Mendelism Lecture 1 PDF

MENDELISM Lecture: 1 Terms to Know and Use  Genetics: the study of heredity. Heredity: the passing of characteristics (traits) from the parents to their offspring. Trait: variations of a gene: (i.e. black or brown hair) determined by alleles. Chromosome: strand of DNA that codes for...

MENDELISM Lecture: 1 Terms to Know and Use  Genetics: the study of heredity. Heredity: the passing of characteristics (traits) from the parents to their offspring. Trait: variations of a gene: (i.e. black or brown hair) determined by alleles. Chromosome: strand of DNA that codes for genes. Locus: location of a gene, or allele, on a chromosome. Gene: a hereditary determinant of a specific biological function. A unit of DNA located in a fixed position chromosome. Allele: One of a pair of alternative forms a gene that occur at a given locus in a chromosome.  Dominant trait: the trait that is observed in the offspring over a recessive trait. Recessive trait: the trait that is not expressed when the dominant trait is present. Homozygous: when both alleles for a trait of a gene are the same (i.e. pure, TT or tt). Heterozygous: when the two alleles for a trait are not the same (i.e. hybrid, Tt). Genotype: the type of alleles on a chromosome (gene makeup) or the gene combination of an organism, e.g., TT or Tt or tt.  Phenotype: the way a genotype is expressed (physical appearance) or the way an organism looks, e.g., red hair or brown hair.  Gamete: A mature male or female reproductive cell (sperm or egg).  Cross: combining gametes from parents (with one or more than one traits).  Monohybrid Cross: crossing parents who differ in only one trait.  Dihybrid Cross: crossing parents who differ in two traits.  True-breeding: organisms that always pass the same genes to their offspring. Gregor Mendel (1822-1884) Father of genetics  GREGOR JOHANN MENDEL Mendel was born in 1822 at Heinzendorf in the former Czechoslovakia. At the age of 21, he entered a Catholic monastery in the city of Brunn. Then he taught at high school in Vienna between 1851 and 1853. After returning to Brunn, he resumed his life as a teaching monk and began his genetic experiments.  Mendel performed experiments with several species of garden plants, he ultimately achieved his success with peas. After completing his work with peas, Mendel published a detailed report in the proceedings of local Natural History Society.  GREGOR JOHANN MENDEL But unfortunately this paper was not accepted until 1900. In 1900, three botanists rediscovered Mendel’s work when they searched for the scientific literature for data supporting their own theories of heredity.  After that Mendel’s ideas gained acceptance through the promotional efforts of William Bateson, a British biologist and he is the man who coined a new term to describe the study of heredity: GENETICS, from the Greek word meaning “to generate”. SITE OF GREGOR MENDEL’S EXPERIME NT AT GARDEN IN THE CZECH REPUBLIC  PARTICULATE INHERITANCE Mendel stated that physical traits are inherited as “particles” Mendel did not know that the “particles” were actually Chromosomes & DNA PUNNETT SQUARE Used to help solve genetics problems MENDEL’S EXPERIMENTS WITH GARDEN PEAS.  ONE REASON FOR MENDEL’S SUCCESS WAS THAT HE CHOSE HIS EXPERIMENTAL MATERIAL ASTUTELY. MENDEL’S EXPERIMENTAL USE OF GARDEN PEA, PISUM SATIVUM, WAS EVIDENTLY NOT AN ACCIDENT, BUT THE RESULT OF LONG CAREFUL THOUGHTS.  WHY MENDEL CHOSE GARDEN PEA? There were several reasons for which Mendel used garden pea as his experimental plant- 1. They could be grown in a small area 2. They produced lots of offspring 3. They produced pure plants when allowed to self- pollinate 4. They could be artificially cross-pollinated 5. They had quick reproductive cycle 6. They had traits that occur in only one of two possible variations 7. They were easy to grow 8. They matured quickly 9. They had both male and female parts Self-pollination Cross pollination  How Mendel Began? Mendel allowed the pea plants to self- pollinate for several generations. As a result of self-pollination of plants, in which the male and female gametes from the same flower unite with each other to produce seeds, individual pea strains displayed very little if any genetic variation from one generation to the next. These strains are said true-breeding. Eventually, Mendel obtained many different true- breeding varieties of peas, each distinguished by a particular variety, e.g., plant height, seed color etc.  Mendel tested the seven characters individually (monohybrid cross) by crossing a variety carrying a particular trait of the a character (e.g., smooth-seed-shape) with another variety carrying a different trait of the same character (e.g., wrinkle-seed-shape). See Table-1.  When he crossed a smooth-seed-shape variety with a wrinkled one, he obtained seeds that were all smooth.  Mendel then noted that the wrinkled character seemed to have disappeared in the progeny of the cross.  Mendel then allowed these smooth seed-shaped plants to undergo self-fertilization. When he examined the progeny, he found that they consisted of both smooth and wrinkled seeds, i.e., the wrinkled seeds reappeared.  The results of Mendel experiments with seven characters are given in table2. Table 1: Seven characters of pea plant tested by Mendel Character Dominant trait Recessive trait Seed shape Round (R) Wrinkled (r) Seed Color Yellow (Y) Green (y) Pod Shape Inflated (S) wrinkled (s) Pod Color Green (G) Yellow (g) Flower position Axial (A) Terminal (a) Plant Height Tall (T) Dwarf (t) Flower color Purple (P) White (p) Table 2: Mendel’s Experimental Results  MENDEL’S EXPERIMENTAL RESULT  For all seven characters tested, the results appeared to fit the following pattern: 1. For any character, the F1derived from crosses between two different varieties showed only one of the traits and never the other. 2. The trait that had disappeared in the F1 reappeared in the F2, but only in a frequency one quarter that of the total number. 3. All the F1 generation plants appear regardless of the way of cross (male and female). Thus, the two reciprocal crosses (male-tall: female-short & female- tall: male-short ) gave the same result.  Did the observed ratio match the theoretical ratio? The theoretical or expected ratio of plants producing round or wrinkled seeds is 3 round : 1 wrinkled Mendel’s observed ratio was 2.96:1 The discrepancy is due to statistical error The larger the sample the more nearly the results approximate to the theoretical ratio GENERATION “GAP”  Parental P1 Generation = the parental generation in a breeding experiment.  F1 generation = the first-generation offspring in a breeding experiment. (1st filial generation)  From breeding individuals from the P1 generation  F2 generation = the second-generation offspring in a breeding experiment. (2nd filial generation)  From breeding individuals from the F1 generation  FOLLOWING THE GENERATIONS Cross 1 Results in Cross 1 Hybrids Pure Plants all Hybrids get TT x tt Tt 3 Tall & 1 Short TT, Tt, tt