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Mendelian genetics

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Mendelian genetics

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Mendelian genetics

  1. 1. Mendelian Genetics An Overview
  2. 2. Mendelian Genetics Gregor Mendel “Father of Genetics” Augustinian Monk at Brno Monastery in Austria (now Czech Republic) Not a great teacher but well trained in math, statistics, probability, physics, and interested in plants and heredity. While assigned to teach, he was also assigned to tend the gardens and grow vegetables for the monks to eat. Mountains with short, cool growing season meant pea (Pisum sativum) was an ideal crop plant.
  3. 3. Contributions in 1860s (US Civil War Era) • Discovered Genes as Particles of Inheritance • Discovered Patterns of Inheritance • Discovered Genes Come from Both Parents Egg + Sperm = Zygote Nature vs Nurture Sperm means Seed (Homunculus) • Discovered One Form of Gene (Allele) Dominant to Another • Discovered Recessive Allele Expressed in Absence of Dominant Allele http://academic.evergreen.edu/v/vivianoc/homunculus.gif
  4. 4. Mendel worked with peas (Pisum sativum) • Good choice for environment of monastery • Network provided unusual varieties for testing • Obligate self-pollination reproductive system  Permits side-by-side genetic barriers  Cross-pollinations require intentional process • Crosses meticulously documented • Crosses numerically/statistically analyzed • Scientists of 1860s could not understand math • Work lost in journals for 50 years! • Rediscovered in 1900s independently by 3 scientists • Recognized as landmark work!
  5. 5. • Pea plants have several advantages for genetics. – Pea plants are available in many varieties with distinct heritable features (characters) with different variants (traits). – Another advantage of peas is that Mendel had strict control over which plants mated with which. – Each pea plant has male (stamens) and female (carpal) sexual organs. – In nature, pea plants typically self-fertilize, fertilizing ova with their own sperm. – However, Mendel could also move pollen from one plant to another to cross-pollinate plants.
  6. 6. • In a typical breeding experiment, Mendel would cross-pollinate (hybridize) two contrasting, true-breeding pea varieties. – The true-breeding parents are the P generation and their hybrid offspring are the F1 generation. • Mendel would then allow the F1 hybrids to self- pollinate to produce an F2 generation. • It was mainly Mendel’s quantitative analysis of F2 plants that revealed the two fundamental principles of heredity: the law of segregation and the law of independent assortment.
  7. 7. • If the blending model were correct, the F1 hybrids from a cross between purple-flowered and white- flowered pea plants would have pale purple flowers. • Instead, the F1 hybrids all have purple flowers, just a purple as the purple-flowered parents. 2. By the law of segregation, the two alleles for a characters are packaged into separate gametes Fig. 14.2
  8. 8. • When Mendel allowed the F1 plants to self- fertilize, the F2 generation included both purple- flowered and white-flowered plants. – The white trait, absent in the F1, reappeared in the F2. • Based on a large sample size, Mendel recorded 705 purple-flowered F2 plants and 224 white-flowered F2 plants from the Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 14.2
  9. 9. Vocabulary • Character –heritable feature • Trait – each variant for a character • True-breeding – plants that self-pollinate all offspring are the same variety • Monohybrid cross – a cross that tracks the inheritance of a single character • P generation – (parental) true-breeding • F1- (first filial) offspring of P generation • F2 – (second filial) offspring from F1 cross
  10. 10. Vocabulary (continued) • Allele- alternate version of a gene • Dominate allele – expressed in the heterozygote • Recessive allele – not expressed in the heterozygote • Homozygote – pair of identical alleles for a character – Homozygous dominant- BB – Homozygous recessive - bb • Heterozygote – two different alleles for a character (Bb) • Genotype – genetic makeup • Phenotype – appearance of an organism
