Key Difference – Monohybrid vs Dihybrid Crosses
Offsprings genetically acquire traits from their parents. This is described as inheritance. Crossing or breeding is the process of deliberately breeding two organisms to find out how characteristics are passed from one generation to the next generation. This is popular among plant species and known as plant breeding. Important characteristics are fixed and maintained in generations by breeding. Monohybrid cross and dihybrid cross are two types of crosses done by breeders. The key difference between monohybrid cross and the dihybrid cross is that monohybrid cross is done to study the inheritance of one trait while dihybrid cross is done to study the inheritance of two different traits in the same cross.
CONTENTS
1. Overview and Key Difference
2. What are Monohybrid Crosses
3. What are Dihybrid Crosses
4. Side by Side Comparison – Monohybrid vs Dihybrid Crosses
5. Summary
What are Monohybrid Crosses?
A monohybrid cross studies the pattern of a particular trait shown in F2 generation. Two homozygous parents are selected for the monohybrid cross and studies are carried out about only one trait, ignoring all other traits. The two true breeding parental lines (homozygous) have two contrasting expressions of the particular trait. Therefore a monohybrid cross can be defined as crossbreeding of two true breeding parental lines to study the inheritance of one trait at a single gene locus.
If we look at an example of a monohybrid cross which studies plant height, homozygous tall plant (TT) and homozygous dwarf plants (tt) are crossed with each other. These two parents are named as the parental generation. In this cross, the tall allele is dominant over the dwarf allele. The resulting offspring or new generation is the first hybrid generation which is named as F1 generation, and they all show the same phenotype (tall plants) with a genotype which is heterozygous for the trait (Tt). When F1 generation is allowed to self-pollinate, the resulting offsprings are known as the F2 generation. Then the F2 generation is analyzed for target trait, which is plant height. In F2 generation, the phenotypic ratio (tall: dwarf) can be observed as 3:1 while genotype ratio (TT:Tt:tt) is observed as 1:2:1. This example is illustrated below in figure 01 for further explanation.
Figure 01: Monohybrid cross
What are Dihybrid Crosses?
Dihybrid cross is a cross which is done to study the inheritance of two traits or two pairs of alleles. Parents have different allele pairs for each trait considered. One parent possesses homozygous dominant allele for one trait while the other parent has the homozygous recessive allele for that particular trait. When a cross is done between above two parents, all in the F1 generation will be the same. Then the F1 generation is self-pollinated, and the resulting F2 generation will show a phenotypic ratio of 9:3:3:1 and a genotype ratio of 1:2:1:2:4:2:1:2:1.
The father of genetics, Gregor Mendel has done several dihybrid crosses during his experiments. One of his dihybrid crosses involved the study of pea plant pod shape (round or wrinkled) and pod colour (yellow or green). Round (R) and yellow (Y) were dominant over wrinkled (r) and green (y) respectively. The parents used were round yellow (RRYY) and wrinkled green (rryy). F1 population was all round yellow (RrYy) pods. The F2 generation, which resulted from the self pollination of two F1, showed four different phenotypes in 9:3:3:1 ratio as shown in figure 02.
Figure 02: Dihybrid Crosses
What is the difference between Monohybrid and Dihybrid Crosses?
Monohybrid vs Dihybrid Crosses |
|
| Monohybrid cross is a cross between two pure organisms to study the inheritance of one character or a single pair of alleles. | Dihybrid cross is a cross between two pure organisms to study the inheritance of two pairs of alleles or two traits. |
| Characters | |
| Monohybrid cross deals with a single character. | Dihybrid cross deals with two characters. |
| Phenotype Ratio | |
| Monohybrid cross produces phenotypes in ratio 3:1 in F2 generation. | Dihybrid cross produces phenotypes in 9:3:3:1 ratio in F2 generation. |
| Genotype Ratio | |
| Monohybrid cross produces genotype ratio 1:2:2:1 in F2 generation. | Dihybrid cross produces genotypes in 1:2:1:2:4:2:1:2:1 ratio in F2 generation. |
| Test Cross Ratio | |
| Test cross ratio is 1:1. | Test cross ratio is 1:1:1:1 |
Summary – Monohybrid vs Dihybrid Crosses
Inheritance patterns are studied using different crosses. Monohybrid cross is done between two homozygous parents to study a particular trait inheritance to F2 generation. Dihybrid cross is done to study two traits inheritance simultaneously to F2 generation. Monohybrid cross produces offspring phenotypes in 3:1 ratio while dihybrid cross produces phenotypes in 9:3:3:1 ratio. This is the key difference between monohybrid cross and dihybrid crosses.
References
1. Bailey, Regina. “Monohybrid Cross: A Breeding Experiment.” ThoughtCo. N.p., n.d. Web. 27 May 2017. <https://www.thoughtco.com/monohybrid-cross-a-genetics-definition-373473>
2.”Dihybrid cross.” Wikipedia. Wikimedia Foundation, 19 May 2017. Web. 27 May 2017. <https://en.wikipedia.org/wiki/Dihybrid_cross>.
3. Bailey, Regina. “What Is a Dihybrid Cross?” ThoughtCo. N.p., n.d. Web. 27 May 2017. <https://www.thoughtco.com/dihybrid-cross-a-genetics-definition-373463>.
Image Courtesy:
1. “Dihybrid Cross Tree Method” By Tim DeJulio – Own work (Public Domain) via Commons Wikimedia