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Mendelian Genetics Or, what’s the story with this “Mendelian” guy?
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Outline Mendel’s laws Segregation – Dominance and recessivity – Punnett squares Independent Assortment Unusual Patterns of Inheritance – Incomplete dominance and Codominance
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Mendel’s Laws Quantitative analysis of F 2 plants – “selfed” offspring of outcrossed parents The law of segregation – Describes dominance and recessivity The law of independent assortment – We have already learned this…
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The Law of Segregation P Generation White flowered X purple flowered pea plants, all of the F 1 Generation (“hybrids”) All purple flowered F 2 Generation Mostly purple; some white 3:1 ratio Purple is “the dominant heritable factor” white is recessive “Heritable factor” is what we now call a gene EXPERIMENT P Generation (true-breeding parents) Purple flowers White flowers F 1 Generation (hybrids) All plants had purple flowers F 2 Generation 705 purple-flowered plants 224 white-flowered plants
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NO. NO. Ratio depends on who is breeding with whom – RR x RR – Rr x Rr – rr x rr – RR x rr – Rr x rr Punnett Square – Diagrams probabilities of inheritance F 2 Generation Sperm Eggs P P PPPp p p pp 31 Why 3:1? Is it always 3:1?
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Homozygous: two identical alleles for a trait RR / rr “homozygous dominant” or “homozygous recessive” heterozygous : two different alleles for a trait Rr “heterozygous for blah” “carrier” do not always An organism’s expressed traits do not always reveal its genetic composition purple flowers Phenotype: physical appearance (purple flowers) Pp Genotype: genetic makeup (Pp) PPPp phenotypegenotypes In the example of flower color in pea plants, PP and Pp plants have the same phenotype (purple) but different genotypes Useful Genetic Vocabulary
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Phenotype Purple 3 Genotype 1 White Ratio 3:1 (homozygous) (heterozygous) PP Pp pp Ratio 1:2:1 1 1 2 Phenotype Ratio vs. Genotype Ratio
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Allele for purple flowers Homologous pair of chromosomes Locus for flower-color gene Allele for white flowers Mendel’s Model 4-part hypothesis to explain the white flower weirdness 1) Two version of genes (different alleles) account for variations in inherited characters. – Different alleles vary in the DNA sequence at the specific locus of a gene. – Purple and white flower alleles are 2 DNA variations at the flower-color locus. Modern take
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Mendel’s Model 2) Organism inherits two alleles, one from each parent. – Diploid organism= one set of chrms from each parent. – Pair of homologous chrms = two copies of each locus. (Mendel made this deduction without knowing about the role of chromosomes) – The two alleles at a locus on a chromosome may be identical (Mendel’s P generation)… – …or they may differ (Mendel’s F 1 hybrids) dominant allele 3) If two alleles differ, the dominant allele is fully expressed in the organism’s appearance. – Recessive allele has no effect on the organisms appearance. – e.g., F 1 plants had purple flowers
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4) 2 alleles segregate (separate) during gamete production. – Mendel’s Second Law (the Law of segregation) – Corresponds to the distribution of homologous chromosomes to different gametes in meiosis. – Each allele exists as a single copy in all gametes. During meiosis I, tetrads can line up two different ways before the homologs separate. OR Mendel had neither an electron microscope nor a crystal ball, and yet…
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How Can One Tell the Genotype of an Individual Expressing the Dominant Phenotype? Testcross Breed the mystery individual with a homozygous recessive individual – R? x rr Do any offspring display the recessive phenotype? – the mystery parent must be heterozygous
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Fig. 14-7 RESULTS Dominant phenotype, unknown genotype: PP or Pp? Predictions Recessive phenotype, known genotype: pp If PPIf Pp or Sperm ppp p P P P p Eggs Pp pp or All offspring purple 1 / 2 offspring purple and 1 / 2 offspring white Testcross
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Mendelian Inheritance Reflects Rules of Probability Tossing coins or rolling dice – The probability of tossing heads is 1/2 – The probability of rolling a 3 with a six-sided die is 1/6, and the probability of rolling any other number is 1 - 1/6 = 5/6 Tossing a coin has no impact on the outcome of the next toss Each toss is an independent event Gamete from a heterozygous parent has a 50% chance of carrying the dominant allele and a 50% chance of carrying the recessive
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Complex Inheritance Patterns Genotype/phenotype Commonly not as simple as pea plants Deviations from simple Mendelian patterns: – alleles are not completely dominant or recessive – a gene is carried on the X chromosome – a gene has more than two alleles – a gene produces multiple phenotypes
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Sex-Linked Traits Traits encoded on Chromosome X – Females have 2 copies (XX) – Males have 1 (XY) Mothers always donate X – fathers determine offspring sex (donate X or Y) Females can be carriers – Males CANNOT be silent carriers XCYXCYXCXcXCXc XCXcXCXc XCYXCY XCXC XcXc XCXC Y XCXCXCXC XcYXcY color-blind maleNormal vision female
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Degrees of Dominance Complete dominance – phenotypes of the heterozygote and dominant homozygote are identical – Law of segregation Incomplete dominance – phenotype of F 1 hybrids is intermediate between the P phenotypes Red P Generation Gametes White CRCRCRCR CWCWCWCW CRCR CWCW F 1 Generation Pink CRCWCRCW CRCR CWCW Gametes 1/21/2 1/21/2 F 2 Generation Sperm Eggs CRCR CRCR CWCW CWCW CRCRCRCR CRCWCRCW CRCWCRCW CWCWCWCW 1/21/2 1/21/2 1/21/2 1/21/2
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Codominance Two dominant alleles affect the phenotype in separate ways white horse grey horse Appaloosa horse X white bull brown cow Roan calf X X pink camellia white camellia hybrid camellia
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The Law of Independent Assortment Law of Segregation – follows a single trait – monohybrid cross (e.g., flower color) Law of Independent Assortment – Follow two traits – dihybrid cross (e.g., seed color and seed shape) Known information about pea plants from Law of Segregation: – Yellow seed allele (Y) is dominant to green seed allele (y). – Round seed allele (R) is dominant to wrinkled seed allele (r) If you cross homozygous plants (YYRR x yyrr), what do you predict would happen?
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Law of Independent Assortment Do the traits always stay together? – YR and yr? If so… – F 1 and F 2 offspring would still produce yellow, round seeds Doesn’t happen! What is the explanation for this?
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2 pairs of alleles segregate independently- two different chromosomes – Encoded on two different chromosomes has no impact – The presence of one specific allele for one trait has no impact on the presence of a specific allele for the second trait F 1 offspring yellow, round seeds. F 1 gametes all possible allelic combinations – YRYryRyr – YR, Yr, yR, and yr would be produced in equal amounts.
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EXPERIMENT RESULTS P Generation F 1 Generation Predictions Gametes Hypothesis of dependent assortment YYRRyyrr YR yr YyRr Hypothesis of independent assortment or Predicted offspring of F 2 generation Sperm YR yr Yr YR yR Yr yR yr YR YYRR YyRr YYRr YyRR YYrr Yyrr yyRR yyRr yyrr Phenotypic ratio 3:1 Eggs Phenotypic ratio 9:3:3:1 1/21/2 1/21/2 1/21/2 1/21/2 1/41/4 yr 1/41/4 1/41/4 1/41/4 1/41/4 1/41/4 1/41/4 1/41/4 1/41/4 3/43/4 9 / 16 3 / 16 1 / 16 9:3:3:1 Phenotypic ratio approximately 9:3:3:1 31510810132
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