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

The dihybrids crosses, genetic, involve hybridization processes that take into account parental characteristics of each individual. The two characteristics studied must be contrasting with each other and must be taken into account simultaneously when crossing.

The naturalist and monk Gregor Mendel used these types of crosses to enunciate his well-known laws of inheritance. Dihybrid crosses are directly related to the second law or principle of independent segregation of characters.

Source: By Tocharianne (PNG version), WhiteTimberwolf (SVG version) (PNG version), via Wikimedia Commons

However, there are exceptions to the second law. Traits are not inherited independently if they are encoded in genes that are on the same chromosomes, that is, physically together.

The crossing begins with the choice of parents that must differ in two characteristics. For example, a tall plant with smooth seeds is crossed with a short plant with rough seeds. In the case of animals, we can cross a rabbit with white and short fur with an individual of the opposite sex with long black fur.

The principles found by Mendel allow us to make predictions about the outcome of the aforementioned crosses. According to these laws, the first filial generation will consist of individuals exhibiting both dominant traits, while in the second filial generation we will find the proportions 9: 3: 3: 1.

Mendel's Laws

Gregor Mendel managed to elucidate the main mechanisms of inheritance, thanks to the results obtained from different crosses of the pea plant.

Among its most important postulates, they stand out that the particles related to inheritance (now called genes) are discrete and are transmitted intact from generation to generation.

Mendel's First Law

Mendel proposed two laws, the first is known as the principle of dominance and he proposes that when two contrasting alleles are combined in a zygote, only one is expressed in the first generation, being the dominant one and suppressing the recessive characteristic in the phenotype.

To propose this law, Mendel was guided by the proportions obtained in monohybrid crosses: crosses between two individuals that only differ in one characteristic or trait.

Mendel's second law

Dihybrid crosses are directly related to Mendel's second law or principle of independent segregation. According to this rule, the inheritance of two characters are independent of each other.

Since the loci are being segregated independently, they can be treated as monohybrid crosses.

Mendel studied dihybrid crosses combining different characteristics in pea plants. He used a plant with smooth yellow seeds and crossed it with another plant with rough green seeds.

Mendel's interpretation of his dihybrid cross results can be summarized in the following idea:

“In a dihybrid cross, where the combination of a pair of contrasting characters is taken into account, only one variety of each trait appears in the first generation. The two hidden features in the first generation reappear in the second ”.

Exception to the second law

We may perform a dihybrid cross and find that the characteristics are not independently segregated. For example, it is possible that in a population of rabbits the black fur will always segregate with long fur. This, logically, contradicts the principle of independent segregation.

To understand this event, we must explore the behavior of chromosomes in the event of meiosis. In the dihybrid crosses studied by Mendel, each trait is located on a separate chromosome.

In anaphase I of meiosis, the homologous chromosomes separate, which will segregate independently. Thus, the genes that are on the same chromosome will remain together at this stage, reaching the same destination.

With this principle in mind, we can conclude in our hypothetical rabbit example, the genes involved in color and coat length are on the same chromosome and thus segregate together.

There is an event called recombination that allows the exchange of genetic material between paired chromosomes. However, if the genes are physically very close, the recombination event is unlikely. In these cases, the laws of inheritance are more complex than those proposed by Mendel.

Examples

In the following examples we will use the basic nomenclature used in genetics. Alleles - forms or variants of a gene - are denoted with uppercase letters when they are dominant and with lowercase letters when they are recessive.

Diploid individuals, like us humans, carry two sets of chromosomes, which translates to two alleles per gene. A dominant homozygous has two dominant alleles (AA) while a recessive homozygous has two recessive alleles (aa).

In the case of the heterozygote, it is denoted by the uppercase letter and then the lowercase letter (Aa). If the dominance of the trait is complete, the heterozygote will express the trait associated with the dominant gene in its phenotype.

Rabbits' fur color and length

To exemplify dihybrid crosses we will use the color and length of the fur of a hypothetical species of rabbits.

Generally these characteristics are controlled by several genes, but in this case we will use a simplification for didactic reasons. The rodent in question can have a long black coat (LLNN) or a short gray coat (llnn).

First generation filial

The long black furred rabbit produces gametes with the LN alleles, whereas the gametes of the short gray fur individual will be ln. At the time of the formation of the zygote, the sperm and the ovum that carry these gametes will fuse.

In the first generation, we find a homogeneous offspring of rabbits with genotype LlNn. All rabbits will present the phenotype corresponding to the dominant genes: long, black fur.

Second generation subsidiary

If we take two individuals of the opposite sex of the first generation and cross them, we will obtain the well-known Mendelian ratio 9: 3: 3: 1, where the recessive traits reappear and the four traits studied are combined.

These rabbits can produce the following gametes: LN, Ln, lN, or ln. If we make all the possible combinations for the offspring, we find that 9 rabbits will have long black fur, 3 will have short black fur, 3 will have long gray fur and only one individual will have short gray fur.

If the reader wants to corroborate these ratios, he can do so by graphing the alleles, called a Punnett square.

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.