Post on 17-Dec-2015
Using a Punnett Using a Punnett SquareSquare
What is a PUNNETT SQUARE?
• A tool to predict the probability of certain traits in offspring that shows the different ways alleles can combine
• A way to show phenotype & genotype
• A chart that shows all the possible combinations of alleles that can result when genes are crossed
What is a PUNNETT SQUARE?
• Letters stand for dominant and recessive alleles
• An uppercase letter stands for a dominant allele
• Lowercase letters stand for recessive alleles
Example:
Tall is the dominant trait in pea plants
Therefore T = tall and t = short
We also know that an organism gets 1/2 its genes from its mother and1/2 from its father.
So, lets look at some possibilities:
Remember: T = tall t = short
What if Mom gives up a tall gene and Dad gives up a short gene?
What letter represents the tall gene?
What letter represents the short gene?
So the offspring?
What if Mom gives up a tall gene and Dad
gives up a tall gene?
What letter represents the tall gene?
So the offspring?
What if Mom gives up a short gene and Dad
gives up a short gene?
What letter represents the short gene?
So the offspring?
Now for the possibilities
Tt is called hybrid or heterozygous.In a hybrid the dominant trait always shows.The offspring would be tall (phenotype).Phenotype = physical makeup - what it looks like
TT is called pure or homozygous.The offspring would also be tall.
tt is called pure or homozygousThe offspring would be short.This is the ONLY way a recessive trait shows.
Using a PUNNETT SQUARE
To set up a Punnett square, draw a large square, and then divide it into 4 equal sections (also squares). It should look something like this:
Using a PUNNETT SQUARE
Now you need two parents, ones with a known genotype (genetic makeup)
For example, a red flower (genotype Rr) and a white flower (genotype rr). Rr x rr
Rr pure or hybrid?
homozygous or heterozygous
rr pure or hybrid
homozygous or heterozygous
Using a PUNNETT SQUARE
Place one of the parents on top, and one on the left. You should get a something similar to this:
Using a PUNNETT SQUARE
Finally, take each letter in each column and combine it with each letter from each row in the corresponding square. You should now have a picture close to this:
Using a PUNNETT SQUARE
• The two-letter combinations are the possible genotypes of offspring
• They are: Rr, Rr, rr, and rr genotypes• From this it is possible to determine
the probability (chance) that a flower will have a red phenotype (2/4 or 50%) or a white phenotype (2/4 or 50%)