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A Monohybrid cross is a cross between two individuals who have homozygous genotypes for the same genetic characteristic but have the opposite phenotype. Gregor Mendel discovered the rules of genetics in the mid-nineteenth century. He experimented with pea plants, nurturing them and observing the pattern of inheritance at various stages of generation. Mendel is the founder of genetics, Law of Independent Assortment, Law of Dominance, and Law of Segregation.
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Key Terms: Inheritance, Dominant gene, Recessive gene, F1 Generation, Dihybrid, Monohybrid, Genotypes, Chromosome, Gene, Phenotype, Monohybrid Cross
Read More: Principles of Inheritance and Variation
Monohybrid Cross Definition
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Furthermore, in a monohybrid cross, the term "mono" refers to a single creature, while "hybrid" refers to the offspring of two organisms of different species. Finally, the Punnett Square of 2×2 is adequate to establish the monohybrid cross's possibilities. Punnett Square was created by Reginald Punnett, a British geneticist, in an attempt to forecast possible outcomes. The simplest sort of Monohybrid cross is the Punnett Square.
A monohybrid cross is the hybrid of two individuals with homozygous genotypes which result in the opposite phenotype for a certain genetic trait.
The cross between two monohybrid traits (TT and tt) is called a Monohybrid Cross.
Geneticists use the monohybrid cross to see how homozygous offspring express heterozygous genes inherited from their parents.
Read More: Chromosomes and Genes
Steps of the Monohybrid Cross
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The phenotype-to-genotype ratios that are estimated are simply probabilities. The following are the steps for calculating a monohybrid cross:
The video below explains this:
Steps of the Monohybrid Cross Detailed Video Explanation:
Step One – To Find out the Genotype of a Person
The first step in a monohybrid cross is to figure out what genotype you have. This is a non-essential stage because every organism has dominant and recessive characteristics. Furthermore, the genotype's alleles are formed through the determination of these features.
For example, a male's dominant skin colour could be 'Y' and his recessive skin colour could be 'y'. Let the dominant skin colour of the female be 'X,' and the recessive skin colour is 'x.'
As a result, the dominant and recessive combinations of both males and females will be 'Yy' for males and 'Xx' for females. The alleles of the set of genes are combined to form a genotype in this step. The phenotype is determined by the genotype, which is a collection of DNA.
Genotype of Gametes
Step Two: Setting up the Punnett Square
The monohybrid cross's second step is to set up the Punnett Square. Punnett Square is the most simple method for calculating the total number of possible crosses. This also helps in analysing the physical appearance or phenotype.
The Punnett Square is a square with four divisions or a 2×2-grid square. In the first section, there is a single male genotype with both recessive and dominant alleles.
Similarly, the female genotype is presented in the fourth part. Both the male and female have a dominant and recessive allele in the other two sections. For example, the 'Yy' genotype is found in the first segment, while 'XY' is found in the second. In the same way, the third and fourth include the letters 'xy' and 'Xx'. This covers all phenotypic options.
Step Three: To Determine the Off-spring Ratio
The monohybrid cross's final step is to calculate the off-spring ratio. The male dominates the female in the Punnett square. As a result, the offspring's phenotype will be dominated by male characteristics. As a result, the male phenotype to female phenotype ratio is 4:1.
If a guy has dark skin, for example, there is a good likelihood that his kids will have dark skin as well. Only one child out of every four will have a fair complexion.
In practice, however, this is not always the case. There are 7500 people with fair skin out of a total of 1000. This demonstrates that in the real world, this ratio does not hold.
However, we can still predict the phenotype of the offspring. The dominant trait should always have a majority, according to the ratio. In reality, however, it is a matter of probability, as flipping the coin ten times does not guarantee that the result will be five times head and tail.
Read More: Gene Regulation
Gregor Mendel’s Peas
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Mendel started with a pair of pea plants with two contrasting features, one tall and the other dwarf, for a monohybrid cross. Tall plants developed from the cross-pollination of tall and dwarf plants. The hybrid plants were all quite tall. This was referred to as the first hybrid generation (F1), and the progeny was referred to as Filial 1 or F1 progeny.
He tested all seven contrasting pairs and discovered that the entire F1 progeny showed the same pattern of behaviour, i.e., they resembled one of the parents. Another parent character was absent.
