II. Probability and Punnett Squares -Mendel used the principles of probability to explain the results of his genetic crosses. A. Genetics and Probability Probability – the likelihood that a particular event will occur, ex : coins – have a 50/50 shot of heads or tails. Problem : Determine the probability of flipping heads on a coin 3 times in a row. ½ x ½ x ½ = 1/8 *Past outcomes DON’T affect future outcomes. -Segregation of alleles is completely random like coin tossing. -The principles of probability can be used to predict the outcomes of genetic crosses.

B. Punnett Squares *Gene combinations that might result from a genetic cross can be determined by using a Punnett square. Punnett square – a diagram showing the gene combinations that might result from a genetic cross.

In a Punnett square the : 1. Types of gametes produced by P or F1 generations are shown along the top & left sides of the square. 2. Possible gene combinations for the F1 or F2 offspring appear in the 4 boxes that make up the square.

Homozygous vs. Heterozygous -Organisms with 2 identical alleles for a trait (TT or tt) are called homozygous, homo = same. -Organisms with 2 different alleles for the same trait (Tt) are called heterozygous, hetero = different. -Homozygous organisms are true-breeding or pure for a trait & heterozygous organisms are hybrid for a trait.

Genotype vs. Phenotype *Genetic make-up of an organism & its physical characteristics can be referred to as its genotype & phenotype. Genotype – the actual genetic make-up of an organism or the genes it has. Phenotype – physical characteristics that can be seen & described when looking at an organism.

Genotype vs. Phenotype What are the genotypic & phenotypic ratios of the F1 generation of plants in this picture (assuming A = yellow seeds/a = green seeds)? Genotypic = 1 AA : 2 Aa : 1 aa Phenotypic = 3 yellow seeds : 1 green seed

C. Probabilities Predict Averages *Probabilities predict the average outcome of a large # of events. -Probability CAN’T predict the precise outcome of an individual event, ex : flipping a coin 2x & getting heads twice as opposed to heads once & tails once. The same is true of genetics & the probability of expected ratios in a population. -In order to get close to expected values, one must flip a coin many times/in genetics produce many offspring.