Transcript Slide 1
Mendelian Genetics What is Heredity? Female normal wing P- generation The passing on of traits from one generation to the next X Male double wing F1 – normal wing Chapter 9, Section 1: Pre-Mendel Early Ideas about Heredity • Blending Theory (19th century): – Each parent contributed “factors” that were blended in the offspring • What was the problem with this theory? – All individuals of a population would eventually look the same. – Once blended, traits would never separate and show up in later generations. Chapter 9, Section 1: Mendel Gregor Mendel (1822 – 1884) • Augustinian monk who studied pea plants • Established the particulate theory of heredity • Significance: – Developed pure lines – Counted results and kept statistical notes (data) • His work remained undiscovered until 1903. Mendel’s Research Character Dominant Trait Why did he research pea plants? Flower Color 1. They are normally self-pollinated, but can be cross-pollinated. Flower Position 2. They have several qualitative traits that are easy to distinguish Seed Shape i.e.,Tall vs. Short Purple Recessive Trait White Self Pollination Side Top Yellow Green Round Wrinkled Inflated Constricted Seed Color Pod Shape Pod Color Green Cross Pollination Yellow Stem Length Tall Dwarf Mendel’s Research 1. 2. 3. 4. 5. Removed stamens from purple flower. Transferred pollen from stamens of white flower to pistil of purple flower. Pollinated flower matured into a pod. Planted seeds from pod. Examined offspring: All purple flowers… Watch this video clip, and see if you can explain why… Mendel’s Conclusions 1. Law of Segregation a. Factors (genes) for a particular trait occur in pairs b. For each trait, an organism inherits two genes, one from each parent. c. Dominant alleles mask recessive ones i. Exception 1: Incomplete Dominance ii. Exception 2: Co-dominance d. Two alleles for each trait segregate (separate) during gamete production Law of Segregation: Factors for a particular trait occur in pairs Homologous pair of Chromosomes Genes: The “factors” that control traits. Alleles: Different forms of a gene. Allele for White Flowers (p) Locus for Flower Color Gene Allele for Purple Flowers (P) Back to Mendel’s Conclusions W or w w or w W = widow’s peak Ww w = no widow’s peak ww • Each individual is diploid – Diploid: Containing a double-set of chromosomes (2n) • Each gamete is haploid – Gamete: Reproductive cell (egg or sperm) – Haploid: Containing a single-set of chromosomes (n) Law of Segregation: One version of each gene is inherited from each parent Back to Mendel’s Conclusions Dominant alleles mask recessive ones P (Parent) Generation True-breeding parents (PP x pp) 705 plants had purple flowers 224 plants had white flowers F1 (1st Filial) Generation Hybrid Offspring (Pp) What happened to the recessive traits? F2 (2nd Filial) Generation What is the F2 ratio? 705:224 3:1 Dominant alleles mask recessive ones P Generation Phenotype (Appearance) Genotype (Genetic Makeup) Gametes Purple Flowers PP P White Flowers pp p F1 Generation Phenotype (Appearance) Genotype (Genetic Makeup) Gametes Purple Flowers Pp p P Punnett Square F2 Generation What is the Genotypic Ratio of the F2 Generation? PP:Pp:pp 1:2:1 P P PP p Pp p Pp pp What is the Phenotypic Ratio of the F2 Generation? 3: 1 Dominant alleles mask Homozygous: recessive same alleles 1 ones Heterozygous: different alleles Genotype Phenotype PP (homozygous) Purple Pp (heterozygous) Purple Pp (heterozygous) Purple pp (homozygous) White Ratio = 1:2:1 Ratio = 3:1 3 2 1 1 Vocabulary Practice We will now play “Got Gametes?” in order to practice understanding the following new terms: alleles, genotype, phenotype, homozygous, heterozygous Vocabulary Practice Each of you are haploid gametes carrying single alleles – for a trait. Your single letter can be combined with another single letter (i.e., Hh) to form a genotype in a diploid organism. You will observe a series of faces. Come to the front of the classroom if you think you have the right genotype to match the phenotype shown. Make sure to find the corresponding allele for the trait you represent! How do you set up a Punnett Now you try! square? Example 1: Hh X Heterozygous short hair (____) heterozygous short hair (____) Hh Hh Hh H HH h Genotypic Ratio HH:Hh:hh H Hh Hh hh 1:2:1 Phenotypic Ratio Short hair:long hair 3:1 h Dominate alleles mask recessive ones Dominant phenotype, unknown genotype: PP or Pp? Recessive phenotype, known genotype: pp What would your What would your If you have a dominant phenotype hypothesis be if the hypothesis be if the genotype was PP? genotype was Pp? (like purple flowers) how would you determine if it was homozygous (PP) or heterozygous (Pp)? What P p P p Pp Pp P p p p experiment would you design? If PP, then all offspring purple: Pp Pp If Pp, then ½ offspring purple and ½ offspring white: pp Pp pp Pp This is called a test cross Back to Mendel’s Conclusions P Generation Phenotype(s): Red and White Genotype(s): CRCR and CWCW Gamete of Red flower: CR Gamete of White flower: CW CR CW CR CW F1 Generation Phenotype: Pink Genotype: CRCW Gametes: CR and CW F2 Generation Exception to Dominant Alleles Masking Recessive Alleles Incomplete Dominance: Pink Snapdragons Use Root Letter “C” to designate incomplete dominance interaction CR CR CRCR CW CRCW CW CRCW CWCW Back to Mendel’s Conclusions Exception to Dominant Alleles Masking Recessive Alleles: Co-dominance: Blood Types (video) Back to Mendel’s Conclusions Use root letter “I” for dominant alleles of equal strength and “i” for recessive Law of Segregation: Two alleles for each trait segregate (separate) during gamete production Back to Mendel’s Conclusions This occurs in a process called meiosis: Specifically it is “crossing over,” which occurs very early during Prophase I of Meiosis and separation in Anaphase I and II Mendel’s Conclusions The law of segregation followed one single trait at a time, such as seed color. What if two traits were followed, such as seed color and seed shape? Are these genes somehow connected (linked) and inherited together? P Following two traits: Seed shape and seed color Gametes YR F1 Ova yr F2 yr YyRr YR YR Sperm yr Hypothesis: If the genes for seed shape and color are connected in some way, then the dominant R and Y alleles and the recessive r and y alleles will be matched sets in the gametes. For the traits of seed shape and color, this hypothesis is NOT supported by experimental evidence What did the data support instead? What are the genotypes of the P Generation? YYRR What does the P Generation pass on to F1? yr YR F1 Phenotype? F1 Genotype? What does the F1 Generation pass on to F2? Combine the sperm and ova to produce the offspring in F2 yr What do you expect the phenotype will be when YR and YR are combined? What will the Genotype be when YR and YR are combined? Law of Independent Assortment yyrr Gametes YyRr Ova YR Yr yR YR Yr YYRR YYRr YyRR Sperm yR YYRr yr YyRR YYrr Phenotypic Ratios YyRr YyRr Yyrr yyRr 9/16 Yellow & Round 3/16 Green & Round 3/16 Yellow & Wrinkled 1/16 Green & Wrinkled Yyrr yyRR Now try to do it on your own! What are the phenotypic ratios? YyRr YyRr yyRr yyrr Mendel’s Conclusion! Law of Independent Assortment: If the genes are not connected, then they should segregate independently. The alleles are randomly packaged into different gametes during meiosis (For example, genes for seed shape and color were not inherited together.) FOIL YyRr YR Yr yR yr Significance of Mendel’s Research Punnett Square was introduced as a tool to predict or determine the probability of an event Now, let’s practice dihybrid crosses! White board exercises One-trait cross The allele for the hairy trait is “H,” while the allele for the hairless trait is “h.” Make a cross between two homozygotes for each of these traits. Punnett Square Exercise One-trait cross In a population of Wisconsin fast plants, purple color is created by the pigment anthocyan. The gene that codes for this pigment is dominant (A), and without this pigment, the plant is green recessive (a). Purple plant: AA Purple plant: Aaoffspring from Predict the Green plant: the of a purple Greencross plant: aa aa homozygous plant and a greenA plant. a A aa A Aa a WhataAaif the plant Aa purple Aa 50% Purple 100% Purple aa Aa was heterozygous? How Aa 50%be Green aa offspring would the different? Green Purple Punnett Square Exercise One-trait cross In populations of hamsters, brown fur is dominant (B), and white fur is recessive (b). Predict the offspring from Brown hampster: the cross of aBbwhite White hampster: hamster andbba brown hamster ifb the brown B Offspring b hamster's mother was b Bb 50% brown mice bb Bb white. bb 50% white mice Punnett Square Exercise One-trait cross Henry VIII divorced six of his wives (two of whom were executed) for not bearing him any sons. Use Mendelian Genetics to prove to King Henry that it wasn’t his wives’ fault. Remember: Females are XX and Males are XY. Go to Test Crosses White-board exercises Two-trait cross Following two traits: Kernel shape: Plump vs. withered (P and p) and Kernel color Red vs. yellow (R and r) Predict the cross between a homozygous recessive corn plant and a homozygous dominant corn plant. Punnett Square Exercise Two-trait cross Now take the offspring from that cross, and self-pollinate that plant. What is the phenotypic ratio of this cross? Punnett Square Exercise Two-trait cross My pet guinea pigs (Joni and Chachi) are going to have little guinea pigs. What is the possibility that their offspring will have long hair, if I know that Joni and Chachi are heterozygous for the following traits? Black fur (B) White fur (b) Long hair (L) Short hair (l)