Genetics - BIOL 139 BIOL Topic 1 – Mendelian Genetics...

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Unformatted text preview: BIOL 139 BIOL Topic 1 – Mendelian Genetics Transmission Genetics Pages 13-31 iGenetics 13-28 Hartwell 13-28 Genotype vs Phenotype Genotype Genotype: genetic constitution of an organism Phenotype: observable characteristic(s) of an organism product of genotype and interaction with environment Single vs multiple genes commonly referred to as a trait Observable • hair colour • eye colour • balding patterns • height Genetic traits are passed on from one generation to the next generation Artificial selection: • purposeful control over mating purposeful • domesticated plants and animals domesticated • agriculture and pets agriculture Dogs: domesticated wolves . Most compatible wolves were bred –selected for st 1 non-wolf canine skull found dated at 20,000 years old - Arctic Next…..pigs, goats, sheep, cattle ~ 10,000 years ago “like begets like” “like begets unlike” Incredible amount of genetic variation from Canus lupus Transmission Genetics: Science of Heredity: • explanation of mechanisms that determine how how genetic traits are passed on from generation to traits generation Artificial selection –purposeful mating to give domesticated plants and animals But a Puzzle: Why do valued traits sometimes disappear then reappear in only some offspring ? disappear i.e. Morino sheep and wool i.e. What is inherited ? How is it inherited ? What is the role of chance in heredity ? What is inherited? Traits: • units of inheritance –hereditary traits (genes) • give rise to observable traits (phenotype) 4 General Themes from Mendel’s Work: Genetic variation is widespread in nature variation • many alternate forms can exist for traits (alleles) • 100’s of breeds of dogs all originating from wolf To follow inheritance of traits, need observable difference –change in phenotype phenotype Variation not due to chance, governed by genetic laws chance laws Can apply laws of heredity to all sexually reproducing sexually species Prior to Mendel: Prior • theory was one parent contributes most to an offspring’s inherited features (male) Blended Inheritance Blended • theory that parental traits become mixed and forever changed in the offspring + Gregor Mendel (1822-1884) Gregor Augustinian monk and expert plant breeder Experiments with garden peas garden • excellent records of results excellent • rigorous analysis of hereditary rigorous transmission of certain traits transmission ** ** Criteria for Models • Know genetic history Know ** • short life cycle short ** ** • Large # of generations Large in short time period in ** • large # offspring large • easy to handle easy • great genetic variation great exists in population exists • observed in phenotype observed Reasons Why Mendel Succeeded (Terminology) Advantages to Mendel’s Approach Do Peas Fit Criteria? 1. Garden peas: • easy to grow (small amount of space) • large # of offspring • short growing season (from seed to plant to seed) • generally self-fertilize self-fertilize Mendel’s experiments with garden peas Mendel’s Self-fertilizing-selfing Both egg and pollen come from same plant pollen E egg Male/female same plant Mendel’s experiments with garden peas Mendel’s Though Self-fertilize are easy to Cross-fertilize Cross-fertilize Egg from one pea plant with observable trait (purple flowers) can be cross-fertilized with sperm from another pea plant with alternate trait (white flowers) E 2) Anatomy makes easy to cross-fertilize *** 2) cross-fertilize ↑ genetic diversity Advantages to Mendel’s approach Advantages 3. Peas had clear-cut observable alternative forms alternative of particular traits: = expressed in phenotype • purple vs. white flowers • yellow vs. green peas • round vs. wrinkled peas • “either-or” traits (discrete traits) • intermediate forms (continuous traits) • antagonistic pairs -used 7 in total antagonistic Antagonistic Pairs Advantages to Mendel’s approach Advantages 4. Establishment of pure-breeding lines: offspring pure-breeding carry parental trait(s) that remain constant from generation to generation (no surprises!) generation • when pure-breeding plants of same trait are when allowed to self or self • mate (cross) pure-breeding lines of same trait mate pure-breeding same Mendel established true-breeding plants (and thus seeds) for each of the 7 antagonistic traits he