Wednesday
April 16, 2014

Homework Help: Genetics

Posted by Jenna on Wednesday, October 7, 2009 at 3:17pm.

Show the F1 and F2 progeny phenotypic and genotypic ratios of the following dihybrid cross involving unlinked purebred Mendelian traits (tall plants, long stems X dwarf plants, short stems) under the following conditions:
a. the alleles for tall plants and long stems are dominant
b. the alleles for height and stem length are each codominant
c. the allele specifiying plant height is epistatic to the allele specifying stem length

I wrote:
P generation= TTSS x ttss
F1 generation= TtSs
Therefore the F1 progeny are all tall with long stems. How do I write the genotypic and phenotypic ratios? Is it just 1 or is it 1:0 TtSs?

F1= TS, Ts, tS, ts x TS, Ts, tS, ts
I know the genotypic ratio of the F2 progeny is 1:2:2:1:4:2:1:2:1
(TTSS:TTSs:TtSS:TTss:TtSs:Ttss:ttSS:ttSs:ttss)
But the epistatic gene is confusing me on the phenotypic ratio. Does that mean that unless plant height = tt (recessive) the stem length will be long? I don't think I understand that. But if that's the case then the phenotypic ratio would be:
3:3:1:1:1
(Tall, long: medium long: dwarf long: dwarf medium: dwarf short)

Does that look right?

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