  11. 11. Wrinkled P Roundx F1 All Round Phenotype Yet Another Example of Mendel’s Work F1 x F1 = F2 F2 3 /4 Round 1 /4 Wrinkled 1. Round is dominant to Wrinkled 2. Use W/w rather than R/r for symbolic logic: in handwriting make it legible! Ww ww WW Ww Genotype Homozygous Recessive Homozygous Dominant Heterozygous Wrinkled ww Round Ww w Round Ww Round WW W wWPunnett Square: possible gametes possible gametes NEVER use W/R or w/r
  12. 12. Unknown Round Wrinkledx Mendel as a Scientist ww Round Ww Round Ww W Round Ww Round Ww W ww possible gametes possible gametes F1 x F1 = F2F2 Wrinkled ww Round Ww w Round Ww Round WW W wWPunnett Square: possible gametes possible gametes Test Cross: If Unknown is WW: Wrinkled ww Wrinkled ww w Round Ww Round Ww W ww possible gametes possible gametes If Unknown is Ww: Test Progeny All Round Test Progeny Half Round Half Wrinkled
  13. 13. Unknown Round Wrinkledx Mendel as a Scientist ww Round Ww Round Ww W Round Ww Round Ww W ww possible gametes possible gametes Test Cross: If Unknown is WW: Wrinkled ww Wrinkled ww w Round Ww Round Ww W ww possible gametes possible gametes If Unknown is Ww: Actual Results Decision 3 Round 2 Wrinkled Ww 2 Round 3 Wrinkled Ww 1 Round 4 Wrinkled Ww 0 Round 5 Wrinkled Ww 4 Round 1 Wrinkled Ww 5 Round 0 Wrinkled WW <5% chance unknown is Ww 1 /2 • 1 /2 • 1 /2 • 1 /2 • 1 /2 = 1 /32 It only takes 1 wrinkled to be sure the unknown is Ww! Small families do not follow expected ratios perfectly! You could be wrong (rarely)! Rare, but it can happen!
  14. 14. Law of Independent Assortment – Each set of alleles segregates independently
  15. 15. Test cross – designed to reveal the genotype of an organism
  16. 16. Mendelian Inheritance and Rules of Probability • Rule of Multiplication – the probability that two events will occur simultaneously is the product of their individual probabilities • Probability that an egg from the F1 (Pp) will receive p = ½ • Probability that an sperm from the F1 (Pp) will receive p = ½ • Probability that a of offspring receiving two recessive alleles during fertilization ½ x ½ = ¼
  17. 17. Rule Applies to dihybrid Crosses • For a dihybrid cross, YyRr x YyRr, what is the probability of an F2 having the genotype YYRR?
  18. 18. Mendel’s Results and Conclusions • Mendel concluded that: • 1. Each trait is controlled by 2 genes or alleles. • 2. Some alleles are dominant, while other alleles are recessive. • Important terms: • Homozygous • Pure bred • Heterozygous • hybrid • Phenotype • genotype
  19. 19. Mendel’s Conclusions • 3. Law of Segregation —During meiosis, each allele separates in an orderly manner from its partner, the two always go to different gametes. • 4. Law of Independent Assortment—During Metaphase and Anaphase I of meiosis, the alleles for different traits segregate independently. This mixes chromosomes from your father and mother.
  20. 20. More on Mendel • Mutiple alleles— sometimes more than 2 different alleles exist for a trait. (ABO blood type). • Sometimes several gene pairs work together to bring about a trait. Polygenetic inheritance (skin color) Continous variation Epistatis—a process whereby a pair of recessive alleles at one locus masks the effect of genes at other loci.(albinism) Pleiotropy—The effect of one can can have many secondary effects. (cystic fibrosis)
  21. 21. Genetics After Mendel Red x Yellow All Orange When these alleles go walking, they both do some talking (codominance)! OK, so we cannot use R/r nor Y/y so we pick a third letter…P for the petal color gene. Notice: we do NOT mix R/Y or r/y! PR PR PY PY PR PY F1 x F1 = F2 F2 Yellow PY PY Orange PR PYPY Orange PR PY Red PR PR PR PY PRPunnett Square: possible gametes possible gametes P F1 This F2 will NOT have a 3:1 ratio of phenotypes. Instead it shows a 1:2:1 ratio! The exception here proves the rule. After 1900 several scientists tried to replicate Mendel’s crosses using other species including snapdragon.