was studying… He followed these plants for over 8 generations to make sure generations they bred true Monohybrid cross Monohybrid 4. Due to the peas easy ability to cross-fertilize Mendel was able to 4. cross-fertilize cross true-breeding parents with antagonistic traits to produce a hybrid antagonistic hybrid offspring. •This hybrid was called a Monohybrid since Mendel chose to follow This Monohybrid only one phenotypic trait at one time… (i.e. flower colour, pea colour) only Hybrid- offspring of genetically dissimilar parents Hybrid offspring Monohybrid cross Mendel’s mating of parents (true breeding) with one Mendel’s antagonistic trait produce a hybrid offspring hybrid Trait is seed coat Parental (pure breeding) Pea Plants with smooth or wrinkled seeds First filial Monohybrid Monohybrid cross Mating between individuals that differ phenotypically in only Mating one trait (single gene) (single Mating of parents with antagonistic traits produces Mating hybrids hybrids P F1 Appearance of Hybrid Appearance Appearance of Hybrid Appearance Mendel was able to create monohybrid plants for each of the 7 antagonistic traits he was following: In each case… Advantages to Mendel’s approach Carefully controlled breeding (selfed or crossed): 5. Use of reciprocal crosses -reversed traits male/female parents Cross 1 X Egg (ovule) Cross 2 Sperm (pollen) X Egg (ovule) Sperm (pollen) Progeny same colour! Progeny same colour! Theory that one parent contributes most to offspring’s inherited features was disproved by reciprocal crosses - results of reciprocal crosses always same! Carefully controlled breeding. Mendel was able to cross or self when needed. • He was also able to make use of reciprocal crosses –reverse the gender of the traits male/female parents (egg and sperm) and in doing so always found the F1 progeny resembled only one of the parental strains… so this disproved one parent contributed more to the phenotype/genotype of progeny than the other… Progeny was always same (purple) regardless of whether used sperm from white flower of purple flower plant! 6. Meticulous, Statistical Analysis of Monohybrid Crosses plants plants differ in one trait (antagonistic pair –seed colour) Monohybrid plants F2 plants produced two types of peas –those that resembled F1 (yellow peas) and those that resembled the P generation (green peas) which had appeared to be lost! Yellow:Green ratio Theory - parental traits become mixed and forever changed in offspring (disproved through re-appearance of lost trait in F2 Mendel Proposed: • There are diiscrete units of inheritance called particulate screte particulate factors that get transmitted from parent to progeny • These discrete units control appearance of inherited traits • flower colour, seed colour, seed shape etc. • all of our phenotypic traits.. are controlled by these particulate traits (hair, eye, skin colour.. height etc.) Single gene traits come in alternate forms called alleles, each are responsible for expression of a different form of that trait • yellow, green –alternate alleles for seed colour (trait) Mendel proposed that each trait is determined by a particulate factor and that there may be several alleles for that trait that several occur normally in a population. However, only two alleles for any one trait can exist in a diploid individual (i.e. humans) (one maternal/one paternal) Hypothetical example: -lets say each of the following traits is determined by a single gene (like peas) Example of traits: Examples of Alleles Seed; flower colour Eye colour Skin colour Height Hair texture Green/yellow; purple/white Brown or blue Albino or pigmented Tall or short Curly or straight Most traits (including above) are determined by multiple genes with multiple alleles, but a diploid can only have 2 copies of a gene for one trait at a given time. Mendel’s law of segregation: During the formation of gametes, the two alleles for a given gametes the trait separate (segregate) so each gamete receives one or trait so the other. the The traits then unite at random, one from each parent, at random one fertilization to produce the zygote. zygote Gametes: specialized cells carry only one copy of an allele for a given trait one • egg and sperm Zygote: union of egg with a sperm which results in two copies of an allele two for a given trait i.e. embryo or offspring • Each parent carries 2 copies of each gene Each • specific allele