A true breeding strain of squash that produced disk-shaped fruits was crossed to

Question

A true breeding strain of squash that produced disk-shaped fruits was crossed to a true-breeding strain that produced long fruits. All the plants in the F1 have disk-shaped fruits. The progeny from a self-fertilized F1 plant showed a new phenotype , spherical fruits. The number of plants with each phenotype in the F2 is:

Long fruits: 32

Spherical fruits: 178

Disk fruits: 270

1. Assume that the loci you are carrying are 30 cM(centimorgans) apart. What would be the phenotypic ratios ( out of 200 individuals) among the progeny of the self-fertilized F1? A Punnet Square or a line diagram might be helpful.

Explanation / Answer

This is a modified dihybrid cross, where epistasis is seen with Duplicate Interaction. F1 (Disc Fruits) X F1 (Disc Fruits) F2 Genotypes and associated Phenotypes from the infomation provided: D_S_ =Disc [spherical is dominant over D_ss=Disc long ,so ddS_=Spherical spherical=S_ ddss=long long=ss,Disc=D_] Among the F2 plants the allele D_ at one locus inhibits the production of the long shape fruit.The outcome is because a second locus S-(spherical) is dominant over long fruit and the loci D and S are 30centimorgan far and are not linked. Now, F1 X F1 Phenotypes and genotypes in punnet square gametes DS Ds dS ds [D=disc fruit, DS DDSS DDSs DdSS DdSs sp=spherical, (D) (D) (D) (D) l=long fruit] Ds DDSs DDss DdSs Ddss (D) (D) (D) (D) dS DdSS DdSs ddSS ddSs (D) (D) (sp) (sp) ds DdSs Ddss ddSs ddss (D) (D) (sp) (l) The phenotypic ratio is 12:3:1 Out of 200 approximately 148=Disc fruits 40=spherical 12=long fruits

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