MONOHYBRIDISM: WHAT IT CONSISTS OF AND EXERCISES SOLVED - BIOLOGY - 2025

The monohibridismo refers to the cross between two individuals differing in only one feature. Likewise, when making crosses between individuals of the same species and when studying the inheritance of a single trait, we speak of monohybridism.

Monohybrid crosses seek to investigate the genetic basis of traits that are determined by a single gene. The inheritance patterns of this type of interbreeding were described by Gregor Mendel (1822–1884), an iconic character in the field of biology and known as the father of genetics.

Based on his work with pea plants (Pisum sativum), Gregor Mendel enunciated his well-known laws. Mendel's first law explains monohybrid crosses.

What does it consist of?

As mentioned above, monohybrid crosses are explained in Mendel's first law, which is described below:

Mendel's First Law

In sexual organisms there are pairs of alleles or pairs of homologous chromosomes, which are separated during the formation of gametes. Each gamete receives only one member of that pair. This law is known as the "law of segregation."

In other words, meiosis ensures that each gamete contains strictly a pair of alleles (variants or different forms of a gene), and it is equally likely that a gamete contains any of the forms of the gene.

Mendel managed to enunciate this law by making crossbreeds of pea plants. Mendel followed the inheritance of several pairs of contrasting characteristics (purple flowers versus white flowers, green seeds versus yellow seeds, long stems versus short stems), for several generations.

In these crosses, Mendel counted the descendants of each generation, thus obtaining proportions of individuals. Mendel's work managed to generate robust results, since he worked with a significant number of individuals, approximately a few thousand.

For example, in the monohybrid crosses of round smooth seeds with wrinkled seeds, Mendel obtained 5474 round smooth seeds and 1850 wrinkled seeds.

Likewise, crosses of yellow seeds with green seeds yield a number of 6022 yellow seeds and 2001 green seeds, thus establishing a clear 3: 1 pattern.

One of the most important conclusions of this experiment was to postulate the existence of discrete particles that are transmitted from parents to children. Currently, these inheritance particles are called genes.

Punnett square

This chart was first used by geneticist Reginald Punnett. It is a graphic representation of the gametes of the individuals and all the possible genotypes that may result from the cross of interest. It is a simple and fast method to solve crosses.

Solved exercises

First exercise

In the fruit fly (Drosophila melanogaster) the gray body color is dominant (D) over the black color (d). If a geneticist crosses a homozygous dominant (DD) and a homozygous recessive (dd) individual, what will the first generation of individuals look like?

Reply

The dominant homozygous individual only produces D gametes, while the recessive homozygous also produces only one type of gametes, but in their case they are d.

When fertilization occurs, all the zygotes formed will have the Dd genotype. Regarding the phenotype, all individuals will be gray-bodied, since D is the dominant gene and masks the presence of d in the zygote.

As a conclusion we have that 100% of the individuals in F ₁ will be gray.

Second exercise

What proportions result from the crossing of the first generation of flies from the first exercise?

Reply

As we can deduce, the flies of F ₁ have the Dd genotype. All the resulting individuals are heterozygous for this element.

Each individual can generate D and d gametes. In this case, the exercise can be solved using the Punnett square:

In the second generation of flies, the characteristics of the parents reappear (flies with black bodies) that seemed to have been "lost" in the first generation.

We obtained 25% of flies with the dominant homozygous genotype (DD), whose phenotype is gray body; 50% of heterozygous individuals (Dd), in which the phenotype is also gray; and another 25% of homozygous recessive (dd) individuals, with black bodies.

If we want to see it in terms of proportions, crossing heterozygotes results in 3 gray individuals versus 1 black individuals (3: 1).

Third exercise

In a certain variety of tropical silver, one can distinguish between speckled leaves and smooth leaves (without the speckles, unicolor).

Suppose a botanist crosses these varieties. The plants resulting from the first crossing were allowed to self-fertilize. The result of the second generation were 240 plants with mottled leaves and 80 plants with smooth leaves. What was the phenotype of the first generation?

Reply

The key point for solving this exercise is to take the numbers and bring them into proportions, dividing the numbers as follows: 80/80 = 1 and 240/80 = 3.

Evidenced the 3: 1 pattern, it is easy to conclude that the individuals that gave rise to the second generation were heterozygous, and phenotypically had mottled leaves.

Fourth exercise

A group of biologists is studying the coat color of rabbits of the species Oryctolagus cuniculus. Coat color appears to be determined by a locus with two alleles, A and a. Allele A is dominant and is recessive.

What genotype will the individuals resulting from the crossing of a homozygous recessive (aa) and a heterozygous (Aa) individual have?

Reply

The methodology to follow to solve this problem is to implement the Punnett square. Homozygous recessive individuals only produce a gametes, while heterozygous individuals produce A and a gametes. Graphically it is as follows:

Therefore, we can conclude that 50% of the individuals will be heterozygous (Aa) and the other 50% will be homozygous recessive (aa).

Exceptions to the first law

There are certain genetic systems in which heterozygous individuals do not produce equal proportions of two different alleles in their gametes, as predicted by the previously described Mendelian proportions.

This phenomenon is known as distortion in segregation (or meiotic drive). An example of this are selfish genes, which intervene with the function of other genes seeking to increase their frequency. Note that the egoistic element can diminish the biological efficacy of the individual who carries it.

In the heterozygous, the egoistic element interacts with the normal element. The selfish variant can destroy the normal or impede its functioning. One of the immediate consequences is the breach of Mendel's first law.

References

1. Barrows, EM (2000). Animal behavior desk reference: a dictionary of animal behavior, ecology, and evolution. CRC press.
2. Elston, RC, Olson, JM, & Palmer, L. (2002). Biostatistical genetics and genetic epidemiology. John Wiley & Sons.
3. Hedrick, P. (2005). Genetics of Populations. Third edition. Jones and Bartlett Publishers.
4. Montenegro, R. (2001). Human evolutionary biology. National University of Cordoba.
5. Subirana, JC (1983). Didactics of genetics. Editions Universitat Barcelona.
6. Thomas, A. (2015). Introducing Genetics. Second edition. Garland Science, Taylor & Francis Group.