*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 Review:

*Law of Independent Assortment – During gamete formation, segregating pairs of unit factors assort independently of each other

Dihybrid cross- A genetic cross between two individuals involving two characters GGWWggww Example: P1P1 yellow, roundgreen, wrinkled X GW GW gw GgWw F1F1 All yellow, round

F1F1 GgWw F1F1 All yellow, round GgWw X F2F2 9/16 yellow, round 3/16 yellow, wrinkled 3/16 green, round 1/16 green, wrinkled GW Gw gW gw gW GW gw Gw 9:3:3:1 Phenotypic ratio; Genotypic ratio as follows: 1/16 GGWW, 2/16 GGWw, 2/16 GgWW, 4/16 GgWw 1/16 GGww, 2/16 Ggww 1/16 ggWw, 2/16 ggWw 1/16 ggww

Trihybrid cross Example: Theoretical gene pairs represented by the symbols A, B, and C P1P1 AABBCC aabbcc X Gametes: ABC abc 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

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

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

Chi-Square Analysis: p value (probability): consider as a percentage (i.e = 5%)