1 / 21

Mendelian Genetics

Mendelian Genetics. Or, what’s the story with this “ Mendelian ” guy?. Outline. Mendel’s laws Segregation Dominance and recessivity Punnett squares Independent Assortment Unusual Patterns of Inheritance Incomplete dominance and Codominance. Mendel’s Laws.

hanne
Download Presentation

Mendelian Genetics

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Mendelian Genetics Or, what’s the story with this “Mendelian” guy?

  2. Outline • Mendel’s laws • Segregation • Dominance and recessivity • Punnett squares • Independent Assortment • Unusual Patterns of Inheritance • Incomplete dominance and Codominance

  3. Mendel’s Laws • Quantitative analysis of F2 plants • “selfed” offspring of outcrossed parents • The law of segregation • Describes dominance and recessivity • The law of independent assortment • We have already learned this…

  4. EXPERIMENT The Law of Segregation P Generation (true-breeding parents) Purple flowers White flowers • P Generation • White flowered X purple flowered pea plants, all of the • F1 Generation (“hybrids”) • All purple flowered • F2 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 F1 Generation (hybrids) All plants had purple flowers F2 Generation 224 white-flowered plants 705 purple-flowered plants

  5. Why 3:1? Is it always 3:1? • NO. • Ratio depends on who is breeding with whom • RR x RR • RrxRr • rrxrr • RR x rr • Rr x rr • Punnett Square • Diagrams probabilities of inheritance Sperm F2 Generation p P P PP Pp Eggs p pp Pp 3 1

  6. Useful Genetic Vocabulary • 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” • An organism’s expressed traits do not always reveal its genetic composition • Phenotype: physical appearance (purple flowers) • Genotype: genetic makeup (Pp) • In the example of flower color in pea plants, PPand Pp plants have the same phenotype (purple) but different genotypes

  7. Phenotype Ratio vs. Genotype Ratio Phenotype Genotype PP Purple 1 (homozygous) Pp 3 Purple (heterozygous) 2 Pp Purple (heterozygous) pp White 1 1 (homozygous) Ratio 3:1 Ratio 1:2:1

  8. 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. Allele for purple flowers Homologous pair of chromosomes Modern take Locus for flower-color gene Allele for white flowers

  9. 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 F1 hybrids) 3) If two alleles differ, the dominant alleleis fully expressed in the organism’s appearance. • Recessive allele has no effect on the organisms appearance. • e.g., F1 plants had purple flowers

  10. 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…

  11. How Can One Tell the Genotype of an Individual Expressing the Dominant Phenotype? • Testcross • Breed themystery individual with a homozygous recessive individual • R? x rr • Do any offspring display the recessive phenotype? • the mystery parent must be heterozygous

  12. Fig. 14-7 Testcross  Dominant phenotype, unknown genotype: PP or Pp? Recessive phenotype, known genotype: pp Predictions If PP If Pp or Sperm Sperm p p p p P P Pp Pp Pp Pp Eggs Eggs P p pp Pp pp Pp RESULTS or All offspring purple 1/2 offspring purple and 1/2 offspring white

  13. 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

  14. 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

  15. 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 Y Xc XCXC XcY XCXc XCY XC Normal vision female color-blind male XCXc XCY XC

  16. Degrees of Dominance P Generation Red White CRCR CWCW • Complete dominance • phenotypes of the heterozygote and dominant homozygote are identical • Law of segregation • Incomplete dominance • phenotype of F1 hybrids is intermediate between the P phenotypes CR CW Gametes Pink F1 Generation CRCW 1/2 1/2 CR CW Gametes Sperm 1/2 1/2 CR CW F2 Generation 1/2 CR CRCW CRCR Eggs 1/2 CW CRCW CWCW

  17. Codominance X Two dominant alleles affect the phenotype in separate ways white bull brown cow X Roan calf white horse grey horse X white camellia pink camellia Appaloosa horse hybrid camellia

  18. 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?

  19. Law of Independent Assortment • Do the traits always stay together? • YR and yr? • If so… • F1 and F2 offspring would still produce yellow, round seeds • Doesn’t happen! • What is the explanation for this?

  20. 2 pairs of alleles segregate independently- • Encoded on two different chromosomes • The presence of one specific allele for one trait has no impact on the presence of a specific allele for the second trait • F1 offspring yellow, round seeds. • F1gametesall possible allelic combinations • YR, Yr, yR, and yr would be produced in equal amounts.

  21. EXPERIMENT YYRR yyrr P Generation Gametes yr YR  F1 Generation YyRr Hypothesis of dependent assortment Hypothesis of independent assortment Predictions Sperm or Predicted offspring of F2 generation 1/4 1/4 1/4 yr 1/4 YR yR Yr Sperm YR yr 1/2 1/2 1/4 YR YYRr YYRR YyRR YyRr 1/2 YR YyRr YYRR 1/4 Yr Eggs YYRr YYrr Yyrr YyRr Eggs 1/2 yr YyRr yyrr 1/4 yR YyRR YyRr yyRR yyRr 3/4 1/4 1/4 yr Phenotypic ratio 3:1 Yyrr yyRr YyRr yyrr 3/16 1/16 9/16 3/16 Phenotypic ratio 9:3:3:1 RESULTS Phenotypic ratio approximately 9:3:3:1 315 108 101 32

More Related