Microbiology is one of the Biology Topics that involves the study of microorganisms, including bacteria, viruses, and fungi.
Types of Genetic Crosses – Monohybrid, Dihybrid, and Trihybrid Test Cross
The cross and back cross refer to crossing a progeny with any of the parental organisms such crosses initially were performed by Mendel to know the gene (factor) combination of progeny.
A cross between the organism of the first or second filial generation to any organism of the parental generation is called Back Cross.
An example of a back cross may be cited from Mendelian experiments. A cross between the pure breeding tall pea plant and a dwarf pea plant resulted in all tall progeny plants in the F1 generation. If this tall plant of the F1 generation is crossed with either a tall or dwarf pea plant, it will represent a back cross. The back cross may be of two types, for example, a cross between the organism of the filial generation and the member with the dominant feature of the parental generation, and a cross between the organism of the filial generation and the parental member with the recessive feature.
A cross between the member of the filial generation and the recessive variety parental organism is called Test Cross.
As shown, the cross between the FI tall pea plant and the dwarf pea plant represents a test cross. In the case of the monohybrid test cross, two varieties of progeny may appear in the ratio 1 : 1. In the example given in the test cross results in tall and dwarf plants in 1 : 1 ratio. Such a result may be possible due to the segregation of the alleles in equal ratios. As the test cross results may help in elucidating the genotype of the member of the filial generation, the cross is termed a Test cross.
Monohybrid Test Cross
Test Cross in Plant
Test Cross in Animal
Dihybrid Test Cross
Like the monohybrid test cross, the dihybrid test cross helps in determining the genotype of the members of the filial generation produced in the crossing between organisms having pair of contrasting features. Mendelian dihybrid cross involving pure breeding yellow round and green wrinkled pea plants in F1 resulted in only yellow round progeny. Mendel crossed the F1 hybrid yellow round plant with green wrinkled plant and found four types of progeny such as yellow round, yellow wrinkled, green round, and green wrinkled plants in 1 : 1 : 1 : 1 ratio.
This may be an example of a dihybrid test cross. From the result of the dihybrid test cross, it becomes apparent that F! yellow round plants were of hybrid category in which the pairs of alleles Yy and Rr due to independent segregation and random assortment produce four types of gametes – YR, Yr, yR and yr in equal frequency. These gametes being united with yr gamete from yyrr plant produce four types of test cross progeny in equal numbers. Therefore, the result of the dihybrid test cross also supports an independent assortment of genes. In the same way, a dihybrid test cross may also be shown in animal species.
Test Cross in Plants
Gametic union with the help of a checkerboard:
1. Yellow Round: 1/4
2. Yellow Wrinkled: 1/4
3. Green Round: 1/4
4. Green Wrinkled: 1/4
Phenotypic Ratio 1 : 1 : 1 : 1
Test Cross in Animals
1. Grey Long: 1/4
2. Grey Vestigial: 1/4
3. Ebony Long: 1/4
4. Ebony Long: 1/4
Phenotypic Ratio 1 : 1 : 1 : 1
Differences between Back Cross and Test Cross:
|Basis||Back Cross||Test Cross|
|Cross-Type||Cross between a progeny and any parental organism.||Cross between a progeny and recessive form of the parental organism.|
|Purpose||No specific purpose.||To reveal the genotype of the offspring.|
|Typification||All backcrosses cannot be test crosses.||All test crosses are backcrosses.|
|Involvement||The cross involves any type of parental organism.||The cross involves only the recessive form of the parental organism.|
Trihybrid Test Crosses
Hybridization between animals or plants in consideration of three pairs of contrasting features is known as a trihybrid cross. In one trihybrid experiment, Mendel considered the shape of the seed colour of the cotyledon and colour of the flower in the pea plant.
In a pair of plants with contrasting features, one plant showed a yellow cotyledon, round seed, and purple flower, and the other plant showed a green cotyledon, wrinkled seed, and white flower. The parental plants of these two varieties were pure breeding and in the cross-pollination, they produced all plants showing yellow, round, and purple features in F1 Generation. In the next step when the F1 hybrid showing all normal features was allowed to self-pollinate, in the F2 generation plants with 8 phenotypes were produced. The results of such a trihybrid cross may be presented through the following diagram.
Determination of Phenotypes by Branching Method
F1 × F1 cross (YyRrPp × YyRrPp) is actually the combination of three monohybrid crosses (e.g., Yy × Yy, Rr × Rr and Pp × Pp). In each monohybrid cross F2 phenotypic ratio comes to 3 : 1 with two contrasting phenotypes. Therefore, if the results of three monohybrid crosses (F2 phenotypic ratios) are combined together the F2 phenotypes will appear in a ratio 27 : 9 : 9 : 9 : 3 : 3 : 3 : 1.
In the same way, the genotypic ratio of the F2 progeny may be determined by combining the genotypic ratio of each of the monohybrid crosses, when the F2 genotypic ratio of a monohybrid cross is 1 : 2 : 1.