7.2.9 Students know plant and animal cells contain many thousands of different genes and typically have two copies of every gene. The two copies (or alleles) of the gene may or may not be identical, and one may be dominant in determining the phenotype while the other is recessive. I. Principles of Probability Probability- a number that describes the likelihood of a certain event happening Laws of probability predict what is likely to happen but not always what does happen The more times you do the activity the closer the actual results will get to the predicted results Independence of Events- the results of one trial does not effect the next trial II. Probability and Genetics How is probability related to genetics? Mendel’s pea plants had ¾ tall and ¼ short- each time he repeated the cross he got similar results Mendel was the first scientist to recognize that probability can be used to predict results of genetics crosses Punnett squares- a chart that shows all the possible combinations of alleles that can come from a genetic cross Punnett Squares The diagrams show how to make a Punnett square. In this cross, both parents are heterozygous for the trait of seed shape. R represents the dominant round allele, and r represents the recessive wrinkled allele. Punnett square is used to calculate probability Key concept- In a genetic cross, the allele that each parent will pass on to its offspring is based on probability. Remember that a dominate allele masks the recessive allele Look at Figure 8 III. Phenotypes and Genotypes Phenotypes- physical appearance or what it looks like Genotypes- genetic makeup or allele combinations Look at figure 9 Homozygous- two identical alleles for the same trait (TT) Heterozygous- two different alleles for the same trait (Tt) IV. Codominance Codominance- alleles are neither dominant nor recessive so both allelles are present Neither allele is masked Codominant alleles are written as capital letters with subscript What is probability and how does it help explain the results of genetic crosses? What is meant by genotype and phenotype? What is codominance?