous for the trait in question. The typical phenotypic ratio observed in the offspring of a monohybrid cross is 3:1, where three exhibit the dominant trait and one exhibits the recessive trait.
Example:
Consider a cross between two pea plants, both heterozygous for seed shape (Rr), where ‘R’ represents the dominant allele for round seeds and ‘r’ represents the recessive allele for wrinkled seeds.
R r
R RR Rr
r Rr rr
The resulting genotypic ratio is 1:2:1 (RR:Rr:rr), and the phenotypic ratio is 3:1, with three plants having round seeds and one having wrinkled seeds.
Dihybrid Cross:
A dihybrid cross examines the inheritance of two different traits simultaneously. In this type of cross, both parents are heterozygous for both traits. The typical phenotypic ratio observed in the offspring of a dihybrid cross is 9:3:3:1.
Example:
Consider a cross between two pea plants, both heterozygous for seed shape and seed color (RrYy), where ‘R’ represents the dominant allele for round seeds, ‘r’ represents the recessive allele for wrinkled seeds, ‘Y’ represents the dominant allele for yellow seeds, and ‘y’ represents the recessive allele for green seeds.
RY Ry rY ry
RY RRY Y RRY y RrY Y RrY y
Ry RRY y RRyy RrY y Rryy
rY RrY Y RrY y rrY Y rrY y
ry RrY y Rryy rrY y rryy
The resulting phenotypic ratio is:
9 plants with round yellow seeds (R_Y_)
3 plants with round green seeds (R_yy)
3 plants with wrinkled yellow seeds (rrY_)
1 plant with wrinkled green seeds (rryy)
Key Differences:
Number of Traits Studied: Monohybrid crosses involve one trait, while dihybrid crosses involve two traits.
Phenotypic Ratios: Monohybrid crosses typically yield a 3:1 ratio, whereas dihybrid crosses yield a 9:3:3:1 ratio.
Complexity: Dihybrid crosses are more complex due to the involvement of two traits and the potential for independent assortment.
These foundational concepts in Mendelian genetics help explain how traits are inherited and predict the likelihood of certain traits appearing in offspring.
Leave a comment