MONOHYBRID CROSSES: WHAT THEY CONSIST OF AND EXAMPLES - BIOLOGY - 2025

A monohybrid cross, in genetics, refers to the crossing of two individuals that differ in a single character or trait. In more exact terms, individuals possess two variations or "alleles" of the characteristic to be studied.

The laws that predict the proportions of this cross were enunciated by the Austrian naturalist and monk, Gregor Mendel, also known as the father of genetics.

Source: By Alejandro Porto, via Wikimedia Commons

The results of the first generation of a monohybrid cross provide the necessary information to infer the genotype of the parental organisms.

Historical perspective

The rules of inheritance were established by Gregor Mendel, thanks to his well-known experiments using the pea (Pisum sativum) as a model organism. Mendel carried out his experiments between 1858 and 1866, but they were rediscovered years later.

Before Mendel

Before Mendel, the scientists of the time thought that the particles (now we know that they are the genes) of heredity behaved like liquids, and therefore had the property of mixing. For example, if we take a glass of red wine and mix it with white wine, we will get rosé wine.

However, if we wanted to recover the parental colors (red and white), we couldn't. One of the intrinsic consequences of this model is the loss of variation.

After Mendel

This wrong view of heredity was discarded after the discovery of Mendel's works, divided into two or three laws. The first law or law of segregation is based on monohybrid crosses.

In the experiments with peas, Mendel made a series of monohybrid crosses taking into account seven different characters: color of the seeds, texture of the pod, size of the stem, position of the flowers, among others.

The proportions obtained in these crosses led Mendel to propose the following hypothesis: in organisms there are a couple of “factors” (now genes) that control the appearance of certain characteristics. The body is capable of transmitting this element from generation to generation in a discreet way.

Examples

In the following examples we will use the typical nomenclature of genetics, where the dominant alleles are represented by capital letters and the recessive ones by lowercase letters.

An allele is an alternative variant of a gene. These are in fixed positions on the chromosomes, called loci.

Thus, an organism with two alleles represented by capital letters is a homozygous dominant (AA, for example), while two small letters denote the homozygous recessive. In contrast, the heterozygote is represented by the capital letter, followed by the lower case: Aa.

In heterozygotes, the trait that we can see (the phenotype) corresponds to the dominant gene. However, there are certain phenomena that do not follow this rule, known as codominance and incomplete dominance.

Plants with white and purple flowers: first filial generation

A monohybrid cross begins with reproduction between individuals that differ in one characteristic. If it is vegetables, it can occur by self-fertilization.

In other words, the crossing involves organisms that possess two alternative forms of a trait (red vs. white, tall vs. short, for example). The individuals participating in the first crossing are assigned the name "parental".

For our hypothetical example we will use two plants that differ in the color of the petals. The PP (homozygous dominant) genotype translates into a purple phenotype, while the pp (homozygous recessive) represents the white flower phenotype.

The parent with the PP genotype will produce P gametes. Similarly, the gametes of the pp individual will produce p gametes.

The crossing itself involves the union of these two gametes, whose only possibility of offspring will be the Pp genotype. Therefore, the phenotype of the offspring will be purple flowers.

The offspring of the first cross is known as the first filial generation. In this case, the first filial generation is exclusively composed of heterozygous organisms with purple flowers.

Results are generally expressed graphically using a special diagram called a Punnett square, where each possible combination of alleles is observed.

Plants with white and purple flowers: second generation filial

The descendants produce two types of gametes: P and p. Therefore, the zygote can be formed according to the following events: That a P sperm meets a P egg. The zygote will be homozygous PP dominant and the phenotype will be purple flowers.

Another possible scenario is that a P sperm meets a P egg. The result of this crossing would be the same if a p sperm meets a P ovule. In both cases the resulting genotype is a Pp heterozygous with a purple flower phenotype.

Finally, it is possible that sperm p meets an ovum p. The latter possibility involves a homozygous recessive pp zygote and will exhibit a white flower phenotype.

This means that, in a cross between two heterozygous flowers, three of the four possible events described include at least one copy of the dominant allele. Therefore, at each fertilization, there is a 3 in 4 probability that the offspring will acquire the P allele. And since it is dominant, the flowers will be purple.

In contrast, in fertilization processes, there is a 1 in 4 chance that the zygote will inherit the two p alleles that produce white flowers.

Utility in genetics

Monohybrid crosses are often used to establish dominance relationships between two alleles of a gene of interest.

For example, if a biologist wants to study the dominance relationship between the two alleles that code for black or white fur in a herd of rabbits, he is likely to use the monohybrid cross as a tool.

The methodology includes the crossing between the parents, where each individual is homozygous for each trait studied - for example an AA rabbit and another aa.

If the offspring obtained in this cross are homogeneous and only express one character, it is concluded that this trait is the dominant one. If the crossing is continued, the individuals of the second filial generation will appear in 3: 1 proportions, that is, 3 individuals that exhibit the dominant vs. 1 with the recessive trait.

This 3: 1 phenotypic ratio is known as "Mendelian" in honor of its discoverer.

References

1. Elston, RC, Olson, JM, & Palmer, L. (2002). Biostatistical genetics and genetic epidemiology. John Wiley & Sons.
2. Hedrick, P. (2005). Genetics of Populations. Third edition. Jones and Bartlett Publishers.
3. Montenegro, R. (2001). Human evolutionary biology. National University of Cordoba.
4. Subirana, JC (1983). Didactics of genetics. Editions Universitat Barcelona.
5. Thomas, A. (2015). Introducing Genetics. Second edition. Garland Science, Taylor & Francis Group.