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Mendel and Inheritance

Mendel and Inheritance. MUPGRET Workshop December 4, 2004. Genetic variation. In the beginning geneticists studied differences they could see in plants. These differences are called morphological differences.

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Mendel and Inheritance

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  1. Mendel and Inheritance MUPGRET Workshop December 4, 2004

  2. Genetic variation • In the beginning geneticists studied differences they could see in plants. • These differences are called morphological differences. • Individual variants are referred to as phenotypes, ex. tall vs. short plants or red vs. white flowers.

  3. Trait • A broad term encompassing a distribution of phenotypic variation. • Example: • Trait: Disease resistance • Phenotype: resistant vs. susceptible • Morphological differences associated with the trait might include fungal infection, fungal growth, sporulation, etc.

  4. Mendel • Monk at the St. Thomas monastery in the Czech Republic. • Performed several experiments between 1856 and 1863 that were the basis for what we know about heredity today. • Used garden peas for his research. • Published his work in 1866.

  5. Mendel • Results are remarkably accurate and some have said they were too good to be unbiased. • His papers were largely ignored for more than 30 years until other researchers appreciated its significance.

  6. Garden Pea • Pisum sativum • Diploid • Differed in seed shape, seed color, flower color, pod shape, plant height, etc. • Each phenotype Mendel studied was controlled by a single gene.

  7. Terms • Wild-type is the phenotype that would normally be expected. • Mutant is the phenotype that deviates from the norm, is unexpected but heritable. • This definition does not imply that all mutants are bad; in fact, many beneficial mutations have been selected by plant breeders.

  8. Advantages of plants • Can make controlled hybrids. • Less costly and time consuming to maintain than animals. • Can store their seed for long periods of time. • One plant can produce tens to hundreds of progeny.

  9. Advantages of plants • Can make inbreds in many plant species without severe effects that are typically seen in animals. • Generation time is often much less than for animals. • Fast plants (Brassica sp.) • Arabidopsis

  10. Allele • One of two to many alternative forms of the same gene (eg., round allele vs. wrinkled allele; yellow vs. green). • Alleles have different DNA sequences that cause the different appearances we see.

  11. X Parental Lines Round Wrinkled All round F1 progeny Self-pollinate 3 Round : 1 Wrinkled Round 5474 Wrinkled 1850 Principle of Segregation(Mendel’s First Law)

  12. Important Observations • F1 progeny are heterozygous but express only one phenotype, the dominant one. • In the F2 generation plants with both phenotypes are observedsome plants have recovered the recessive phenotype. • In the F2 generation there are approximately three times as many of one phenotype as the other.

  13. Mendel’s Results

  14. 3 : 1 Ratio • The 3 : 1 ratio is the key to interpreting Mendel’s data and the foundation for the the principle of segregation.

  15. The Principle of Segregation • Genes come in pairs and each cell has two copies. • Each pair of genes can be identical (homozygous) or different (heterozygous). • Each reproductive cell (gamete) contains only one copy of the gene.

  16. Mendel’s Principle of Segregation • In the formation of gametes, the paired hereditary determinants separate (segregate) in such a way that each gamete is equally likely to contain either member of the pair. • One male and one female gamete combine to generate a new individual with two copies of the gene.

  17. X Parental Lines Round Wrinkled All round F1 progeny Self-pollinate 3 Round : 1 Wrinkled Round 5474 Wrinkled 1850 Round vs. Wrinkled

  18. Round vs. wrinkled • The SBEI causes the round vs. wrinkled phenotype. • SBEI = starch-branching enzyme • Wrinkled peas result from absence of the branched form of starch called amylopectin. • When dried round peas shrink uniformly and wrinkled do not.

  19. Round vs. wrinkled • The non-mutant or wild-type round allele is designated W. • The mutant, wrinkled allele is designated w. • Seeds that are Ww have half the SBEI of wild-type WW seeds but this is enough to make the seeds shrink uniformly. • W is dominant over w.

  20. Round vs. wrinkled • An extra DNA sequence is present in the wrinkled allele that produces a non-functional SBEI and blocks the starch synthesis pathway at this step resulting in a lack of amylopectin.

  21. A Molecular View Parents F1 F2 Progeny WW ww Ww ¼WW ¼Ww ¼wW ¼ww 1: 2 : 1 Genotype = 3: 1 Phenotype

  22. Dihybrid crosses reveal Mendel’s law of independent assortment • A dihybrid is an individual that is heterozygous at two genes • Mendel designed experiments to determine if two genes segregate independently of one another in dihybrids • First constructed true-breeding lines for both traits, crossed them to produce dihybrid offspring, and examined the F2 for parental or recombinant types (new combinations not present in the parents).

  23. Mendel and two genes Round Yellow Wrinkled Green x All F1 Round, Yellow Wrinkled Yellow 101 Wrinkled Green 32 Round Yellow 315 Round Green 108

  24. Dihybrid cross produces a predictable ratio of phenotypes genotype phenotype number phenotypic ratio • Parent Y_R_ 315 9/16 • Recombinant yyR_ 108 3/16 • Recombinant Y_rr 101 3/16 • Parent yyrr 32 1/16 Ratio of yellow (dominant) to green (recessive)=3:1 (12:4) Ratio of round (dominant) to wrinkled (recessive)=3:1 (12:4)

  25. Ratio for a cross with 2 genes • Crosses with two genes are called dihybrid. • Dihybrid crosses have genetic ratios of 9:3:3:1.

  26. Mendel and two genes Wrinkled Yellow 101 Wrinkled Green 32 Round Yellow 315 Round Green 108 Yellow = 416 Green = 140 Round = 423 Wrinkled = 133 Each gene has a 3 : 1 ratio.

  27. Summary of Mendel's work • Inheritance is particulate - not blending • There are two copies of each trait in a germ cell • Gametes contain one copy of the trait • Alleles (different forms of the trait) segregate randomly • Alleles are dominant or recessive - thus the difference between genotype and phenotype • Different traits assort independently

  28. Rules of Probability Independent events - probability of two events occurring together What is the probability that both A and B will occur? Solution = determine probability of each and multiply them together. Mutually exclusive events - probability of one or another event occurring. What is the probability of A or B occurring? Solution = determine the probability of each and add them together.

  29. PRODUCT RULE From James Birchler

  30. SUM RULE Mutually exclusive ways! From James Birchler

  31. From James Birchler

  32. All Dominant Dominant Recessive All Recessive From James Birchler

  33. Punnett Square method - 24 = 16 possible gamete combinations for each parent Thus, a 16  16 Punnett Square with 256 genotypes That’s one big Punnett Square! Loci (Genes) Assort Independently - So we can look at each locus independently to get the answer. Branch diagrams are also convenient tools

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