1. *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:

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

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

4. 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

5. 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

6. The Forked-Line Method (branch diagram):

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

8. 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

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

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

11. Chapter 4: Modification of Mendelian Ratios Allele *Wild-type allele *Mutant allele Conventional symbols for alleles: recessive allele- initial letter of the name of the recessive trait, lowercased and italicized dominant allele- same letter in uppercase Genetic nomenclature is extremely diverse!

12. Incomplete or Partial Dominance Cross between parents with contrasting traits: Red flowers or white flowers Offspring with an intermediate phenotype: pink flowers

13. Codominance: Example: MN Blood group- red blood cells contain a transmembrane glycoprotein (glycophorin); two different forms of this protein exist, M and N

14. Examples: *Table 4.1: over 100 alleles at a given locus in Drosophila *ABO Blood group in humans Multiple Alleles: *Characterized by the presence of glycoprotein antigens on the surface of red blood cells *Distinct from the M and N antigens *Also exhibits codomiance