gneet    TOPIC : INHERITANCE AND VARIATION Number of questions are 382
Q1) Dominate epistasis ratio is ... [ kerala 2009 ]


Epistasis occurs when one gene is able to mask the phenotype of another gene. Dominant epistasis is when only one allele of the gene that shows epistasis can mask alleles of the other gene. Recessive epistasis is where two alleles have to be inherited in order for the phenotype of the second gene to be masked.

a dominant allele at one locus may mask the phenotype of a second locus. This is called dominant epistasis, which produces a segregation ratio such as 12:3:1, which can be viewed as a modification of the 9:3:3:1 ratio in which the A_B_ class is combined with one of the other genotypic classes that contains a dominant allele. One of the best known examples of a 12:3:1 segregation ratio is fruit color in some types of squash . Alleles of a locus that we will call B produce either yellow (B_) or green (bb) fruit. However, in the presence of a dominant allele at a second locus that we call A, no pigment is produced at all, and fruit are white. The dominant A allele is therefore epistatic to both B and bb combinations as shown in figure.

One possible biological interpretation of this segregation pattern is that the function of the A allele somehow blocks an early stage of pigment synthesis, before neither yellow or green pigments are produced.
Ratio 9:3:4 is for Recessive epistasis
Answer : (d)

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