*Unit factors in pairs- genetic characteristics are controlled by unit factors that exist in pairs in individual organisms *Principle of Dominance and Recessiveness- one factor is dominant over the recessive one *Law of Segregation- During gamete formation the paired unit factors segregate randomly so that each gamete receives one or the other *Law of Independent Assortment – During gamete formation, segregating pairs of unit factors assort independently of each other Review:

Dihybrid cross F1F1 All yellow, round GGWWggww Example: P1P1 yellow, roundgreen, wrinkled X GgWw

Dihybrid cross con’t: F1F1 All yellow, round GgWw F1F1 All yellow, round GgWw X F2F2

The Trihybrid Cross: Trihybrid cross Example: Theoretical gene pairs represented by the symbols A, B, and C P1P1 AABBCCaabbcc X Gametes: ABCabc F1F1 AaBbCc Gametes: ABCABc aBC AbC aBc Abc abCabc

The Forked-Line Method (branch diagram): Recall: *The F 1 that result from a monohybrid cross (AA x aa) all have the genotype Aa and the phenotype represented by A *The F 2 that result from a cross between 2 individuals from the F 1, have a phenotypic ratio of 3:1

The Forked-Line Method (branch diagram):

*NOTE: We are assuming that independent assortment of these 3 gene pairs is a random process! The Forked-Line Method (branch diagram):

Mendel Rediscovered: Why did Mendel’s work go unnoticed for so long? *1879 Walter Flemming Charles Darwin and Alfred Russel Wallace: continuous variation – offspring were a blend of parents’ phenotypes *early 20 th century *Hugo de Vries *Karl Correns *Erich Tcshermak *1902 Walter Sutton and Theodor Boveri Chromosomal Theory of Inheritance

How do we account for genetic variation? *Independent assortment *Crossing over *Random fertilization Independent Assortment: Cross over:

Human Pedigrees Pedigree = Female = Male =Unknown *Proband (p)