exists for each gene specific • each individual receives 1 from each parent each Mendel’s law of segregation: Mendel’s Example: Trait is pea shape = smooth Each parent carries 2 copies of a particulate Each copies factor (gene) factor they can be the same alleles (pure-breeding) or different alleles (hybrid) egg or sperm S = smooth determining allele of seed shape gene s = wrinkle determining allele of seed shape gene During the formation of gametes (egg or sperm which carry only one gametes copy of an allele for a given trait), the two alleles for a given trait separate copy the (segregate) so each gamete receives one so The traits then unite at random, one from each parent, at fertilization The random to produce the zygote (results in 2 copies of an allele for any given trait) . copies The law of segregation following alleles The Particulate Factors (Genes) occur in Pairs: Pure-breeding Grows into plant Produces gametes locus Pure breeding Pair of ID factors EGGS Example: Trait is pea shape cross SPERM Each gamete contains one copy of allele for trait one S = Let S stand for smooth determining allele of seed shape gene s = Let s stand for wrinkle determining allele of seed shape gene The law of segregation following alleles Two gametes, one from each parent, unite at random at fertilization If parents are true breeding they contain fertilization ID copies for only one allele of trait (S or s) The F1 progeny will have a copy of each alternate allele (both S and s) • hybrid -monohybrid monohybrids Two different alleles for a single trait But how to explain 3:1 ratio of F2 progeny and missing trait? The law of segregation following alleles The Alleles segregate Egg Sperm The law of segregation following alleles The SS ss Ss The law of segregation following alleles The selfed selfed cross Use of the Punnett square in crosses Use P F1 F2 monohybrids Two different alleles for a single gene that determines a single feature or trait Law of Segregation: • During formation of gametes, paired unit factors separate or segregate randomly • each gamete receives one or other with equal likelihood F1 YY Y All Yy Yy Y yy y y Genotype versus Phenotype for seed shape Genotype Unit pair SAME alleles for given trait, individual is homozygous SAME Unit pair DIFFERENT alleles for given trait = heterozygous DIFFERENT When 2 unlike unit factors for a single trait are present in an individual, one factor is dominant to the other which is said to be recessive Genotype versus Phenotype Genotype Plants showing a dominant trait (Yellow) can be either purebreeding (YY) or hybrid (Yy) Dominant and Recessive are terms used to describe the phenotypic effect of different alleles Dominant alleles mask the phenotypic effect of recessive alleles Confirming the Principle of Segregation: 2 Yellow F peas : Pure breeding and hybrid Yellow Monohybrid 1/ 4 2/ 4 1/ 4 2/ 4 phenotype Hybrid genotypes occur in a ratio of 1:2:1 and phenotypes of 3:1 (yellow:green) 1/ 4 Plants showing a dominant trait (Yellow) can be either purebreeding (YY) or hybrid (Yy). How can you tell the difference between them? What if you have an organism that cannot self-fertilize? How can you tell genetic make-up (i.e. pure-breeding or hybrid?) heterozygous Usually denote by Y_ Test Cross: a mating between an individual showing the dominant phenotype with an individual that is showing the recessive • homozygous dominant homozygous phenotype i.e. S_ x ss. (homozyogous recessive) Test Crosses Establish Genotype ! Test At the DNA level different alleles differ in nucleotide sequence. This results in changes in the amino acid sequence or expression level of proteins. Round vs Wrinkled Seeds starch sugar Trait: seed shape Alleles: round (R) vs wrinkled (r) Gene: codes for enzyme starch-branching enzyme1 (SBE1) RR large amounts of branched starch rr no branched starch In rr, sucrose is not used and therefore builds up. This causes excess water to enter the young peas and when they mature, they lose this water and shrink, appearing wrinkled Rr R allele produces enough branched starch to prevent wrinkling the R allele is dominant over the r allele for the gene encoding SBE1 ...
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This note was uploaded on 10/04/2011 for the course BIOL 139 taught by Professor Christinedupont during the Spring '10 term at Waterloo.

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