Mendel’s Monohybrid Crosses

What is a Monohybrid Cross? A monohybrid cross is a cross between two organisms with different variations at one genetic locus of interest. The characters being studied in a monohybrid cross are governed by two or multiple variations for a single location of a gene.

Each parent is chosen to be homozygous or true breeding for a given trait. When a cross satisfies the conditions for a monohybrid cross, it is usually detected by a characteristic distribution of second-generation (F2) offspring that is called the monohybrid ratio.

What Are Monohybrid Crosses Used For?

A monohybrid cross helps in the determination of the dominance relationship between two alleles. The cross starts with the parental generation in which one is homozygous for one allele, and the other parent is homozygous for the other allele. The offspring produced through this cross is part of the first filial generation, i.e. F1. All the members of the F1 generation are heterozygous, and the phenotype of the F1 generation expresses the dominant trait.

The second filial generation, i.e. F2, is produced by crossing any two members of the F1 generation. According to the prediction of Probability Theory, three-quarters of the F2 generation will have the dominant allele’s phenotype, and the rest will have the recessive allele’s phenotype. The predicted 3:1 phenotypic ratio assumes Mendelian inheritance.

Mendel’s Experiment With Pea

Gregor Mendel (1822–1884) was an Austrian monk who theorized and proposed inheritance rules. He analyzed the offspring of the matings of garden peas that he bred in his garden for eight years, from 1858 to 1866. He experimented on garden peas because a great number of varieties were available that bred true for qualitative traits, and their pollination could be manipulated. The seven variable characteristics Mendel investigated in pea plants are as follows:

• Seed Texture (Round vs Wrinkled)
• Seed Colour (Yellow vs Green)
• Flower Colour (White vs Purple)
• Growth Habit (Tall vs Dwarf)
• Pod Shape (Pinched or Inflated)
• Pod Colour (Green vs Yellow)
• Flower Position (Axial or Terminal)

First Cross

In Mendel’s Experiment, all the peas produced in the second or hybrid generation were round. Every pea of the F1 generation has an Rr genotype. The haploid sperm and eggs produced by meiosis received one chromosome 7. The zygotes received one R allele (from the round seed parent) and one r allele (from the wrinkled seed parent). The phenotype of all the seeds was round because the R allele is dominant to the r allele. The phenotypic ratio in this case of the Monohybrid cross is 1.

Second Cross

The wrinkled trait reappeared in 25% of the new crop of peas when Mendel let his hybrid peas to self-pollinate. This trait did not appear in his hybrid generation. A random union of equal numbers of R and r gametes produced an F2 generation with 25% RR and 50% Rr, both with the round phenotype and 25% rr with the wrinkled phenotype.

Third Cross

After the second cross, Medel again let some of each phenotype in the F2 generation to self-pollinate. In this cross, the offsprings of the wrinkled seeds in the F2 generation were only wrinkled seeds in the F3. One-third, i.e. (193/565) of the round F1 seeds produced only round seeds in the F3 generation. Two-thirds, i.e. (372/565) of them produced both types of seeds in the F3 generation. The ratio was once again 3:1.

One-third of the round seeds and all of the wrinkled seeds in the F2 generation were homozygous and produced only seeds of the same phenotype. But two-thirds of the round seeds in the F2 were heterozygous, and their self-pollination produced both phenotypes in the ratio of a standard F1 cross.

Phenotype Ratios Are Approximate

The union of sperm and eggs is random. As the sample size gets larger, chance deviations become minimized, and the ratios approach the theoretical predictions more closely. The table given below shows the actual seed production by ten of Mendel’s F1 plants. Even though Mendel’s individual plants differed widely from the desired 3:1 ratio, the group as a whole approached it quite closely.

Mendel’s Hypothesis

Mendel formulated a hypothesis to explain the results of his experiments. The hypothesis included the following: 

• In the organism, a pair of factors control the appearance of a given characteristic, and they are called genes. 
• The organism inherits these factors from its parents, one from each. 
• A factor is transmitted from generation to generation as a discrete, unchanging unit.
• When the gametes are formed, the factors separate and are distributed as units to each gamete. 
• This statement is often called Mendel’s rule of segregation. 
• If an organism has two unlike factors, i.e. alleles for a characteristic, one may be expressed to the total exclusion of the other (dominant vs recessive).

To test his hypothesis, Mendel predicted the outcome of a breeding experiment that he had not carried out yet. He crossed heterozygous round peas (Rr) with wrinkled (homozygous, rr) ones. According to his hypothesis, in this case, one-half of the seeds produced would be round (Rr) and one-half wrinkled (rr).

Test of Mendel’s Hypothesis

The cross appeared similar to the P cross described above, i.e. round-seeded peas being crossed with wrinkled-seeded peas. But Mendel predicted that he would produce both round and wrinkled seeds in a 50:50 ratio this time. He performed the cross and harvested 106 round peas and 101 wrinkled peas. Mendel tested his hypothesis with a type of backcross called a testcross. An organism has an unknown genotype, one of two genotypes (like RR and Rr) that produce the same phenotype. The result of the test identifies the unknown genotype.

Mendel continued to cross pea varieties that differed in six other qualitative traits the results of the experiment supported his hypothesis every single time. He crossed peas that differed in two traits and discovered that the inheritance of one trait was independent of the other, framing his second rule: the rule of independent assortment. This rule does not apply to some genes due to genetic linkage.

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