What is dihybridism?

The dihybridism, in genetics, it defines the simultaneous study of two different hereditary traits, and by extension, of those whose manifestation depends on two different genes even if it is the same trait

The seven traits that Mendel analyzed were useful to him in putting forward his theory of the inheritance of characters because, among other things, the genes responsible for their manifestation had contrasting alleles whose phenotype was easy to analyze, and because each one determined the expression single character.

That is, they were monogenic traits whose hybrid condition (monohybrid) allowed determining the dominance / recessivity relationships between the alleles of that single gene..

When Mendel analyzed the joint inheritance of two different characters, he proceeded as he had with the single characters. He obtained double hybrids (dihybrids) that allowed him to check:

• That each one complied with the independent segregation that he had observed in the monohybrid crosses.
• Furthermore, in dihybrid crosses the manifestation of each character was independent of the phenotypic manifestation of the other. That is, their inheritance factors, whatever they were, were independently distributed.

We now know that the inheritance of characters is a bit more complex than what Mendel observed, but also that in its fundamentals Mendel was completely correct..

The subsequent development of genetics made it possible to demonstrate that dihybrid crosses and their analysis (dihybridism), as Bateson was initially able to demonstrate, could be an inexhaustible source of discoveries in this powerful and nascent science of the 20th century..

Thanks to their clever use, they were able to give the geneticist a somewhat clearer idea of ​​the behavior and nature of genes..

Article index

• 1 Dihybrid crosses of different characters
• 2 Alternative phenotypic manifestations of dihybrid crosses
• 3 A little more epistasis
• 4 References

Dihybrid crosses of different characters

If we analyze the products of a monohybrid cross Aa X Aa, we can notice that it is equal to developing the remarkable product (TO+to)^two= AA + 2Aa + aa.

The expression on the left includes the two types of gametes that one of the parents heterozygous for the gene can produce TO/to; when squaring we indicate that both parents are of identical constitution for the gene under study [that is, it is a monohybrid cross (TO+to) X (TO+to)].

The expression on the right gives us the genotypes (and therefore the phenotypes are deduced) and expected proportions derived from the crossing.

Hence, we can directly observe the genotypic proportions derived from the first law (1: 2: 1), as well as the phenotypic proportions explained by it (1AA+twoAa= 3TO_ for every 1aa, or phenotypic ratio 3: 1).

If we now consider a cross to analyze the inheritance of a gene B, the expressions and proportions will be the same; in fact, it will be so for any gene. In a dihybrid cross, therefore, we actually have the development of the products of (TO+to)^two X (B+b)^two.

Or what is the same, if the dihybrid cross involves two genes that participate in the inheritance of two unrelated characters, the phenotypic proportions will be those predicted by the second law: (3TO_:1aa) X (3B_:1baby) = 9TO_B_:3TO_baby:3aaB_:1aabb).

These, of course, are derived from the ordered genotype ratios 4: 2: 2: 2: 2: 1: 1: 1: 1 resulting from the product of (TO+to)^two X (B+b)^two= (AA + 2Aa + aa) X (BB + twoBb + baby).

We invite you to check it out for yourself in order to analyze now what happens when the phenotypic ratios 9: 3: 3: 1 of a dihybrid cross "deviate" from these clear and predictable mathematical relationships that explain the independent inheritance of two encoded characters. by different genes.

Alternative phenotypic manifestations of dihybrid crosses

There are two main ways that dihybrid crosses deviate from what is “expected”. The first is the one in which we are analyzing the joint inheritance of two different characters, but the phenotypic proportions observed in the progeny give a clear predominance to the manifestation of parental phenotypes..

Most likely it is a case of linked genes. That is, the two genes under analysis, although they are at different loci, are so close to each other physically that they tend to be inherited together and, obviously, they are not being distributed independently..

The other circumstance, which is also quite common, derives from the fact that a small minority of hereditary traits are monogenic..

On the contrary, more than two genes participate in the manifestation of most hereditary traits..

For this reason, it is always possible that the genetic interactions that are established between the genes that participate in the manifestation of a single character are complex and go beyond a simple relationship of dominance or recessivity as observed in the relationships allelic typical of monogenic traits.

For example, in the manifestation of a trait, about four enzymes may participate in a particular order to give rise to the final product responsible for the phenotypic manifestation of the wild phenotype..

The analysis that makes it possible to identify the number of genes from different loci that participate in the manifestation of a genetic trait, as well as the order in which they act, is called epistasis analysis and is perhaps the one that most typically defines what we call genetic analysis. in its most classic sense.

A little more epistasis

At the end of this post, the phenotypic proportions observed in the most common cases of epistasis are presented - and this only taking into account dihybrid crosses..

By increasing the number of genes that participate in the manifestation of the same trait, the complexity of gene interactions and their interpretation obviously increases..

In addition, which in turn can be taken as the golden rule for a correct diagnosis of epistatic interactions, the appearance of new phenotypes not present in the parental generation can be verified..

Finally, apart from allowing analysis of the appearance of new phenotypes and their proportion, the analysis of epistasis also allows determining the hierarchical order in which the different genes and their products must manifest in a given pathway to account for the phenotype associated with them..

The most basic or early manifestation gene is epistatic above all the others, since without its product or action, for example, those downstream of it will not be able to express themselves, which will therefore be hypostatic to it..

A gene / product in the third place in the hierarchy will be hypostatic to the first two, and epistatic to any others remaining in this gene expression pathway..

References

1. Bateson, W. (1909). Mendel's Principles of Heredity. Cambridge University Press. Cambridge, U. K.
2. Brooker, R. J. (2017). Genetics: Analysis and Principles. McGraw-Hill Higher Education, New York, NY, USA.
3. Cordell, H. (2002). Epistasis: What it means, what it doesn't mean, and statistical methods to detect it in humans. Human Molecular Genetics, 11: 2463-2468.
4. Goodenough, U. W. (1984) Genetics. W. B. Saunders Co. Ltd, Pkiladelphia, PA, USA.
5. Griffiths, A. J. F., Wessler, R., Carroll, S. B., Doebley, J. (2015). An Introduction to Genetic Analysis (11^th ed.). New York: W. H. Freeman, New York, NY, USA.