SLE111 C13 — Mendel and the Gene Idea Learning Objectives - De ne the terms: homozygous, heterozygous, allele, phenotype, genotype, monohybrid cross, dihybrid cros - Describe the relationship between dominant and recessive genes and how it affect phenotyp - Describe Mendel's law of segregation and independent assortmen - Relate the law of segregation and independent assortment to the behaviour of chromosomes during meiosi - Use Punnett squares to predict the ratio of offspring of particular phenotypes for monohybrid, dihybrid and test crosse - Use the multiplication and addition rules of probability to solve genetics related proble What principles account for the passing of traits from parents to offspring? - The “blending” hypothesi • Heredity materials from the two parents blends togethe • (like blue and yellow paint blends to make green • Over many generation would results in uniform populatio - The “Particulate” hypothesi • Discreet heritable units (genes Mendel and the Gene Idea - Mendelian Genetic - Name derives from Austrian monk. Gregor Mendel - Mendel documented a particulate mechanism - Controlled mating of pea plant - Observed distinct patterns of inheritanc - Developed rules that applied to manner in which genetic material is passed from generation to generation Terminology - Heritable features that vary among individuals is called a character (or characteristic - Each variant is called a trai - For examples Flower (Character), Colour — purple, white (Trait Mendel used a scientific approach to identify to laws of inheritance - Mendel only studied characters that varied in an either-or manne s s e m ) t r ) n r ) e ) s s t s s s fi 1 SLE111 - Peas have a short generation time and produce many offsprin - Mendel used started with plants that had self-pollinated over many generations and only gave the same trait – truebreedin - And... Mendel could strictly control mating between plant - Using this method Mendel crossed a true-breeding purple- owered plant with a true-breeding white- owered plan - Note: stamen = sperm producing part of plant.Carpel = egg producing part of plan The offspring of the first cross were all purple Mendel allowed the offspring to self-pollinate and the white trait reappeared in the offspring g fl t fl g 2 SLE111 Mendel repeated this experiment studying other characters in peas and always observed a 3:1 ratio in the F^2 - Mendel used the results of these experiments to make a model of inheritance. He referred to “heritable factor” which we now call gene Mendel’s model of inheritance Concept 1: alternate versions of genes account for variations in inherited character - We refer to these alternate version as alleleThe gene for ower colour in peas has two alleles; purple & white fl 3 SLE111 Concept 2: for each character an organism inherits two copies of the gene, one from each parent - Mendel made this deduction without knowing about the role of chromosome Concept 3:If an organism has two alleles for a gene then one determines the organisms appearance & the other has no noticeable effect - The allele that determines appearance is referred to as dominant and the masked allele is referred to as recessive Concept 4:The Law of Segregation 4 SLE111 Useful genetic vocabulary - Homozygous: An organism with two identical alleles for a character - Heterozygous: An organism that has two different alleles for a gene • Because of the different effects of dominant and recessive alleles, an organism’s traits do not always reveal its genetic compositio - Therefore, we distinguish between an organism’ • Phenotype; physical appearanc • Genotype: genetic makeu - It is possible to have the same phenotype but a different genotyp Does Mendel’s segregation model account for the 3:1 ratio he observe? - Possible combinations of sperm and egg can be shown using a Punnett square - A diagram for predicting the results of a genetic cross between individuals of known genetic makeu In a Punnett square the genes are represented by letter p e s e p n 5 SLE111 How to tell the genotype of an individual with the dominant phenotype? What about if we look at two different characteristics at the same time? - Mendel derived the law of segregation by following a single character • E.g. Flower colou • The F1 offspring produced in the cross of true-breeding parents were monohybrids, heterozygous for one character (Pp • A cross between such heterozygotes is called a monohybrid cros - But what about if we look at two characters at the same time? Eg pea colour(green or yellow) & pea shape (round or wrinkled)? - Will the two characters be inherited together or separatel The dihybrid cross: Mendel followed two characters at the same time s y r ) ? 6 SLE111 Do alleles for one character segregate into gametes dependently or independently of the alleles for a different character Mendel’s second law: the law of independent assortment 7 SLE111 Laws of probability govern Mendelian inheritance - Mendel’s laws re ect the rules of probabilit - When tossing a coin, the outcome of one toss has no impact on the outcome of the next tos - In the same way, the alleles of one gene segregate into gametes independently of another gene’s allele Multiplication rule –Monohybrid crosses Addition rule –Monohybrid crosse - The rule of addition states that the probability that any one of two or more exclusive events will occur is calculated by adding together their individual probabilities s s y fl 8 SLE111 Solving complex genetic problems - We can apply the laws of probability to predict the outcome of crosses involving multiple character • A dihybrid cross is equivalent to two or more independent monohybrid crosses occurring simultaneousl - In calculating the chances for various genotypes, each character (gene) is considered separately, and then the individual probabilities are multiplied togethe Try this.. - Consider the cros - PpYyRr X Ppyyr - What fraction of offspring from this cross would be predicted to exhibit the recessive phenotype for at least two of the three characters List all the genotypes that fit the condition - ppyyR - ppYyr - Ppyyr - Ppyyr - ppyyr - The condition: exhibit the recessive phenotype for at least two of the three characters - Note: these are not all the possible genotypes these are just the ones that t the above condition y s ? fi r ? s r r r r r r 9