Gregor Mendel’s Peas
He continued his experiment with F1 offspring plants that were self-pollinated. Surprisingly, one of the four plants he noticed was small, while the other three were tall. The tall and short plants were arranged in a 3:1 ratio.
He also noted that none of the progeny was of intermediate height, indicating that no blending had occurred. The result was the same for other plant features as well, thus he called the offspring Filial 2 or F2 progeny and called the second hybrid generation.
Traits that were absent in the F1 generation reappeared in the F2 generation, according to Mendel. Such suppressed traits were referred to as recessive traits, while expressed traits were referred to as dominant traits. He also concluded that some 'factors' are inherited through generations by offspring from their parents.
These 'factors' were later called genes. The inheritance of qualities from one generation to the next is controlled by genes. Genes are made up of two alleles that code for various features. Homozygous pair refers to identical alleles (e.g., TT or tt), whereas heterozygous pair refers to alleles that are different or non-identical (e.g., Tt).
Mendel looked at pea plant pairings that had one contrasting feature. Mendel looked at the following seven characters, each with their own set of characteristics:
- Flower colour: Violet/white
- Flower position: Axial/terminal
- Pod colour: Green/yellow
- Pod shape: Inflated/constricted
- Seed colour: Yellow/green
- Seed shape: Round/wrinkled
- Stem height: Tall/dwarf
Things To Remember
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- Lower case letters can be used to represent recessive alleles. Upper case letters are used to signify dominant alleles.
- Traits may be inherited from the F1 generation to the F3 generation without appearing in the F2 Generation.
- Dominant genes are always expressed in presence of the recessive genes.
- Recessive genes can be expressed only in homozygous conditions.
- For a monohybrid cross, both parents should be heterozygous in nature for one trait.
- Albinism is a good example of a monohybrid disorder.
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Sample Questions
Ques. What is the difference between monohybrid and dihybrid? (2 Marks)
Ans. The number of traits studied in the offspring can be used to distinguish between a monohybrid and a dihybrid cross. Because the parents are homozygous, the inheritance of a single gene is predicted in a monohybrid cross, whereas in a dihybrid cross, the parents differ in two different traits.
Ques. What does a monohybrid cross focus on? (2 Marks)
Ans. To evaluate the dominance relationship between two alleles, the monohybrid cross is commonly used. The parental generation is the starting point for the cross. One parent is homozygous for one allele while the other is homozygous for the other. The first filial (F1) generation is made up of offspring.
Ques. Does a monohybrid cross have one parent? (2 Marks)
Ans. A monohybrid cross is a genetic cross between two individuals who have homozygous genotypes for a single feature or trait, with the phenotype often being the polar opposite. When one parent is homozygous for one allele and the other is homozygous for the other, a cross occurs between the parents.
Ques. What is a test cross? Why is it done? (2 Marks)
Ans. The test cross is used to assess if a dominant parent is heterozygous or homozygous for its genotype. It is possible to estimate which genotype the parent has based on the data obtained in the ratio of the offspring. By evaluating the proportions of offspring traits, the former's zygosity can be determined.
Ques. What is the advantage of test cross? (2 Marks)
Ans. A test cross is a cross between a person with an unknown dominant phenotype and a person (parent) who is homozygous recessive for that trait. The dominant character is determined by a test cross to see if it comes from a homozygous dominant genotype or a heterozygous dominant genotype. As a result, it is more practical than a back cross.
Ques. How does test cross help detect linkage? (2 Marks)
Ans. If the genes displayed independent assortment, the heterozygous parent with the dominant phenotype should produce four gametes in equal frequency: AB, Ab, aB, and ab. If more testcross progeny received the parental gametes from the dominant individual than would be expected by chance alone, the linkage will be identified.
Ques. What is the monohybrid ratio explained for example? (2 Marks)
Ans. A monohybrid cross, for example, is a cross between tall pea plants and dwarf pea plants that considers only the height of the parents. It will produce tall offspring, but they will be hybrid. Character in focus is the height in this representation of the monohybrid cross. TT X tt.
Ques. What are pure gametes? (2 Marks)
Ans. The haploid gametes are known as pure gametes. The gametes are always pure, and their chromosomes are not identical. Instead, they have one of each type of chromosome, either recessive or dominant. The second law given by Mendel is the law of gamete purity. It describes how gametes are unique and how they are passed down from parents to children. Mendel is known as the founder and father of genetics since he solved the mystery of gametes and inheritance.
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