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Mendelian Genetics & Beyond

Mendelian Genetics & Beyond. Chapter 10.2 & 10.3 Chapter 11.2. Today, we understand much about the inheritance of traits. But this was not always so… 150 years ago we had no knowledge of how traits were passed from parents to offspring…..

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Mendelian Genetics & Beyond

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  1. Mendelian Genetics & Beyond Chapter 10.2 & 10.3 Chapter 11.2

  2. Today, we understand much about the inheritance of traits. But this was not always so… 150 years ago we had no knowledge of how traits were passed from parents to offspring….. UNTIL….

  3. GREGOR MENDEL – 1822-1884 • Born in Austria - monk - loved nature • Interested in plants, meteorology and theories of evolution discovered 3 basic laws which govern the passage of traits today he is known as the… “FATHER OF GENETICS” (postmortem)

  4. GREGOR MENDEL studied - pea plants observed - 7 traits (each with 2 forms) by tracing these 7 traits, Mendel discovered 3 basic laws which govern the passage of traits

  5. PEA PLANT

  6. before Mendel could start his experiments….. he had to get pure (or true-breeding) plants ALWAYS produce offspring with the same trait Why was this important?

  7. Mendel got pure plants by self-pollinating (or inbreeding) plants for several generations.

  8. eventually, he had 14 pure strains (7 traits X 2 contrasting forms) each pure strain he called a parental generation (P) Now, he was ready to begin his experiments

  9. MENDEL’S EXPERIMENTS Mendel crossed pure (P) X contrasting pure (P) P X P = F1 (first filial generation) RESULTS only 1 form of the trait appeared in the F1 generation Mendel repeated his experiment s many times – all with the same results

  10. then, he crossed… F1 x F1 = F2 (second filial generation) RESULTS both forms of the trait appeared in a ratio of 3:1 Mendel repeated his experiment s many times – all with the same results

  11. MENDEL concluded that the patterns of inheritance are governed by 3 principles PRINCIPLE OF . . . . 1. Dominance and Recessiveness 2. Segregation 3. Independent Assortment

  12. Principle of Dominance and Recessiveness Mendel concluded… each trait is controlled by a pair (2) of factors… a dominant factor will prevent a recessive factor from being expressed

  13. PRINCIPLE OF SEGREGATION Mendel concluded… each pair of “factors” must segregate (separate) during the formation of gametes So that….. only one “factor” is inherited from each parent

  14. Principle of Segregation

  15. PRINCIPLE OF INDEPENDENT ASSORTMENT Mendel concluded… the inheritance of 1 trait is independent of the inheritance of another trait * the factors for different traits are distributed independently from one another * this principle requires the observation of 2 or more traits at the same time

  16. Principle of Independent Assortment

  17. Important Terms to know • Genetics: study of how traits are passed from parent to offspring (heredity) • Gene: unit of inheritance that usually is directly responsible for one trait or characteristic • Allele: an alternate form of a gene; formerly called factors by Mendel • Represented by letters yellow = Y purple = P Examples of Alleles Smooth Wrinkled

  18. Alleles can be…Homozygous or Heterozygous • When alleles from each parent are the same, they are called homozygous (pure) • Written as double letters that are the same size • Ex: PP, pp, YY, yy, BB, bb • When alleles from each parent are different, they are called heterozygous (hybrid) • Written as double letters that are different sizes • Ex: Pp, Yy, Bb

  19. Alleles can be…Dominant or Recessive • A dominant allele is expressed no matter what the second allele is • Represented by a capital letter • Ex: PP, Pp, YY, Yy, BB, Bb • A recessive allele is only expressed when the second allele is the same • Represented by a lower case letter • Ex: pp, yy, bb

  20. Phenotype is the physical expression or appearance of the trait Ex. Purple flower yellow seeds blond hair Genotype refers to the alleles (genes) pairs Ex. PP, Pp YY, Yy bb Phenotype vs. Genotype This may be changed. (Coloring your hair) (Plastic surgery) This can NEVER be changed.

