Re: Would the ratio from tall to short plants in Mendel's 3rd generation be 3:1

Hi Samantha,

It depends on the genotype of the plants in the P1, or first generation.  If 
you start with one parent who has two "tall" alleles (genotype = TT ; 
phenotype = tall) and the other parent with two "short" alleles (genotype = 
tt;  phenotype = short), they will produce offspring (the F1 generation) who 
are all tall, but carry one of each type of allele (Tt).  In the 3rd, or F2 
generation, the phenotypic ratio would be 3 tall to 1 short, and the 
genotypic ratio would be 1:2:1.  One of the tall plants would have two 
"tall" alleles (TT), and the other two would have one "tall" and one "short" 
(Tt).  Since "tall" is dominant and "short" is recessive, they would appear 
tall. The fourth genetic combination, tt, would give a short phenotype.

 The link below will take you to a nice diagram of this mendelian cross.  
Instead of tall and short plants, it uses the example of smooth and wrinkled 
peas, with smooth being dominant over wrinkled, but the concept of how these 
alleles segregate in the different generations, and the resulting 
phenotypes, is identical to the problem you asked about. One word of 
caution:  Beware of memorizing these ratios, because they will vary 
depending on the genotype of the parents in the original cross (the P1 
generation). 
 http://
www.accessexcellence.org/AB/GG/mendel.html    

I hope this is helpful. It sounds like you are on the right track, so keep 
up the good work!    
Warm Regards,

Jen