Download 3. Punnett Square Notes - Hamilton Local Schools

Punnett Squares Notes
If you have ever had a cat or dog that has had babies or known someone who has you know that by
looking at the mother and father you can determine to a reasonable degree what the offspring may look
like. Today, we are going to take this one step further and actually determine all the possible genotypes
and phenotypes for a mating cross.
How do we determine the genotypes and phenotypes of a mating cross?
Using a Punnett Square!
•
A punnett square will tell us the possible ____________________ and the ___________ of those
______________________.
•
From the genotypes we can than determine the possible ____________________ and _________
for those ____________________.
________________________________________________________________________
First, we must determine the ____________________ of the parents.
Example: A cat who is heterozygous for hair length is crossed with a long haired individual.
Step 1) What is the dominant allele and what is the recessive allele.
Ans:
Dominant = ____________________ = ______
Recessive = ____________________ = ______
Step 2) The genotype of each individual parent can be determine by the clues given in the
problem.
Parent A is ____________________; therefore, they must is have _________ _________
allele and ________ _________ allele.
Parent A = ________
Parent B is long haired. We know that long hair is ________________, and recessive
traits are only shown when someone possess ________ copies of that allele.
Parent B = _________
Now, we can determine that our cross is as follows:
_________ X _________
Now that we know the genotypes of our two parents, we can set up our punnett square. In this case it is
called a ________________________ cross.
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The possible gametes of one parent go on the top of the table while the possible gametes of the
other parent go on the left of the table.
*Remember, each letter represents a possible gamete that the individual can produce. In this
instance, we are only looking at ________ gene/trait so it is only one letter long.
•
Next, we fill in the rest of the table by figuring out what the offspring’s ____________________
would be if the two gametes came together and produced offspring.
Parent A’s Possible Gametes
Parent B’s Possible
Gametes
•
The possible genotypes of the offspring produced from this cross (mating) are:
_______ and _______.
•
The genotypic ratio is 2 ______:2 ______ or ______ chance for _______________________
individuals and _______ chance for ____________________ recessive individuals.
•
The possible phenotypes of the offspring produced from this cross (mating) are:
____________ _______ and ____________ _________.
•
The phenotypic ratio is 2 __________:2 __________ or ______ chance of short haired cats and
_______ chance of long haired cats.
Try one on your own.
Troll hair color is a genetic trait. It can either be blue or green. What are the possible genotypes and
phenotypes that can be produced by a crossing between a heterozygous blue haired troll and a
homozygous dominant blue haired troll? What are the genotypic and phenotypic ratios.
Step 1) Figure out the dominant and recessive alleles.
Dominant = T = _____________ hair
Recessive = t = _____________ hair
Step 2) Figure out the genotypes of each parent.
Parent A = _______
Parent B = _______
So, the crossing being performed is:
_________ X_________
Step 3) Complete your punnett square.
Parent A’s Possible Gametes
Parent B’s Possible
Gametes
Step 4) Determine the genotypes possible and the ratio of those genotypes.
______ = _________________________; _____ = _____________________
Ratio: ___ ______ :___ _______
Step 5) Determine the phenotypes possible and the ratio of those phenotypes.
Possible Phenotypes: _____________________
Ratio = _____ _________: ____ __________