Transcript Punnett Square

Review of Terminology
Genotype
• Hereditary Information
• internally coded, inheritable information
• Genotype can be determined by direct
observation of DNA
Phenotype
• A phenotype describes any observed quality of an
organism, such as its shape, development, or
behavior
• outward, physical appearance
• Traits are small parts of the phenotype of an
organism, such as the red color seen in the flower
petals
genotype + environment → phenotype
Heterozygous
• Having two different alleles for a single
trait.
• Rr
Homozygous
• Having identical alleles for a single
trait.
• RR or rr
Constructing a Punnett
Square
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
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 to mate, ones
with a known genotype
For example, a red flower (genotype Rr)
and a white flower (genotype rr).
Rr x rr
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
Probability
• Total possible combinations (number of
squares in the table) becomes the
denominator
Probability
• Count the number squares for each combination
• Rr = 2 (numerator)
• rr = 2 (numerator)
Probability
• Put the pieces together
• Probability (chance) that a flower will
have:
– red phenotype, Rr = 2/4 or 50%
– white phenotype, rr = 2/4 or 50%
Probability
• Flip a coin ONCE
• Number of total possible outcomes (this
becomes the denominator)
• Count the number of times you get a head or
tail (numerator)
• Probability = numerator/denominator
Probability
• Flip a coin ONCE
• Number of total possible outcomes (this becomes the
denominator) = 2 (head or tail)
• Count the number of times you get a head or tail
(numerator)
– Head = 1
– Tail = 0
• Probability of getting Head = numerator/denominator =
½ = 0.5
Probability
•
Flip a coin TWICE
•
Number of total possible outcomes (this becomes the denominator) = 4
(HH, HT, TH, TT)
H
T
•
Count the number of times you get each
combination (numerator) :
– HH = 1
– TT = 1
– HT or TH = 2
•
•
•
H
HH
T
TH
HT
Probability of both HEADS = numerator/denominator = 1/4 = 0.25
Probability of both TAILS = numerator/denominator = 1/4 = 0.25
Probability of HEAD AND TAIL = numerator/denominator = 2/4 = 0.5
TT