CHAPTER 3 Mendelian Genetics

1. CHAPTER 3Mendelian Genetics 3.1 Mendel�s Experimental Approach 3.2 Monohybrid Cross 3.3 Dihybrid Cross 3.4 Trihybrid Cross 3.5 Mendel�s Work Rediscovered in 20th Century 3.6 Mendels Influence on Modern Transmission Genetics 3.7 Independent Assortment and Variation 3.8 Laws of Probability 3.9 Chi Square Analysis 3.10 Pedigree Analysis

3. 3.1 Mendel�s Experimental Approach Johann Mendel born in 1822 to a peasant family in Heinzendorf Austria. In 1843 admitted to Augistinian Monastery of St. Thomas in Brno Austria 1851-1853 attended University of Vienna. He studied Physics and Botany. Returned to Brno to teach. He was supported by Monastery. He did this for 16 years. In 1856 Mendel conducted his first hybridization experiments with the garden pea. In 1868 he was elected Abbot of the Monastery and did not have time to continue his experimentation. In 1884 he died of a kidney disorder. READ THE TRIBUTE PAID TO HIM STARTING ON THE BOTTOM OF PAGE 46. Why was his experimentation a success ? 1. Picked a great organism to study, great characteristics, much data ! 2. He was a great statistician. 3. Limited research to a few contrasting traits. How about viewing a quick you-tube video on Mendel�s Life !

4. 3.2 Monohybrid Cross Cross that involves one pair of contrasting traits. Original parents called P(1) generation. Offspring called filial generation F(1). F1 X F1 is called F2. Mendel crossed pure tall with pure dwarf and got all tall (F1). When he crossed the F1�s he ended up with a 3 to 1 ratio of tall to short. He also performed reciprocal crosses and the results were the same. ( Not sex dependent) He studied other traits and found the 3 to 1 ratios held true.

5. Mendel� Monohybrid Cross

7. Mendel proposed that unit factors serve as the basis of heredity. He said they are passed from generation to generation. Mendel proposed three postulates 1. Genetic characters are controlled by unit factors that exist in pairs. 2. When two unlike unit factors responsible for a single characteristic are present in a single individual, one is dominant while the other is recessive. 3. During the formation of gametes, the paired unit factors separate or segregate randomly so that each gamete receives one or the other we equal likelihood. Modern Genetic Terminology Genotype-Genetic make up of an individual Phenotype- Physical expression of the trait Alleles- Alternative forms of a single gene. Genes- Mendel�s unit factors Homozygous Dominant� Both dominant alleles AA Homozygous Recessive� Both recessive alleles aa Heterozygous� One dominant and one recessive allele Aa

8. Punnett Squares Used first by Reginald C. Punnett Set up the box or square to conduct monohybrid and dihybrid crosses. Some examples : We will work some crosses on the board, copy them down.

11. Figure: 03-04 Caption: Test cross of a single character. In (a), the tall parent is homozygous. In (b), the tall parent is heterozygous. The genotype of each tall parent can be determined by examining the offspring when each is crossed to the homozygous recessive dwarf plant. Figure: 03-04 Caption: Test cross of a single character. In (a), the tall parent is homozygous. In (b), the tall parent is heterozygous. The genotype of each tall parent can be determined by examining the offspring when each is crossed to the homozygous recessive dwarf plant.

12. 3.3 Dihybrid Cross Mendel�s dihybrid cross revealed his fourth postulate Independent Assortment Dihybrid crosses involve two or more contrasting traits. If two heterozygous plants for two traits are crossed you always get a 9 Dom, Dom 3 Dom, Rec 3 Rec, Dom 1 Rec, Rec Ratio Independent Assortment During gamete formation segregating pairs of unit factors assort independently from each other. * After we look at the Independent Assortment Diagrams Let�s work on some dihybrid crosses.

14. Figure: 03-06 Caption: Computation of the combined probabilities of each F2 phenotype for two independently inherited characters. The probability of each plant bearing yellow or green seeds is independent of the probability of it bearing round or wrinkled seeds. Figure: 03-06 Caption: Computation of the combined probabilities of each F2 phenotype for two independently inherited characters. The probability of each plant bearing yellow or green seeds is independent of the probability of it bearing round or wrinkled seeds.

18. 3.4 Trihybrid Cross Mendel�s postulate on Independent Assortment can apply to 3 unit factors. We call this a trihybrid cross. These can get a bit cumbersome to work out using the punnett square. Instead we use the fork line method or branch diagram. Also, let�s take a look at pg. 55 Table 3.1

21. 3.5 Mendel�s Work Rediscovered in 20th Century Mendel�s work went unnoticed for 35 years. WHY ? Mathematical analysis not widely accepted Discontinuous variation was not widely accepted Theory of Evolution by Darwin clouded transmission genetics for many years. ( Why phenotypes continue)

22. 3.6 Mendels Influence on Modern Transmission Genetics Flemmings discovery of chromosomes brought about a resurgence in Mendel�s work as did hybridization experiments by three botanists, DeVries,Correns and Tschermak. In 1902 Sutton and Boveri (Cytologists) publish work that is verified by Mendel�s Postulate on Independent Assortment. This gave rise to Chromosomal Theory of Heredity.

23. Unit Factors, Genes and Homologus Chromosomes Mitosis/Meiosis Review Point of emphasis involving Independent Assortment. One chromosome in homologus pair comes from maternal side and the other paternal. Also, many more genes than chromosomes so chromosomes must carry more than one gene. The point on a chromosome where any specific gene is located is called a locus. Different forms of a gene are an allele. Criteria necessary to classify two chromosomes as homologs. 1. During mitosis and meiosis when chromosomes are visible as distinct structures, both members of the pair are the same size, same centromere location. 2. During early stages of meiosis homologs pair together or synaps. 3. Homologs contain the same linearly ordered gene loci.

51. Let�s take a look at the sample problems in the text.