Interalleic Gene Interaction

Patterns of Inheritance

The phenomenon in which a single gene controls one character in Mendel's laws of Inheritance, but various exceptions has been noticed in which different types of interactions are possible between the genes. The complete concept of gene interaction was suggested by Bateson and this concept is called Bateson's factor hypothesis.

Gene Interactions

Gene interaction is the influence of allelic or non-allelic genes on normal phenotypic expression of the trait. In other words, cases where two genes of the same allelic pair or genes of two or more different allelic pairs influence one another is called gene interaction. It is of two types:

1. Inter-allelic or allelic gene interaction (Intragenic interaction)
2. Intra-allelic or non-allelic gene interaction (Intergenic interaction)

1) Inter-allelic or allelic gene interaction-

The gene interaction in which two alleles present on the same gene locus on the two homologous the chromosome of a gene interact together for phenotypic expression is called interallelic gene interaction. This allelic gene interaction modifies the Mendelian monohybrid, phenotypic F₂ ratio i:e 3:1 to 1:2:1. The examples of this interaction is:

• Incomplete dominance
• Co-dominance
• Multiple alleles

• Incomplete dominance:

An allelic interaction where the dominant allele is unable to express its character completely in the presence of its recessive allele in heterozygous or hybrid, as a result an intermediate character or a fine mixture of expression of two alleles is called incomplete dominance. This phenomenon was discovered by Corens in the flower color of Mirabilis Jalapa(4 O' clock plant).

When the Mirabilis plant with red flower (RR) is crossed with the plant with white flower (rr), F₁ hybrids (Rr) are formed that bear pink color flowers instead of red. The development of pink flower of intermediate phenotype (Rr) in F₁ hybrid is due to the expression of both dominant (R) and recessive (r) character to some degree. It means the blending of phenotype takes place but not genotype in the hybrid.

When these F₁ hybrids (Rr) are allowed to self-breed, in F₂ generation; red (RR), pink (Rr) and white (rr) flowers are formed in the ratio 1:2:1 instead of typical Mendelian monohybrid phenotypic F₂ ratio i:e 3:1. The phenotypic and genotypic ratio in incomplete dominance is same i:e

phenotypic ratio= 1 red : 2 pink : 1 white = 1:2:1

genotypic ratio = 1(RR) : 2(Rr) :1(rr) = 1:2:1

• Co-dominance:

An allelic interaction where the dominant and recessive relationship is not found between alleles in heterozygous condition but both of them express themselves equally, simultaneously and side by side in F₁hybrid is called co-dominance. Example- A, B, O blood group in the human being, rain coat color of cattle e.t.c.

When a man having homozygous blood group A(I^AI^A) is married to a woman having homozygous blood group B(I^BI^B), they produce hybrid offspring with blood group AB(I^AI^B) in F₁ generation. An allele I^A produces specific glycoprotein called anti-gene A and I^B produces specific glycoprotein, called anti-gene B. Both anti-gene are equally present in the blood group AB. So, the contribution of allele I^A is equal to the allele I^B for expressing both hybrid blood group AB(I^AI^B). In this way, blood group AB shows the phenomenon of co-dominance.

NOTE-I -stands for Isohaemagglutinogens. The blood group O [I^O] doesn't produce any anti-gene and functions as recessive along with allele I^Aand I^B in the hybrid.

• Multiple Alleles:

The two alternation forms of a gene that controls the single character are called allele.

More than two alternative forms of a gene present in the same gene locus that controls a single character in same cases are called multiple alleles. It is derived by the mutation of wild type gene.

⇒Characters of multiple alleles:

1) There are more than two alternative forms of a gene for a single character. Example- Human blood group has 3 alleles, eye color of fruit fly has 15 alleles.

2) All the multiple alleles are located on the same gene locus on the same chromosome.

3) They do not undergo crossing over because they all lie in the same locus.

4) Each trait consists of two similar or dissimilar alleles but gamete has single allele.

5) They show co-dominance and dominance-recessive relationship.

6) Genotypes of multiple alleles are determined by a formula: $$\frac{n\left ( n+1 \right )}{2}$$ , where n= number of alleles

Reference

Keshari, Arvind K. and Kamal K. Adhikari. A Text Book of Higher Secondary Biology(Class XII). 1st. Kathmandu: Vidyarthi Pustak Bhandar, 2015.

Mehta, Krishna Ram.Principleof biology.2nd edition.Kathmandu: Asmita, 2068,2069.

Jorden, S.L.principle of biology.2nd edition . Kathmandu: Asmita book Publication, 2068.2069.