punnett squares

Biology Tutorial

punnett squares


The two things a Punnett square can tell you are the genotypes and phenotypes of the offspring. A genotype is the genetic makeup of the organism. This is shown by the three genetic conditions described as(BB, Bb, bb) and phenotype is the trait those genes express.
How do you fill out punnett squares?

Sample Problem

Draw a p-square.
“Split” the letters of the genotype for each parent & put them “outside” the p-square.
Write the genotypes of the parents in the form of letters (ex: Tt x tt).
Determine the possible genotypes of the offspring by filling in the p-square.


“Cross a short pea plant with one that is heterozygous for tallness”. Heterozygous always means one of each letter, so we’d use “Tt” (where “T” = tall, & “t” = short). The only way for a pea plant to be short is when it has 2 lowercase “t’s”, so that short parent is “tt”. So the cross ends-up the same as in my first example: Tt x tt.

Brown is dominant (B), and white is recessive (b).”Predict the offspring from the cross of a white hamster and a brown hamster if the brown hamster’s mother was white”. First things first: the only way for the white hamster to be white (the recessive trait) is if it’s genotype is homozygous recessive (2 little letters), so the white hamster is “bb”. Now, the brown hamster’s genotype could be either “BB” or “Bb”. If its mommy was white (bb), then this brown hamster MUST have inherited a little “b” from its mommy. So the brown one in our cross is “Bb” (not “BB”), and our hamster cross is: Bb x bb.

1.draw your p-square
2. “Split” the letters of the genotype for each parent & put them “outside” the p-square. (For an example cross we’ll use these parental genotypes: Tt x tt.)
Take the genotype letters of one parent, split them and put them on the left, outside the rows of the p-square; the hetrozygous tall plant (Tt) and put its big “T” out in front of the top row, and the little “t” out in front of the bottom row. Now, when it comes time to filling things in, those lowercase “t’s” will each be copied into the two boxes directly below them. So after the next step, each little box will have two letters in it (one “tee” from the left & one “tee” from the top). These new 2 letter combos represent possible genotypes of the offspring.
3.In this example, where our parent pea plants were Tt (tall) x tt (short), we get 2 of our 4 boxes with “Tt”, and 2 of our 4 with “tt”. The offspring that are “Tt” would end up with tall stems (the dominant trait) and the “tt” pea plants would have short stems (the recessive trait).

Parent Pea Plants
(“P” Generation)
Tt x tt
tall x short

(“F1” Generation)
50% (2/4) Tt
50% (2/4) tt
50% tall
50% short

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