Trihybrid cross, punnet square help?

ABC; ABc; AbC; aBC; Abc; abC; aBc; abc - goes along the top

abC; abc; - only need these two down the side since the others are repeats.

If you really want to do a Punnet square, you can make your life a whole lot easier. If you look at the gametes from the second parent, you'll see that there are only two kinds of gametes: abC and abc in equal proportions. The first parent does produce all 8 types of gametes that you have listed. So, you could make your Punnet square an 8 X 2 punnet square rather than an 8X8 one.

Or, you can do what is called a branching diagram of the cross. In this, you look at each trait individually. So, for Aa X aa, you will produce 1/2 Aa and 1/2 aa offspring. Then you look at the next trait and see that you will produce 1/2 Bb and 1/2 bb. For the third trait, you will produce 1/4 CC, 1/2 Cc and 1/4 cc.

(I tried to find a good image of a branching diagram, but couldn't quickly...If you spend a bit of time with google, you might find a good one...)

But the end result is that for the offspring:

1/2 X 1/2 X 1/4 = 1/16 will be AaBbCC
1/2 X 1/2 X 1/2 = 1/8 will be AaBbCc
1/2 X 1/2 X 1/4 = 1/16 will be AaBbcc

Continuing through all possible combinations will give you your final answer, perhaps...

First parent - yes, all of those gametes go along the top of the punnet square.
In general, there are 2^n possible gametes for n genes, so 8 is the right number for this cross.

Second parent - note that it doesn't matter which a allele the gamete gets, because they are identical. Same for b. This parent produces only two types of gamete: abC and abc.

The dimensions of your "square" are 8x2.


As long as the three genes sort independently, you can combine them mathematically instead of drawing the big punnet square.

Ca x Cc produces CC, Cc and cc in a 1:2:1 ratio.
Aa x aa produces Aa and aa in a 1:1 ratio. So within each of the CC group, the Cc group, and the cc group, there is a 1:1 ratio of Aa to aa.
This makes a 1:1:2:2:1:1 ratio of AaCC:aaCC:AaCc:aaCc:Aacc:aacc

Adding in the B's makes it 1:1:1:1:2:2:2:2:1:1:1:1