  21. Mendel’s studies • Studied garden peas because: • they grew fast • made lots of offspring with short generation times • few traits that were easily seen • traits showed complete dominance • Usually self-pollinate/fertilize • 7 well-defined garden pea traits • Counted offspring of each phenotype and analyzed the results mathematically – saw patterns You MUST KNOW THESE!!

  22. Fig. 10.4, Mendel’s 7 garden pea characters.

  23. Practice assigning alleles 1. PP 2. TT 3. GG 4. Gg 5. ss 6. AA 7. Aa 8. aa 9. yy 1. Homozygous purple flower 2. Homozygous tall 3. Homozygous green pod 4. Heterozygous green pod 5. Homozygous wrinkled seed 6. Homozygous axial flower 7. Heterozygous axial flower 8. Homozygous terminal flower 9. Homozygous green seeds

  24. Probability The chance that an event will occur Probability = # of 1 kind of event total # of events What is the probability of… • A coin landing on heads? • Drawing a king from a deck of cards? • Having a baby boy? • A die landing on the number “3”?

  25. Punnett Squares • Dr. Reginald Punnett, early 1900s • Graphical way to show probability • 5 steps: • Assign P genotypes • Remember: use the letter of the dominant trait (homozygous = same size; heterozygous = different sizes) • Split alleles • Perform cross • Report F1 genotypes • Report F1 phenotypes

  26. Types of Crosses • Monohybrid cross = cross of two different alleles for a single trait. • Ex: crossing eye color x eye color • Ex: crossing hair color x hair color • Dihybrid cross = cross of two different alleles for two traits. • Ex: crossing eye & hair color x eye & hair color 4 squares 16 squares

  27. Monohybrid Crosses can be used with… • Complete dominance of traits • Incomplete dominance of traits • Codominance of traits • Sex-linked traits • Multiple allele traits

  28. Let’s Practice Step 1. P = SS x ss Step 2. Split alleles S S s s Step 3: perform Punnett Step 4: F1 genotype 100% Ss Step 5: F1 phenotype 100% smooth seeds

  29. Some practice – Show all 5 steps 1. In gerbils, brown fur is completely dominant over white fur. Cross a heterozygous brown-furred gerbil with a white-furred gerbil. 2. Cross two heterozygous tall plants. 3. In pigs, curly tails are completely dominant over straight tails. Cross a homozygous curly-tailed pig with a heterozygous curly-tailed pig. What are the possible phenotypes of the offspring?

  30. INCOMPLETE DOMINANCE • when neither allele is completely dominant, • both alleles influence the trait • there is a BLENDING of the alleles EXAMPLE – Four O’clock Flowers • red (r) and white (w) exhibit incomplete dominance • a heterozygous individual (rw) will be pink

  31. CODOMINANCE • when both alleles are dominant • both alleles are fully expressed – NO BLENDING EXAMPLE - Some cattle and horses exhibit codominance in their coat color. • red (R) and white (W) are codominant alleles • in the heterozygous individual (RW), both be fully expressed

  32. SEX-LINKED GENES ….are genes that are linked to (found on) the sex chromosomes X chromosome is LARGE + carries many genes genes on the X chromosome are called X linked genes discovered by Thomas Hunt Morgan when working with the fruit fly, Drosophila melanogaster

  33. Sex-linked Traits • Males affected more often; cannot be heterozygous • Females less affected; can be heterozygous (carriers) Examples: Red-green colorblindness Hemophilia (blood clotting disorder) Duchenne Muscular Dystrophy

  34. COLORBLINDNESSa recessive trait found on the X chromosomeresults in an inability to distinguish certain colors

  35. Duchenne MUSCULAR DYSTROPHYa recessive trait found on the X chromosomes results in the weakening and wasting away of muscle tissue

  36. MULTIPLE ALLELES • occur when there are 3 or more forms of a trait • although there are 3+ alleles, only 2 are inherited EXAMPLE – 3 alleles (A, B and o) influence blood type Alleles A and B are codominant. Allele i is recessive. 4 possible blood types – What is a person’s blood type if their allele pair is? AA, Ao, BB, Bo, AB, oo

  37. Sometimes with multiple alleles • Epistasis occurs • One allele hiding the effect of another allele Example: coat pigmentation on animals – see p. 305

  38. POLYGENIC TRAITS Traits that result from the interaction of several genes (several allele pairs). Skin color, hair color, eye color are polygenic traits.

