Punnett Square
This worksheet will guarantee you mastery of the punnett square. The punnett square
is a probability tool. It shows what COULD happen not what WILL happen.
For a given individual’s genotype, remember that each letter represents one allele on one
of the homologous chromosomes. So Bb means that one chromosome has “B” and the
second chromosome, from the other parent has the “b”.
During gametogenesis, meiosis usually insures that only one “blended” chromosome of
each homologous pair is sent to 50% of the gametes and the other member of that pair
goes to the other 50%. (principle of segregation).
So for the above genotype of Bb, half of this individual’s gametes would have the B allele
and the other half, the b allele. Let’s assume the mate of this individual is also Bb.
Knowing this, we can easily calculate the probability (what might happen) for different
offspring genotypes based on the parental genotypes. Bb x Bb.
First draw your “tic-tac-toe”lines. This is the punnet square for a monohybrid cross.
Second, above the upper row of boxes, write the female parent’s genotype, to the left of
the left hand boxes write the male parent’s genotype.
Put an X or what generation of cross this is (F1) etc in the uppermost left corner. This
will prevent you from using it for anything else.
Now the remaining upper boxes represent the possible gametes of the female. Look at
her genotype and put the first of the alleles in the first open upper box and the second
allele in the remaining upper box.
Do the same using the remaining left hand boxes for the male’s possible gametes.
To see what the offspring genotypes could be, just put the gametes together as your
clever teacher tells you. Each offspring box in these circumstances, corresponds to a
25% probability (1 of 4 boxes). Remember this is just a probability of occurrence.
Now let’s try some do a punnett square for a female with genotype Rr and a male also
Rr. In this example we will let R stand for red and r stand for yellow flowers.
What is the genotype probability of an offspring of RR_______, Rr_______ or rr______?
What is the phenotype probability of red_______ or yellow________?
Now here is one that will make you think a little more. If you have a mother with blood
type B and the father who is type A, which blood types are possible? Unless you know
otherwise, always assume the genotype is heterozygous (this will always provide all
possible combinations).
When we do questions involving alleles or genes on the sex chromosomes, it will almost
always be on the “X”. Very few of the “Y” alleles are known. It would be smart to
write the sex chromosomes as well as the allele for the genotype and in the gamete
boxes. Often these traits are said to be sex linked or X linked, since they involve a sex
(usually the X) chromosome. Try this one. Cross a colorblind male with a homozygous
normal vision female.
Monohybrid crosses involve calculations for one trait, Rr flower color, whatever. If you
are given a genotype for two traits, SsYy (seed shape and seed color) this would be a
dihybrid cross, perhaps it is not as obvious regarding possible gametes.
The best way to determine all gamete possibilities is to F.O.I.L. the genotypes (Ss)(Yy).
In this example you would get these: SY,Sy, sY and sy. FOIL the other parents genotype
and this will give the other gametes.
A traditional 2x2 tic-tac-toe grid won’t be large enough. You will have to draw a 4x4.
Use the same rules as the monohybrid cross. In the offspring boxes, keep similar (Yy)
allele letters together. For example if both parents were heterozygous for both traits
(SsYy) the first offspring box (there will be a possible 16 boxes here) would be, SSYY,
keep the r’s together and the t’s together it will be easier to determine phenotypes this
way, this offspring is homozygous dominant for both traits. Try it below with the above
parental genotypes. Make your 4x4 large enough so you can fit in 4 letters into each
offspring box. Check your work. See pg. 149 POL