lecture 15

Deviation from Mendelian inheritance – Non allelic interaction without modification in Mendelian ratio – Bateson and Punnett’s experiment on fowl comb shape. Non allelic interaction with modification in Mendelian ratio – i.) Dominant epistasis (12:3:1)

Gene interaction:

   For the determination of single phenotypic character, two alleles of a single gene interacted in various way. E.g.: complete dominance, incomplete dominance or codominance. These kinds of genetic interactions occur in between the two alleles of a single gene is referred as Allelic interaction or intra genetic interaction.

Epistasis: Interaction of two or more genes, thus involving two or more loci

Dominant Epistasis: Gene interaction in which a dominant allele at one locus can mask the expression of both alleles (dominant and recessive) at another locus resulting in 12: 3: 1 ratio. Also referred as simple epistasis.

Types of gene action:

   Gene action refers to the manner in which genes control the phenotypic expression of various characters in an organism. Alleles of the gene may interact with one another in a number of ways to produce variability in their phenotypic expression. The dominant and recessive relationship is fundamental and is essentially constant with each pair of alleles.

Gene interaction or epistasis:

    When expression of one gene depends on presence / absence of another gene in an individual, it is known as gene interaction. Interaction of genes at different loci that affect the same character is called epistasis. The term epistasis was first used by Bateson in 1909 to describe two different genes which control the same character, out of which one masks / suppresses the expression of another gene. Gene that masks the action of another gene is called epistatic gene while the gene whose expression is being masked is called hypostatic gene.

Gene interaction without modification of Mendelian ratio (9:3:3:1)

   Two pairs of alleles affecting the same character and producing the F2 four different phenotypes in the ratio of 9:3:3:1 was discovered in fowls by Bateson and Punnett.

   Each breed of poultry possesses a characteristic type of comb. Each of these breeds are true

• The Wyandotte breed has ‘rose’ comb.

• The Bramha has a ‘pea’comb.

• The Leghorn has a ‘Single’ comb.

• Malay breed has a comb ‘walnut’ comb.

    Cross between rose comb and single combed types show that rose in dominant to single comb and that there is a segregation of 3 rose: 1 single comb in the F 2. In mating between pea combed with single combed and 3:1 ratio appears in F2. In mating between pea combed with single combed bird, pea combed is found to be dominant over single comb and 3:1 ratio appears in F2. When a rose combed fowl is crossed with a pea combed one, all the F1 birds showed a new comb known as walnut comb. When the walnut combs are inbred there appears in F2 walnut, rose pea and single in the ratio of 9:3:3:1. The rose comb is due to the presence of R gene and pea due to p gene. Walnut comb is due to the presence of the dominant genes. R and P and single comb are due to the presence of recessive of r and p. The ratio expressed in F2 is 9:3:3:1.

Types of epistasis:

    Epistasis leads to modification of normal dihybrid or trihybrid segregation ratio in F2 generation. various types of epistatic gene interaction are:

• Dominant epistasis (12:3:1)

•  Recessive epistasis (9:3:4)

• Duplicate gene with cumulative effect (9:6:1)

• Duplicate dominant epistasis (15:1)

• Duplicate recessive epistasis (9:7)

• Duplicate recessive epistasis (13:3)

 1.     Dominant epistasis (12:3:1):

   Dominant epistasis occurs when a dominant allele of one gene masks or overrides the expression of alleles at another gene locus, leading to a modified phenotypic ratio, such as 12:3:1. This means that a dominant allele at one gene (the epistatic gene) can prevent a trait from being expressed, even if the other gene (the hypostatic gene) has alleles that would normally produce that trait.