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9.3.1. Gene interaction between allelic genes

Gene interaction between allelic genes

According to Mendel, when crossing two different individuals (pure races) for the same character, the resulting hybrids manifested in their phenotype the dominant character (that of one of their parents). This type of dominance is known as complete dominance .

But this is not always true, since it is possible that this dominant character is not expressed in F1, but rather that it does so in a diluted way (intermediate inheritance) or that another different character appears in which the two alleles are expressed (codominance).

Review: Types of genetic inheritance (4th ESO).


Codominance occurs when each of the two alleles of a gene from a heterozygous individual manifest independently. That is, in the phenotype of the hybrid, the two codominant alleles are manifested simultaneously, they both have the same intensity. By crossing these individuals, a 1: 2: 1 phenotype (AA: 2Aa: aa) is obtained.

For example, blood group AB or Short-horn cattle , whose hair can be red, white and roan (half red and half white).

Nomenclature for genetic problems

Since uppercase letters represent dominance and lowercase letters, recessivity, in the case of codominance problems, we will represent the different alleles by means of subscripts or superscripts of the same representative letter of the gene. This letter will be capitalized, since both alleles are expressed to a greater or lesser degree. For example, A1, A2.

In the case of blood groups the MN system, the gene is represented by the letter L and the two alleles are LM and LN. Possible genotypes are LM LM , LN LN and LM LN.

Intermediate inheritance

The phenotype of the heterozygous individual is different from that of the two homozygous, due to the fact that neither allele totally cancels out the other, the two have the same intensity. The heterozygous phenotype may be an intermediate between that of the two parents, or a new trait may appear.

We can see examples of intermediate dominance in the dragon's mouth (Antirrhinum maius) or in the marvel of Peru (Mirabilis jalapa), when when crossing a homozygous plant with red flowers and another homozygous plant with white flowers, all the hybrid offspring have pink flowers and F2 produces 1 red: 2 pink: 1 white, rather than the 3:1 phenotypic segregation typical of complete dominance.

Nomenclature for genetic problems

In cases of intermediate inheritance, a new phenotype appears, although it does not necessarily have to be intermediate. They are different alleles of the same gene, so we will use a single letter for all of them.

We can use an uppercase letter (A) and another lowercase letter (a), since in this case there is neither dominance nor recessivity, or use a capital letter for the two alleles, with a subscript or superscript to differentiate them (C1, C2).

Questions that have come out in University entrance exams (Selectividad, EBAU, EvAU)

Genetics problems solved: Codominance and intermediate inheritance problems (4th ESO). It contains genetic problems that have come out in EvAU exams from various universities.

Aragon. June 2018, option B, question 3

Regarding Mendelian inheritance: (2.5 points)

c) Codominant inheritance (0.5 points) and intermediate inheritance (0.5 points)

Aragon. September 2017, option B, question 2 .

In guinea pigs there are three varieties for the coat: yellow, cream and white. (2.5 points)

By crossing two cream-colored guinea pigs, descendants of all three varieties are obtained. Deduce what type of inheritance the character presents by raising the cross. (2 points)


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