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Classical Genetics. Classical genetics deals with the study of heredity Often referred to as Mendelian genetics Based on the principles first set forth.

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Presentation on theme: "Classical Genetics. Classical genetics deals with the study of heredity Often referred to as Mendelian genetics Based on the principles first set forth."— Presentation transcript:

1 Classical Genetics

2 Classical genetics deals with the study of heredity Often referred to as Mendelian genetics Based on the principles first set forth by Gregor Mendel in 1866 –Austrian monk studying traits of pea plants Early analysis of organismal traits was based on morphological characteristics –Prior to Mendel, blending of traits was standard idea Later rejected due to subsequent reappearance of traits in offspring Mendel studied seven traits: seed shape, seed color, flower color, pod shape, pod color, flower and pod position, and stem length –Traits occurred in two different forms His results were largely ignored until 1900

3 Mendels Peas Petal Stamen Carpel

4 Pea plant traits Flower color Flower position Seed color Seed shape Pod shape Pod color Stem length PurpleWhite AxialTerminal YellowGreen RoundWrinkled InflatedConstricted GreenYellow Tall Dwarf

5 Mendel theorized that discrete heritable factors are passed from parent to offspring Following traits through multiple generations provided evidence to predict how traits could be passed on Mendel cross-fertilized peas of his choosing based on desired characteristics through controlled matings –Called first generation the P1 (parental) generation Differed by one trait (hybrid) –Offspring of the P generation were F1 generation –Offspring of F1generation were F2 generation (dihybrid) He made several important discoveries based on observations of experiments

6 LE 9-2c Removed stamens from purple flower White Carpel Parents (P) Purple Transferred pollen from stamens of white flower to carpel of purple flower Stamens Pollinated carpel matured into pod Planted seeds from pod Offspring (F 1 )

7 LE 9-3a P generation (true-breeding parents) Purple flowersWhite flowers All plants have purple flowers F 1 generation F 2 generation Fertilization among F 1 plants (F 1 F 1 ) of plants have purple flowers 3434 of plants have white flowers 1414

8 Mendels Observations & Hypotheses Some traits disappeared in F1 generation but reappeared in F2 generation in particular ratios Some traits mask or dominate expression of the other trait: dominant form Some traits are masked by expression of dominant trait: recessive form Offspring get alternative forms of discrete heritable factors (genes) that account for variation in traits passed down: alleles Organisms receive one allele from each parent –Supported by law of segregation: one allele on each chromosome of a (homologous) pair

9 Homologous Chromsomes Alternate forms of the genes (alleles) reside at the same locus on homologous chromosomes Supports law of segregation Either allele may be present, location is the constant Identical alleles: homozygous Differing alleles: heterozygous

10 LE 9-4 Gene loci PaB Dominant allele Pab PP Bb Genotype: Homozygous for the dominant allele Homozygous for the recessive allele Heterozygous aa Recessive allele

11 Mendels Conclusions Organisms appearance doesnt necessarily reflect its genetic makeup Genotype is genetic makeup Phenotype is expression of traits In monohybrid cross, 3:1 phenotypic ratio and 1:2:1 genotypic ratio Used a Punnett square to demonstrate possible combinations of crosses

12 LE 9-3b P plants Gametes Genetic makeup (alleles) All Pp F 1 plants (hybrids) F 2 plants Sperm Phenotypic ratio 3 purple : 1 white Gametes PPpp All PAll p Eggs 1212 Genotypic ratio 1 PP : 2 Pp : 1 pp P 1212 p p P P p PPPp pp

13 Dihybrid Crosses Mating a parental generation differing in two traits Results of mating F1 generations produce F2 generation Can be used to demonstrate independent assortment: alleles of two different genes segregate independently of one another Demonstrates alleles segregating independently of one another during gametogenesis Phenotypic ratio of 9:3:3:1, genotypic ratio of 1:2:2:4:2:1:1:2:1 (forget it!)

14 LE 9-5a rryy RrYy RRYYrryy RY ry RrYy Sperm Eggs RY rY Ry ry RYrYRyry RRYYRrYYRRYyRrYy RrYYrrYYRrYyrrYy RRYyRrYyRRyyRryy RrYyrrYyRryyrryy Yellow round Green round Yellow wrinkled Green wrinkled Actual results support hypothesis ry RY RRYYrryy P generation RY Hypothesis: Dependent assortment Hypothesis: Independent assortment Gametes Actual results contradict hypothesis F 1 generation F 2 generation Gametes

15 Independent Assortment Phenotypes Genotypes Mating of heterozygotes (black, normal vision) Phenotypic ratio of offspring Black coat, normal vision B_N_ Black coat, blind (PRA) B_nn Blind Chocolate coat, normal vision bbN_ Chocolate coat, blind (PRA) bbnn BbNn 9 black coat, normal vision 3 black coat, blind (PRA) 1 chocolate coat, blind (PRA) 3 chocolate coat, normal vision Example: Coat color and vision in Labs Black or chocolate coat: B or b Normal vision or PRA: N or n

16 Discovering Genotypes Test crosses can be used to determine genotype Homozygous recessive organism is mated with organism of dominant phenotype to determine genotype Phenotypic ratio of F1 generation will allow determination of genotype

17 LE 9-6 Testcross: Genotypes Gametes Offspring All black 1 black : 1 chocolate Two possibilities for the black dog: or B_ bb Bb Bb bb B BB Bb bb

