Genetics - 03 Mendelian Genetics

HUMAN GENETICS AND CYTOGENETICS • MENDELIAN OR TRANSMISSION GENETICS LESSON 1: OVERVIEW OF MENDELIAN GENETICS JOHANN GREGOR MENDEL (1822–1884) • • • 1851 to 1853 – attended the University of Vienna, studied physics and botany 1853 – returned to Brno, next 15 years thought physics and natural science. 1856 – performed first set of hybridization experiments with garden pea. 1884 – died of chronic kidney disease and heart failure. MENDEL’S EXPERIMENTAL APPROACH • • • • • • • • Often called the “Father of Genetics,”Gregor Mendel a teacher, lifelong learner, scientist, and a man of faith. When he was young, he had difficulty in pursuing his education because he just came from a family with a very limited resource. He suffered depression and physical illness but he was determined to pursue his studies until he graduated from high school and entered university, from when he joined the Augustinian Abbey of St. Thomas in Brno (the Czech Republic now), The Monastery happened to be the center for the cultural and intellectual hub of the region where he didn’t find any difficulty exposing to new teachings and ideas. Because he wanted to continue his passions on Scientific interest, he was being supported by the monastery where he taught Physics, Botany, and Natural Science Courses. • 1865 – Mendel first reported the results of simple genetic crosses between strains of garden peas. o Chose an organism that was easy to grow and hybridize artificially → easy to cross-breed during experiments. o Worked with 4 characters with 7 contrasting traits. Garden Pea: The 4 Characters with 7 Contrasting Traits LESSON 2: MONOHYBRID CROSS MONOHYBRID CROSS • • • Genetics is a science that deals with the study of Heredity and variation among organisms. Performed by Mendel. In a monohybrid cross, organisms with only one trait are crossed. (For example, seed color) MENDELIAN GENETICS OR TRANSMISSION GENETICS (PPT) MAJOR CONCEPTS • • • • Inherited characteristics are under the control of particulate factors → genes Genes are transmitted from one generation to another by vehicles called chromosomes. The crosses (mating) are affected by chance deviation and evaluated using statistical analysis i.e., chi-square method. Human traits are initially studied using pedigrees. GREGOR JOHANN MENDEL • • Born in 1822 to a peasant family in Heizendorf (now part of the Czech Republic) Admitted to the Augustinian Monastery of St. Thomas in Brno in 1843. TERMS TO REMEMBER • • • • • • • • • • Phenotype – physical appearance of a trait in an individual. Genotype – genetic makeup of an individual whether it is diploid or haploid. Allele – alternative forms of a single gene. Homozygous (Homozygote) – when both alleles are the same i.e., DD or dd Heterozygous (Heterozygote) – when the alleles are different i.e., Dd Character – heritable feature, ex. stem height Trait – variant of a character, ex. tall or dwarf P – parental generation F1 – first filial generation F2 – second filial generation MONOHYBRID CROSS • • Only involves one pair of contrasting traits. Mating individuals from two (2) parent strains which exhibits one (1) of two (2) contrasting forms of the character under study. DIHYBRID CROSS AND MENDEL’S FOURTH POSTULATE • • • Uses two characters that are under study. → two (2) pairs of contrasting traits. Also called two-factor cross. DIHYBRID CROSS • • How to Use the Punnet Square o Developed by Reginald C. Punnet in 1967 o Each possible gametes is assigned to a column or row o Generally: o X-axis/Horizontal column – male parent o Y-axis/Vertical column – female parent Use of the punnet square in generating the F2 ratio of the F1 using another trait of garden pea (Color of the pod) o P= Gg x Gg o Results: o PR= 3 green:1 yellow o GR= 1GG:2Gg:1gg MENDEL’S FIRST THREE POSTULATES • • • Unit Factors in Pairs o Genetic characters are controlled by unit factors that exist in pairs in individual organisms. Concept of Dominance and Recessiveness o When two unlike unit factors responsible for a single character are present in a single individual, one unit factor is dominant to the other, the recessive one. Segregation o During the formation of gametes, the paired unit factor separate or segregate randomly so that each gamete receives one or the other. TEST CROSS • • An organism expressing a dominant phenotype but unknown genotype. Crossed with a homozygous recessive individual MENDEL’S FOURTH POSTULATE: INDEPENDENT ASSORTMENT • • • During gamete formation, segregating unit factors assort independently of each other. Any pair of unit factor segregate independently of all other unit factor. Therefore, each gamete receives one member of every pair of unit factor. TRIHYBRID CROSS • • • • Apply to three pairs of contrasting traits Also referred to as three-factor cross. Somewhat more complex than a dihybrid cross Easily calculated if the principles of segregation and independent assortment are followed. FORMATION OF P1 AND F1 GAMETES IN A TRIHYBRID CROSS. • Inheritance of albinism LESSON 3: DIHYBRID CROSS AND LAW OF INDEPENDENT ASSORTMENT LECTURE DISCUSSION EVALUATING GENETIC DATA: CHI-SQUARE ANALYSIS • • • • • • The ratios generated from Mendel's work are merely hypothetical predictions based on the following assumptions: Each allele is dominant or recessive. Segregation is operative. Independent assortment occurs. Fertilization is random. The last 3 are influenced by chance events and are therefore subject to random fluctuation. HUMAN PEDIGREES • • • The pattern of inheritance of a specific type of phenotype can be studied even in humans. Able to predict how the gene controlling the trait is inherited. o Circles – females; squares – males; if sex is unknown a diamond is used. o If phenotype is expressed the shapes representing the sex are shaded; for those who fail to express a recessive trait when known to be heterozygote, only the left half of the shape is shaded. Pedigree Showing Autosomal Dominant Trait • After the discussion on the monohybrid cross, the next line of discussion I want you to focus on is all about the Dihybrid cross. Explain the Law of Independent Assortment and demonstrate its application in solving a dihybrid Cross. LESSON 4: TRIHYBRID AND POLYHYBRID CROSS • It was a quite long journey had been discussing the Monohybrid, Dihybrid, and Law of Independent assortment, but still, I know you can manage to read and study about the last topic for this module, these will focus on Trihybrid and Polyhybrid crosses. Let us define the first trihybrid cross, an individual or strain that is heterozygous for three pairs of genes. While poly hybrid is a hybrid whose parents differ in a number of characters: an individual or group heterozygous for more than one pair of genes.