Monohybrid Cross

A monohybrid cross is a breeding experiment involving two organisms in the parent generation with homozygous genotypes. The parents have entirely dominant or completely recessive alleles of a gene, forming opposite phenotypes for a particular genetic trait. Traits are characteristics determined by discrete DNA segments called genes. Each individual inherits two alleles of a particular gene. 

It is done between plants having gone through one hybrid generation. Geneticists perform monohybrid cross to observe how homozygous parents’ offspring express the heterozygous genotypes they inherit from their parents. A monohybrid cross also signifies a genetic mix between the two individuals having heterozygous genotypes. It is represented using a Punnett square.

The word ‘mono’ means ‘single’, and ‘hybrid’ signifies heterozygous parents for the trait or character under study. Thus, each parent has a dominant and a recessive allele of a gene. In a monohybrid cross, only one pair of contrasting traits or characters is studied.

Given the genotype of any two parents, all possible genotypes of offspring obtained through meiosis can be predicted. British geneticist Reginald Punnett invented a convenient way of calculating the expected genotypic and phenotypic ratios from a cross in the early 1900s.

A Punnett square is a matrix where all possible gametes produced by one parent are listed along one axis. The gametes obtained from the other parent are listed on the other. All possible combination of gametes is shown at the intersection of each row and column. The most straightforward Punnett square is a monohybrid cross.

1. Signify each allele using characters. Capital letters are used for the dominant allele, lower case letters for the recessive allele.
2. Note down the genotype and phenotype of the parents, which are the parental (P) generation.
3. Note the genotype of the parental gametes. They will be haploid due to meiotic division.
4. Use a Punnett square to chalk out probable combinations of gametes. Due to random fertilization, all possible combinations have equal opportunity to express.  
5. Note the genotype and phenotype ratios of the potential offspring. The outcome is the first filial (F1) generation. The F2 and F3 form the subsequent second and third generations.

Mendel’s Experiment

Mendel performed seven types of monohybrid crosses, each with contrasting traits for different characteristics.

• Flower Color: Purple/white
• Flower Position: Axial/terminal
• Seed Shape: Round/wrinkled
• Seed Color: Yellow/green
• Pod Color: Green/yellow
• Pod Shape: Inflated/constricted
• Stem Height: Tall/dwarf

He experimented with all seven pairs of contrasting characters. The entire F1 progeny showed a single pattern in their behavior, resembling one parent, while the other parent character is absent.

F1 Generation

A pair of pea plants with contrasting characters is chosen, one bearing blue flower with genotype (BB) and the other bearing white flower with genotype (bb). In this cross, the allele for blue flower (B) is totally dominant over the recessive allele for white flower (b). The blue flower-bearing plant genotype is (BB), and the genotype of white flower-bearing plant genotype is (bb). The cross-pollination between the true-breeding plants results in offspring all with blue-bearing plants.  All the genotypes are found to be (Bb). The F1 generation plants’ offspring all bear blue flowers because the dominant blue character obscures the recessive dwarf white character (see the diagram above).   

Similarly, a tall plant having genotype TT and another dwarf plant with genotype tt is crossed. The organisms are thus true-breeding for stem height. In this cross, the allele for tall stem height (T) is completely dominant over the recessive allele for dwarf stem height (t). The tall stem height plant’s genotype is (TT), and the genotype for the dwarf stem height plant is (tt). The cross-pollination between the true-breeding homozygous dominant tall stem height plant and the true-breeding homozygous recessive dwarf stem height plant results in offspring with phenotypes as tall stem height plants. All the genotypes are (Tt). The F1 generation plants’ offspring are tall because the dominant tall character obscures the heterozygous genotype’s recessive dwarf character. He observed no intermediate height plants and thus confirmed no blending of characters in the result. The result was the same for the other six pairs of traits in F2 progeny plants.

Based on the result, Mendel proposed that each monohybrid cross’s parent contributed one of the two units to each offspring. All possible combinations of factors are equally likely. The outcome of the experiment confirms the dominance of an allele. 

F2 Generation

Mendel continued with his experiment with the self-pollination of F1 progeny plants. To his surprise, he observed that one out of the four F2 generation plants was white, while the other three were blue. The genotypes were found to be (BB, Bb, and bb) with a ratio of 1:2:1. One-fourth of the F2 generation offspring was homozygous dominant (PP). One half was heterozygous (Bb). The rest one-fourth was homozygous dominant (bb). The blue and white flower plants were found to be in the phenotypic ratio of 3:1, with three-fourths bearing blue flower (BB and Bb) and one-fourth bearing white flower (bb).

Similarly, in the cross between blue and white flower-bearing plants, the genotypes were found to be (TT, Tt, and tt) with a ratio of 1:2:1. One-fourth of the F2 generation offspring was homozygous dominant (TT). One half was heterozygous (Tt). The rest one-fourth was homozygous dominant (tt). The tall and short plants were found to be in the phenotypic ratio of 3:1, with three-fourths having tall stem height (TT and Tt) and one-fourth having dwarf stem height (tt). The genotypes of the F2 offspring expressing dominant phenotype were obtained using a test cross.

Huntington’s disease

It is a progressive degenerative disorder commonly found in the US. The genetic nature of Huntington’s disease is determined using a monohybrid cross. Everyone having the disease carries the Huntingtin gene, which is responsible for the complication. It has no cure.

Scientists pair the Huntingtin gene of an individual with a homozygous dominant allele (HH) with another homozygous recessive allele (hh). All the offspring carried the dominant allele for Huntington’s disease. This result proves the dominant nature of the Huntingtin gene.

It is a cross that helps to explore the genotype of an organism. When an organism’s genotype expressing a dominant trait is not known to be homozygous or heterozygous, we need to perform a test cross. A monohybrid test cross is done involving a single pair of contrasting characters.

Here an individual with an unknown genotype is crossed with another individual that is homozygous recessive for a particular trait. The unknown genotype can be obtained by analyzing the phenotypes in the offspring. The result of a monohybrid test cross-ratio is represented using a Punnett square. If the unknown genotype is heterozygous, a test cross with a homozygous recessive individual will result in a 1:1 ratio of the offspring’s phenotypes.

Examples

Test Cross 1: Using the tall stem height plant from Mendel’s monohybrid cross example, a cross between a plant with recessive dwarf stem height plant (tt) and a plant heterozygous for tall stem height (Tt) produces both tall and dwarf plants. Half are dwarf (tt), and half are tall (Tt).

Test Cross 2: A cross between a plant with recessive dwarf stem plant (tt) and a plant homozygous dominant for tall stem (TT) produces all tall offspring with heterozygous genotype (Tt).