  39. Some ways to check yourself…. • In monohybrid crosses • heterozygous x heterozygous crosses: • Genotypic ratio ALWAYS = 1:2:1 • Phenotypic ratio ALWAYS = 3:1 • In dihybrid crosses • heterozygous x heterozygous crosses: • Phenotypic ratio ALWAYS = 9:3:3:1 TRY SOME INTERACTIVE PUNNETT SQUARES @ http://glencoe.mcgraw-hill.com/sites/dl/free/0078695104/383934/BL_05.html

  40. Dihybrid Punnett Squares • Follow the same steps as in monohybrids • Extra step (FOIL the alleles) • Assign P genotypes • Remember: use the letter of the dominant trait (homozygous = same size; heterozygous = different sizes) • FOIL alleles • Split alleles • Perform cross • Report F1 genotypes • Report F1 phenotypes

  41. Practice with FOILing 1. Homozygous purple & axial x white & terminal P= PPAA x ppaa FOIL = PA PA PA PA x pa pa pa pa 2. Homozygous smooth & yellow seeds x heterozygous smooth & yellow seeds P= SS YY x Ss Yy FOIL = SY SY SY SY x SY Sy sY sy 3. Heterozygous green & inflated pods x heterozygous green & inflated pods P= Gg Ii x Gg Ii FOIL = GI Gi gI gi x GI Gi gI gi

  42. Cross #1 • P= PPAA x ppaa • FOIL = PA PA PA PA x pa pa pa pa PA PA PA PA pa pa pa pa F1 phenotypes: 100% purple & axial flowers F1 genotypes: 100% PpAa

  43. Cross #2 P= SS YY x Ss Yy FOIL = SY SY SY SY x SY Sy sY sy SY SY SY SY SY Sy sY sy F1 genotypes: 25% SSYY, 25% SSYy 25% SsYY, 25% SsYy F1 phenotypes: 100% smooth, yellow seeds

  44. Cross #3 P= Gg Ii x Gg Ii FOIL = GI Gi gI gi x GI Gi gI gi GI Gi gI gi GI Gi gI gi F1 genotypes: 1 GGII, 2 GGIi, 2 GgII, 4 GgIi, 1 GGii, 2 Ggii, 1 ggII, 2 ggIi, 1 ggii F1 phenotypes: 9 green & inflated pods 3 green & contricted pods 3 yellow & inflated pods 1 yellow & constricted pods (9:3:3:1) There should always be a 9:3:3:1 phenotypic ratio in a heterozygous x heterozygous dihybrid cross.

  45. Polyploidy • One or more extra sets of all chromosomes in an organism • Triploid organism (3n) • Rarely occurs in animals • Always fatal in humans • Plants OFTEN exhibit polyploidy • Exhibit more vigor and size • oats & wheat = 6n • Sugar cane = 8n

  46. Genetic Recombination understood After studying meiosis and After Mendel’s studies… Why don’t you look identical to any other human being? *the possible number of allele combinations is 223 x 223 70 trillion (not including variation from crossing over) Scientists now use this knowledge to artificially recombine genes to breed plants and animals with desirable traits.

  47. Gene Linkage • Is an exception to the Law of Independent Assortment • Genes that are closer together on a chromosome are more likely to travel together during gamete formation. • Scientists studied the Drosophila melanogaster (fruit fly) to demonstrate gene linkage • Chromosome maps are used to show this frequency • 1 map unit = 1% cross over

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