# Punnett Square during exam

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#### w1ll

##### Junior Member
10+ Year Member
5+ Year Member
So theres often one or two of these on the practice exams. I almost always find myself starting one of these and thinking "I don't have time to sit here and draw this thing out right now!" and I just end up guessing. Are you guys actually sitting there drawing these things out? Or are there faster ways to predict "how many will be tall and short", and the answer is usually a fraction. I'm not talking about the X-linked ones either. I mean like the TtRr x ttRr kinda ones.

#### Doc Henry

##### blowin' in the wind
10+ Year Member
So theres often one or two of these on the practice exams. I almost always find myself starting one of these and thinking "I don't have time to sit here and draw this thing out right now!" and I just end up guessing. Are you guys actually sitting there drawing these things out? Or are there faster ways to predict "how many will be tall and short", and the answer is usually a fraction. I'm not talking about the X-linked ones either. I mean like the TtRr x ttRr kinda ones.

I don't draw the big ones out. I look at it just like a long calculation, its only worth 1 point just like everything else, so I'm not going to waste my time on it. I just try to reason my way through the question, but yea, those big punnet square ones are so lame!

#### BloodySurgeon

##### Full Member
Moderator Emeritus
15+ Year Member
I look at things in parts... like ur example TtRr x ttRr (T=tall, r=white), how many will be tall and white?

Tt x tt = 50% or 1/2 tall

Rr x Rr = 25% or 1/4 white

therefore: 1/2 x 1/4 = 1/8 = 12.5%

I could draw the punnett squares for each one but it just seems like a waste of time when its not hard visualizing 4 boxes.

#### RSAgator

##### Junior Member
10+ Year Member
5+ Year Member
I look at things in parts... like ur example TtRr x ttRr (T=tall, r=white), how many will be tall and white?

Tt x tt = 50% or 1/2 tall

Rr x Rr = 25% or 1/4 white

therefore: 1/2 x 1/4 = 1/8 = 12.5%

I could draw the punnett squares for each one but it just seems like a waste of time when its not hard visualizing 4 boxes.

Pretty much how I do it. The only thing i write down is the combination of alleles (Xx, Tt, etc). I then do each of them separately and multiply the results of interest.

#### w1ll

##### Junior Member
10+ Year Member
5+ Year Member
Pretty much how I do it. The only thing i write down is the combination of alleles (Xx, Tt, etc). I then do each of them separately and multiply the results of interest.

Thanks a lot for the help guys. I'm an idiot and didn't think of doing them that way. I use EK books and they just kinda tell you to draw a punnett square.