I'm doing this not because the school said, but because I want to get good grades in my GCSE's so please don't say "do it yourself" because I DID try, I even used the answer sheet but I still do not understand. Could anyone help me with this question?

 

 

https://gyazo.com/35c495b42eea7b952acb56997df92fa0

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If the initial batch of plants is a cross between tall and short plants, and they are all tall, then tallness is a dominant trait and shortness is recessive, so only double-short plants will be tall and everything else will be short.

 

If T is tall and t is short, all of the first generation of plants are Tt.

 

Crossed with each other, you will get a mix of TT, Tt and tt. Only tt will be short.

 

You can figure out exactly what the percentages are with a basic Punette square. Just divide a box into four parts. Since all of the offspring are the result of a cross between a Tt plant and a Tt plant, label one column T and the other t, and one row T and the other t. Wherever a T and a T intersect, you get TT. Where T and t intersect you get Tt and where t and t intersect, you get tt.

 

Each part of the total box represents 25% of the total population for that generation, so you should get the total for each of TT, Tt and tt. As a sanity check, you know that only tt will be short and that there are seventy short plants and 210 tall plants in this generation (that will be a mix of TT and Tt).

 

Once you have the numbers for each possible combination, you can make three more diagrams. You know the final generation is crossing the previous one with short plants, and short plants are always tt.

 

So you can set up Punnett squares to cross tt with TT, Tt and tt plants. Then assign each of the four boxes in each Punnett square a number representing 25% of the value that you got for that combination in the previous round, add all the like combinations together and you should get the totals for TT, Tt and tt in that generation.

 

Then just remember that tt is short and Tt and TT are tall, and you should be able to answer the question.