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Monohybrid and Dihybrid

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What are Monohybrid and Dihybrid Crosses?

The modern principles of genetics are based on the concept of the gene which is the unit of heredity. Gregor Johann Mendel, known as the father of genetics discovered the existence of genes during his pea plant experiments. In the first generation F1, Mendel carried out self-pollination between pure lines of purple and white-flowered plants, and the new color of the new generation of flowers was completely purple. In the second cross, the pollination was carried between the purple plants and the F2 results came as a ratio of 3:1 between purple and white plants. In crosses between yellow and green seeded plants, the ratio was 1:2:1. 


From these conclusions, Mendel concluded the concepts of genes. The concepts of phenotypes and genotypes were also written down. Mendel also outlined the law of segregation based on the results of the F1 and F2 crosses. It states that two members of a gene pair separate from each other during gamete formation into equal numbers of gametes. Therefore, each gamete carries only one gene from each gene pair. The union of gametes to form a zygote is random in regards to which allele the gametes carry. 


In dihybrid crosses, where Mendel crossed between two or more characteristics of the pea plant, the outcome ratio was 9:3:3:1 in F2 which are really 3:1 ratios crossed at random. From these results, Mendel inferred that alleles in a pair of genes in a dihybrid cross behave independently of each other which gave rise to Mendel's second law. It applies to genes on separate chromosomes. The basis for the law is the independent segregation of different chromosomes at meiotic cell division.


Monohybrid and Dihybrid Cross Definition

Monohybrid cross:  A monohybrid cross can be defined as a genetic mix between two individuals who have homozygous genotypes or genotypes which have completely dominant or recessive alleles.  This results in opposite phenotypes for a specific genetic trait.


Monohybrid cross experiments are carried out by geneticists to study how the offspring of homozygous individuals express the heterozygous genotypes they inherit from their parents. The cross also signifies a genetic mix between two individuals who have heterozygous genotypes which confirm the dominance of an allele. 


Example: A well-noted example of a monohybrid cross is Mendel’s experiments on pea plants which helped him narrate the concept of genes and the law of segregation and independent assortment. 


Another example is Huntington’s disease which is a fatal genetic disorder. The Huntingtin gene is responsible for the disease it is present in all the individuals of a generation. The homozygous dominant allele of the gene was paired with the homozygous recessive allele of the gene and the dominant allele was carried forward to the next generation.


Di-hybrid Cross: A dihybrid cross is another experiment in genetics that is carried out to follow the behavior of the phenotypes of two genes through the mating of individuals carrying multiple alleles at those gene loci.  Simply put, it’s a cross between two observed states, where the homozygous dominant traits are crossed with homozygous recessive traits and in the first generation, all are heterozygous where the dominant phenotypic traits are observed. However, all the offspring will be carriers of the recessive traits. The name di-hybrid indicates that there are two traits involved and each trait has two different alleles. 


Example: Mendel's experiments on the pea plants can be taken as an example here as well. The traits that were taken were yellow and round seeds and green and wrinkled seeds. Two different phenotypes were crossed which gave rise to four different phenotypes in the F2 generation with a ratio of 9:3:3:1 and the genotypic ratio was 1:2:2:4:1:2:1:2:1. The observations shed light on the mode of inheritance in an organism. Based on the results, Mendel also created the laws of independent assortment.


 The list of difference between the dihybrid and monohybrid cross are listed below:


Differentiate Between Monohybrid and Dihybrid Cross

Dihybrid cross

Monohybrid cross

It is a cross between two pure lines or organisms to study the inheritance of two pairs of contrasting characters.

It’s a cross between two pure lines or organisms to study the inheritance of a single pair of contrasting characters.

The cross is carried out between individuals with different alleles for two gene loci of interest.

The cross is carried out between homozygous individuals with different alleles for a single gene locus of interest.

The phenotypic ratio in the F2 generation is 9:3: 3:1

The phenotypic ratio in the F2 generation is 3:1

The genotypic ratio in the F2 generation is 1:2:2:4:1:2:1:2:1

The genotypic ratio in the F2 generation is 1:2:1

The test cross-ratio is 1:1

The test cross-ratio is 1:1:1:1

It is important to assess the independent assortment of alleles.

It Is important to determine the dominant relationship between alleles.

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FAQs on Monohybrid and Dihybrid

1. What is a Test cross?

A test cross is a method to expel the genotype of an organism. The use of testcross as an experimental mating test allows geneticists to determine what alleles are present in the genotype and whether a dominant phenotype is homozygous or heterozygous for a specific allele. 


To identify this, the organism in question is crossed with an organism that is homozygous for a recessive trait and the offspring of this cross are examined.  If the test cross results in phenotypically dominant offspring then the parent organism is homozygous dominant for the allele in question. If the cross results in any recessive offspring then the parent organism is heterozygous for the allele in question.

2. What is a Mendelian cross?

Di-hybrid crosses are also known as Mendelian crosses. It is a cross between two different lines or genes that differ in a pair of different traits. According to Mendel, between the alleles of both loci, there is a relationship of complete dominance which can be recessive.

3. What is Gene loci?

Gene loci are the plural form of gene locus which is a specific position on a chromosome where a particular gene or gene marker is located. Gene loci are used in the construction of a gene map.

4. What is the example of a monohybrid cross?

For monohybrid cross, Mendel experimented with a pair of pea plants with a pair of  contrasting traits that was one tall and the other dwarf.  Furthermore, the tall and dwarf cross pollination will result in the tall plants. All the hybrid plants were tall. He named this as a first hybrid generation (F1) and the  Filial1 or F1 progeny were the offspring.

An  experiment was conducted  with all seven contrasting pairs and observed that the entire F1 progeny showed one pattern in their behavior, i.e., they resembled one of the parents. Another parent character was completely absent

5. What is the example of Dihybrid Cross?

Example of Dihybrid cross is the cross between pea plant, wrinkled green seeds and round yellow color seeds. The round yellow color  seeds are represented by RRYY alleles, and the wrinkled green seeds are represented by rryy. When the two parents  cross, the  RrYy allele is formed. The hybrids hold round yellow seeds (RrYy) with the dominant R allele for roundness and the dominant Y allele for the yellow color.

The four alleles can be mixed in four different combinations; RY, Ry, rY, and ry. The four alleles are assorted randomly to create four types of gametes. The gametes unite at random during fertilization to  form sixteen types of individuals in the F2 generation. The hybrids are present in the ratio of 9 round yellow, 3 round green, 3 wrinkled yellow, and 1 wrinkled green.

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