11-2 Probability and Punnett Squares
Mendel realized that the principles of probability could be used to explain the results of genetic crosses. He repeated a particular cross and obtained the same results.
Probability The likelihood that a particular event will occur
A coin flip Possible outcomes: Heads Tails Either outcome are equal.
One coin flip, makes 1 chance in 2 This is ½ or 50%
The probability of flipping three heads in a row: ½ X ½ X ½ = 1/8 1 chance in 8 flips
The principle of probability can be used to predict the outcome of genetic crosses.
Punnett Square The gene combinations that might result from a genetic cross.
F1 Parents are shown along the top and left side of the square. It also contains the possible gene combination for F2 offspring and the allele
The punnett square can be used to predict and compare the genetic variations that will result from a cross.
Homozygous Organisms that have two identical alleles for a particular trait –TT or tt. Org. for true breeding in particular trait
Heterozygous Organisms that have two different alleles for the same trait. Tt Organisms are hybrid for a particular trait.
Phenotype Plants have the same physical characteristics.
Genotype Genetic makeup, Genotype of one third of the tall plants is TT Two thirds of the tall plant is Tt.
Plants have the same phenotype but different genotype. What would happen if you cross this two plants? Short or Tall??
T t TT 25% Tt
Probability and Segregation ¼ of the F2 plants have two alleles for tallness 2/4 or ½ have one allele for tallness ¼ one allele for shortness ¾ are going to be tall and ¼ short Ratio 3:1
Predicted Ratio For each seven crosses ¾ of the plants showed the trait controlled by the Dominant. ¼ showed the trait controlled by the recessive allele.
The larger the number of offspring, the closer the resulting number will get to expected values.
Homework To turn in: Make a punnett square with your family. Use only one trait, like eye color, tallness, skin color, etc. Add pictures to illustrate.
T t Tt 25% tt