Download Punnett Square Word Notes

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11.2—Punnett Squares and Probability
I.
Genetics and Probability
A. Probability—the likelihood a particular event will occur
B. “Principles of Probability” in genetics
1. Are used to PREDICT outcomes of genetic crosses
2. Account for RANDOM segregation of alleles
II.
Probability and Segregation
A. When two heterozygous (Tt X Tt) generations are crossed:
1. Alleles segregate at random
2. The PREDICTED ratio is always:
a. 3 dominant : 1 recessive or a 3:1 ratio
b. This ratio is called the “Mendelian Ratio”
III.
Probability and Predicted Averages
A. Probability predicts average outcomes in large events
B. HOWEVER, probability cannot predict outcomes in individual
events
C. More offspring present in the F1 generation (100’s—1000’s of
offspring)
1. The closer the actual ratios come to the predicted values
IV.
Punnett Squares and Probability
A. Reginald Punnett (British biologist—1905)
1. Developed a visual way of showing genetic crosses
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B. Punnett Squares
1. Diagrams that show possible gene combinations
2. Can be used to:
a. Predict genetic variations
b. Compare genetic variations
3. IMPORTANT GENETIC TERMS ****
a. Genotypegenetic makeup represented by alleles.
i. Homozygousidentical alleles for a
particular trait (remember that “homo”
means “same”)
 Homozygous dominantAA
 Homozygous recessive aa
ii. Heterozygousdifferent alleles for the same
trait (remember that “hetero” means
“different”)
 Heterozygous dominant Aa
b. Phenotypephysical characteristics that result
from the genotypeTHIS IS WHAT YOU SEE!
i. For example: height, length, color, size,
shape etc.
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4. Completing a Punnett Square
Given the following cross and information:
Tt x Tt
T = tall (dominant allele)
t = short (recessive allele)
 Set up the cross in the following way:
T
t
T
t
 Once you have properly set up your Punnett
Square, you are ready to begin your cross.
** Steps **
1. In the first square, bring together both alleles.
T
T

T
T
TT
2. Once you have determined which alleles belong in the
box, the DOMINANT ALLELE is written first.
3. The RECESSIVE ALLELE follows the dominant
allele.
T
t
4. Repeat for each box.
T

t
Tt
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 You final Punnett Square should look like this:
T
t
T TT
Tt
Tt
tt
t
C. Punnett Squares can tell you:
1. Probable genotypes
a. These are the “letters” found in each box
b. For the example cross the genotypes are:
i. TT (homozygous dominant)
ii. Tt (heterozygous dominant)
iii. tt (homozygous recessive)
2. Probable phenotypes
a. These are determined by looking at genotype.
b. For the example cross the phenotypes are:
i. TT  tall
ii. Tt  tall
iii. tt  short
3. Probable averages
a. What percentage of the offspring will be tall?
i. 3 of 4 will be tall or ¾ = 75%
b. What percentage of the offspring will be short?
i. 1 of 4 will be short or ¼ = 25%
c. You can answer those questions by looking at the
phenotypes/genotypes and using your Punnett
Square.