Monohybrid, Dihybrid and Test Cross NEET Notes | EduRev

NEET : Monohybrid, Dihybrid and Test Cross NEET Notes | EduRev

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Gregor Mendel, after rigorous studies on peas, found that genetic traits are passed from parents to their offspring in a specific manner. Known as father of modern genetics, his experiments led to the discovery of Mendel's Laws of Inheritance.
Monohybrid: Definition
In biology, a monohybrid cross is defined as the breeding experiment conducted between parents, which differ in one specific trait ('mono' meaning one). Or in other words, the parents are heterozygous (having dissimilar alleles) at only one locus. Over here, one parent has a dominant gene for a specific phenotypic character (e.g. tall trait), while the other has recessive gene for the particular phenotype (e.g. dwarf trait).
Dihybrid: Definition
Contrary to monohybrid cross, parents that differ in two traits ('di' meaning two) are bred in a dihybrid cross. To be more precise, the parental organisms are heterozygous for two different characters. In this case, one parent has dominant genes for two characters (e.g. tall plant bearing red flowers) and the other parent has recessive genes for the two characters (dwarf plant bearing white flowers) in the chromosome.
Difference Between the Two
Needless to remind, genetic science would not have been the same without the findings of Gregor Mendel. He did several experiments to study the trend for genetic inheritance in green peas. By applying basic rules of inheritance, you can explain the difference between the two.
Application
In biology experiments, a monohybrid cross is used to study the pattern of inheritance in the first and second generation offspring. The first filial generation (F1 generation) yields all heterozygous offspring that shows dominant character. From these, two heterozygous organisms are used for crossing, and the result is 3 offspring with dominant character and one offspring with recessive trait. Whereas, dihybrid is helpful for studying inheritance pattern of dominant and recessive characters for two different traits. Over here, we can find out all possible genotypic combinations.
Examples
Monohybrid
When a cross is made between a tall plant (TT) and a dwarf plant (tt), the two resulting F1 offspring are tall (Tt). Then, a cross is again made between the two tall members of the F1 generation (Tt x Tt). In this F2 generation, the expected offspring along with their genetic component are three tall plants (TT, Tt, Tt) and one dwarf plant (tt). Thus, the ratio of dominant to recessive phenotype is 3:1.
Dihybrid
In dihybrid cross, a tall plant having red flower (TTRR) is crossed with another dwarf plant having white flower (ttrr). After the first F1 generation, we get four heterozygous offspring that exhibit both tall height and red color flower (TtRr). A second cross is made between offspring of the F1 generation (TtRr x TtRr), resulting in 16 offspring. The ratio of phenotype character is 9:3:3:1, with 9 tall plants having red flower, 3 tall plants with white flower, 3 dwarf plants with red flower and 1 dwarf plant with white flower.
Thus, with monohybrid cross, inheritance of a single trait can be traced easily, while dihybrid type is useful for studying the pattern of genetic inheritance in organisms for two different traits.
test cross
A test cross is a way to explore the genotpye of an organism. Early use of the test cross was as an experimental mating test used to determine what alleles are present in the genotype. An organism's genetic makeup is called its genotype, and it reflects all of the alleles, or forms of the gene, that are carried by the organism. Consequently, a test cross can help determine whether a dominant phenotype is homozygous or heterozygous for a specific allele.
Diploid organisms, like humans, have two alleles at each genetic locus, or position, and one allele is inherited from each parent. Different alleles do not always produce equal outward effects or phenotypes. One allele can be dominant and mask the effect of a second recessive allele in a heterozygous organism that carries two different alleles at a specific locus. Recessive alleles only express their phenotype if an organism carries two identical copies of the recessive allele, meaning it is homozygous for the recessive allele. This means that the genotype of an organism with a dominant phenotype may be either homozygous or heterozygous for the dominant allele. Therefore, it is impossible to identify the genotype of an organism with a dominant trait by visually examining its phenotype.
To identify whether an organism exhibiting a dominant trait is homozygous or heterozygous for a specific allele, a scientist can perform a test cross. The organism in question is crossed with an organism that is homozygous for the recessive trait, and the offspring of the test cross are examined. If the test cross results in any recessive offspring, then the parent organism is heterozygous for the allele in question. If the test cross results in only phenotypically dominant offspring, then the parent organism is homozygous dominant for the allele in question.

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