por What is a monohybrid cross?
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 have two variations or «alleles» of the trait 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.
The results of the first generation of a monohybrid cross provide the information necessary to infer the genotype of the parental organisms.
historical perspective
The rules of heredity were established by Gregor Mendel, thanks to his well-known experiments using the pea as a model organism (pisum sativum). Mendel carried out his experiments between 1858 and 1866, but they were rediscovered years later.
Before Mendel
Before Mendel, scientists at the time thought that the particles (we now know that they are 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 retrieve the colors of the parents (red and white), we couldn’t. One of the intrinsic consequences of this model is the loss of variation.
after Mendel
This erroneous 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 carried out a series of monohybrid crosses taking into account seven different characters: seed color, pod texture, stem size, flower position, 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 organism is able to transmit this element from generation to generation in a discreet way.
Examples of monohybrid cross
In the following examples we will use the typical nomenclature of genetics, where the dominant alleles are represented with capital letters and the recessive ones with lower case letters.
An allele is an alternate variant of a gene. These are found at fixed positions on the chromosomes, called loci.
Thus, an organism with two alleles represented by capital letters is a homozygous dominant (oh, for example), while two lowercase letters denote the homozygous recessive. In contrast, the heterozygote is represented by the capital letter, followed by the lowercase: Aah.
In heterozygotes, the character 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 breeding between individuals that differ in one characteristic. If it is about vegetables, it can occur by self-fertilization.
In other words, crossbreeding involves organisms that possess two alternative forms of a trait (red vs. white, tall vs. short, for example). The individuals that participate in the first crossing are assigned the name of “parents”.
For our hypothetical example we will use two plants that differ in the color of the petals. the genotype PP (homozygous dominant) results in a purple phenotype, while the pp (homozygous recessive) represents the white-flowered phenotype.
The parent with the genotype PP will produce gametes P. Similarly, the gametes of the individual pp will produce gametes p.
The crossing itself involves the union of these two gametes, whose only possibility of descendants will be the genotype. pp. Therefore, the phenotype of the offspring will be purple flowers.
The offspring from the first cross are known as the first filial generation. In this case, the first filial generation is formed exclusively from heterozygous organisms with purple flowers.
Generally, the results are 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 filial generation
Offspring produce two types of gametes: P and p. Therefore, the zygote can be formed according to the following events: That a sperm P meet an ovum P. The zygote will be homozygous dominant. PP and the phenotype will be purple flowers.
Another possible scenario is that a sperm P find an egg p. The result of this crossing would be the same if a sperm p find an egg Q. In both cases the resulting genotype is a heterozygote. pp with phenotype of purple flowers.
Finally, maybe sperm p meet an ovum p. This last possibility involves a homozygous recessive zygote. pp 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 chance 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 both alleles. p They 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 flock of rabbits, he or she would likely use the monohybrid cross as a tool.
The methodology includes crossing between the parents, where each individual is homozygous for each character studied – for example a rabbit. AA and other oh.
If the offspring obtained in said cross is homogeneous and only expresses one character, it is concluded that this trait is the dominant one. If the cross is continued, the individuals of the second filial generation will appear in proportions 3:1, that is, 3 individuals that exhibit the dominant characteristic vs. 1 with the recessive trait.
This 3:1 phenotypic ratio is known as “Mendelian” after its discoverer.
References
Elston, RC, Olson, JM, & Palmer, L. (2002). Biostatistical genetics and genetic epidemiology. John Wiley & Sons.
Hedrick, P. (2005). Genetics of Populations. Third edition. Jones and Bartlett Publishers.
Montenegro, R. (2001). Human evolutionary biology. National University of Cordoba.
Subirana, JC (1983). didactics of genetics. Editions University Barcelona.
Thomas, A. (2015). Introducing Genetics. Second edition. Garland Science, Taylor & Francis Group.