  22. 22. In addition to this, there are multiple alleles possible: PR = red PY = yellow p = no pigment The combination of alleles in a diploid determine the flower color: PR PR = red PR PY = orange PY PY = yellow PR p = pink PY p = cream pp = white Human hair color follows a similar pattern: Alleles: HBn = brown HBd = blonde hR = red hbk = black The combinations of these alleles determine the base hair color: HBn HBn = dark brown HBn HBd = sandy brown HBn hR = auburn/reddish brown HBn hbk = dark brown HBd HBd = blonde HBd hR = strawberry blonde HBd hbk = blonde hR hR = red hR hbk = red hbk hbk = black Dominant does NOT mean frequent! Recessive can be common!
  23. 23. Incomplete Dominance
  24. 24. Incomplete Dominance in Carnation Coloration
  25. 25. Codominance • Two alleles affect the phenotype in separate and distinguishable ways. • Neither allele can mask the other and both are expressed in the offspring and not in an “intermediate” form. • Example: red flowers that are crossed with white flowers that yield red and white flowers.
  26. 26. Codominance. Human Blood Types Codominance is a condition where two non-identical alleles of a pair specify two different phenotypes, yet one cannot mask the expression of the other (blood types in humans) Blood types in humans are an example of a multiple allele system
  27. 27. • 1) In cattle, roan coat color (mixed red and white hairs) occurs in the heterozygous (Rr) offspring of red (RR) and white (rr) homozygotes. When two roan cattle are crossed, the phenotypes of the progeny are found to be in the ratio of 1 red:2 roan:1 white. Which of the following crosses could produce the highest percentage of roan cattle? • A) roan x roan • B) red x white • C) white x roan • D) red x roan • E) All of the above crosses would give the same percentage of roan.
  28. 28. Pleiotropy • Most genes have multiple phenotypic effects. The ability of a gene to affect an organism in many ways is called pleiotropy.
  29. 29. EpistasisEpistasis • Epistasis occurs when a gene at one locus alters or influences the expression of a gene at a second loci. In this example, C is for color and the dominate allele must be present for pigment (color) to be expressed.
  30. 30. Phenotype = Genotype + Environment Crop Yield = Genotype + Minerals + Water + Light - Pests etc. Human Skin Color = Genotype + Sun (UV) Exposure - Aging Factors The sun exposure effect is most obvious in people of intermediate skin base color but everyone can have “tan lines.” Optimizing these factors determines agricultural productivity…
  31. 31. Who Gets To Mate With Whom? …Two Extremes Inbreeding Depression: related parents give same recessives to children Hemophilia: Queen Victoria’s Mutation and Diseased Grandchildren Tay-Sachs: Jewish Populations Bipolar: Irish Populations Dog Diseases: German Shepherd hip dysplasia Hybrid Vigor: recessives of one family are “covered” by dominant of other family Wild Corn A x Wild Corn B High Yield Hybrid Corn!
  32. 32. Going Outside the Box of a Species: Bread Wheat AA (diploid wheat) x DD (diploid grass) AD (sterile diploid) similar to mule! AADD (fertile tetraploid) Triticum urartu Aegilops tauschii colchicine or spontaneous BB (diploid grass) BAD (sterile triploid) colchicine or spontaneous BBAADD Fertile hexaploid Bread Wheat Triticum aestivum Bread Wheat Created 7500 BC with spontaneous doubling of DNA Levy A. A. and M. Feldman. 2002. The impact of polyploidy on grass genome evolution. Plant Physiol. 130: 1587-1593. Aegilops speltoides
  33. 33. Environmental Impact on Phenotype pH of the soil will change the color of hydrangea flowers from blue to pink

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