18 Genetics and Probability Mendels laws follow predictable rules of probability Events following rules of probability occur independently of one another Current genetic configuration does not influence future outcome: sex in subsequent offspring Multiplication rule: probability of two events occurring simultaneously is product of the probabilities of the separate events( ½ X ½ = ¼ ) Addition rule: probability that event can occur in two or more alternative ways is the sum of the separate probabilities of the different ways (¼ + ¼ = ½) Can be used to predict probability of combinations of traits occurring in offspring

19 LE 9-7 F 1 genotypes Bb female Bb male Formation of sperm Formation of eggs F 2 genotypes Bb B BBBb b bBbb

20 Variations on Mendels Laws Mendels laws can be applied to all sexually reproducing organisms, but… Some patterns of inheritance dont follow Mendels laws – too complex! Complete vs. Incomplete Dominance –Complete dominance: dominant allele exerts its affect regardless of number of copies –Incomplete dominance: heterozygote shows intermediate characteristics of two homozygous conditions Not the same as blending Hypercholesterolemia in humans, flower color in snapdragons

21 LE 9-12a P generation F 1 generation F 2 generation Gametes Eggs White rr Pink Rr R R r r Sperm R 1212 r 1212 Red RR Pink rR Pink Rr White rr Red RR Rr

22 LE 9-12b HH Homozygous for ability to make LDL receptors Genotypes: Hh Heterozygous Phenotypes: LDL receptor Cell Normal Mild diseaseSevere disease hh Homozygous for inability to make LDL receptors

23 Genes and Multiple Alleles Many genes have more than 2 alleles: multiple alleles Example: ABO blood types in humans: A, B, AB, O Codominance of A and B alleles in heterozygotes phenotype Six possible genotypes in ABO system

24 LE 9-13 Blood Group (Phenotype) Genotypes Antibodies Present in Blood Reaction When Blood from Groups Below Is Mixed with Antibodies from Groups at Left OABAB O A B ii Anti-A Anti-B Anti-A I A or I A i I B or I B i IAIBIAIB

25 Gene Linkages Inheritance patterns inconsistent with Mendelian laws first noted in 1908 Sweet peas failed to show predicted ratios in the F2 generation Genes located close together on the same chromosome are linked Dont follow Mendels laws of independent assortment Usually inherited together

26 LE 9-19 Experiment Purple flower Purple long Purple round Red long Red round Explanation: linked genes Parental diploid cell PpLl Most gametes P L p l Meiosis P L p l Fertilization Sperm P L p l P L p l P L p l P L 3 purple long : 1 red round Not accounted for: purple round and red long Most offspring Eggs Long pollen PpLl Phenotypes Observed offspring Prediction (9:3:3:1)

27 Crossing Over New combinations of alleles produced from crossing over Occurs during meiosis between homologous chromosomes Results in new combinations of alleles in gametes

28 Crossing Over T.H. Morgan used fruit flies for genetic studies in early 1900s Easy to grow, short generation time, very inexpensive Studied mutant and wild-type phenotypes Was able to determine genes were on chromosomes: chromosome basis of inheritance Didnt know about crossing over, but something breaks linkages according to Morgan Crossover data used to help map genes: determine their relative positions on chromosomes Nucleotide distances used to determine gene maps now

29 Fruit Flies Experiment Gray body, long wings (wild type) GgLl Female Black body, vestigial wings ggll Male Offspring Gray longBlack vestigialGray vestigialBlack long Parental phenotypes Recombinant phenotypes Recombination frequency = 391 recombinants 2,300 total offspring = 0.17 or 17% Explanation GgLl (female) ggll (male) gl gl g l g L G l g l G L g l EggsSperm Offspring G L g l G lg L g l

30 Drosophila crosses Drosophila melanogaster can be used to study Mendelian patterns of inheritance Many mutant strains available to study Mutations found on various chromosomes Maintained as inbred lines for study Linked and non-linked mutations available Linkages occur on various chromosomes

31 Sex Linkage Genes unrelated to sex determination, but located on sex chromosomes X-linked in humans Inheritance follows peculiar patterns –Eye color in fruit flies: three possible patterns of inheritance

32 LE 9-23b FemaleMale XRXRXRXR X r Y Sperm XRXrXRXr XRYXRY XrXr Y XRXR Eggs R = red-eye allele r = white-eye allele

33 LE 9-23c FemaleMale XRXrXRXr XRYXRY Sperm XRXRXRXR XRYXRY XRXR Y XRXR Eggs XrXr XrXRXrXR X r Y

34 LE 9-23d FemaleMale XRXrXRXr X r Y Sperm XRXrXRXr XRYXRY XrXr Y XRXR Eggs XrXr X r X r Y

35 Disclaimer This workforce solution was funded by a grant awarded under the Presidents Community-Based Job Training Grants as implemented by the U.S. Department of Labors Employment and Training Administration. The solution was created by the grantee and does not necessarily reflect the official position of the U.S. Department of Labor. The Department of Labor makes no guarantees, warranties, or assurances of any kind, express or implied, with respect to such information, including any information on linked sites and including, but not limited to, accuracy of the information or its completeness, timeliness, usefulness, adequacy, continued availability, or ownership. This solution is copyrighted by the institution that created it. Internal use by an organization and/or personal use by an individual for non-commercial purposes is permissible. All other uses require the prior authorization of the copyright owner.


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