Determining (P) plants from F2 data

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Determining (P) plants from F2 data

Post by aiyana1228 » Mon Mar 19, 2012 3:36 am

Note: This is HS biology so please remember I am just a 15 yr old kid with answers.

Our teacher likes to make us think but I cannot figure out where to go with this. We have not done any reverse problems like this and I am lost. Here is the problem from the lab:
Materials: F2 generation corn ears (dihybrid cross)
Two breeding parental (P) corn strains were crossed by dusting the flowers of one strain with the polen of the other, resulting in the first generation of offspring (F1). Then two identical F1 plants were crossed in the same manner, to form F2 generation offspring. The task is to use the ears from these F2 individuals to figure out the identities of the origonal parental (P) plants.
1. What do you observe about the F2 corn ears.
2. Try to pick two types of characteristics which are evident for the kernals of the F2 corn ear.
3. Note: we are assuming in this activity that each of these traits is determined by a single gene pair.
4. Score the phenotypes of the kernals on the F2 ear, grouping each according to the 2 characteristics above.
5. Calculate the adjusted count by dividing each count number in the kernal count column by the smallest count number in that column. Round to nearest whole number.

Here is my chart data:
Phenotype Kernal count Adjusted count
Yellow 203 2 (1) recessive
Purple 467 6 (3) dominant
Plump 513 6 (3) dominant
shriveled 157 2 (1) recessive

Identify the alleles: BB-homozygous Dominant Purple
Bb-heterozygous dominant purple
bb-homozygous recessive yellow
DD-homozygous dominant plump
Dd-hererozygous doninant plump
dd-homozygous recessive shriveled


bd BbDd BbDd BbDd BbDd

bd BbDd BbDd BbDd BbDd

bd BbDd BbDd BbDd BbDd

bd BbDd BbDd BbDd BbDd
F2=100% BbDd

Parental genotypes: BBDDx bbdd

Parental phenotypes:
homozygous dominant purple plump
homozygous recessive yellow shriveled.

I think I have my conclusions all wrong. I have tried every combination I could think of. I have done a million punnent squares. I really do not think I am doing it right because I am not sure how to go from the F2 generation to the P generation. Any help in putting me in the right direction would be greatly appreciated.

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Post by JackBean » Mon Mar 19, 2012 2:49 pm

First of all, there is nothing like heterozygous dominant. Heterozygous organism is simply heterozygous ;)

so, the color is yeallow : purple - 203 : 467 - 1 : 2,3, that seems pretty close to 1 : 3
the other trait is plump : shriveled - 513 : 157 - 1 : 3,27, -"-

since purple and plump both start with p, I will avoid this letter, but you can use any. Thus Y is purple and y is yellow. S is plump and s is shriveled.
So, the F2 generation seems to be YY : Yy : yy = 1 : 2 : 1 and SS : Ss : ss = 1 : 2 : 1
this seems like result of YySs * YySs cross, right?
How will you get YySs plants?
If the parental plants were "true-breeding", i.e. producing always the same offspring, the only two possibilities are either YYSS * yyss or YYss * yySS.

Cis or trans? That's what matters.

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