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MENDELIAN GENETICS

MENDELIAN GENETICS

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MENDELIAN GENETICS

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  1. MENDELIAN GENETICS What is genetics? The study of how traits are inherited or how genetic information is passed from one generation to the next. It also explains biological variation

  2. Gregor Mendel • 1850’s Grew up in a farm wanting to garden • Austrian monk (Flunked out of college twice) but became a mathematician • Experimented with garden pea plants • Using pea plants looked at seven different characters (height of plants, seed color, texture, flower color) and found evidence of how parents transmit genes to offspring • Mendel’s statistical analysis provided a model for predicting what the next generation would be like

  3. What was the prevalent believe about inheritance before Mendel? • People believed in “spontaneous generation” and in the “blending of characters” • Blending theory • Problem: • Would expect variation to disappear • Variation in traits persists Ex: Yellow and green parakeets should have all blue babies. This is not what you observe.

  4. The gene theory • An alternative idea is the “gene” idea. Parents pass on discrete individual heritable units: genes

  5. Petal Stamen Carpel Figure 9.2 A Figure 9.2 B • Experimental genetics began in an abbey garden • Modern genetics • Began with Gregor Mendel’s quantitative experiments with pea plants

  6. The Garden Pea Plant • Mendel chose to work with the pea plant because he could control which plant mated with which. Pea plants are • Self-pollinating • True breeding (different alleles not normally introduced) • Can be experimentally cross-pollinated

  7. Mendel crossed pea plants that differed in certain characteristics • And traced traits from generation to generation • Mendel started his experiments with plants that were “true breeding”. 1Removed stamens from purple flower White 2 Transferred pollen from stamens of white flower to carpel of purple flower Stamens Carpel Parents(P) Purple 3 Pollinated carpel matured into pod 4 Planted seeds from pod Offspring(F1) Figure 9.2 C

  8. Purple Flower color White Terminal Axial Flower position Green Yellow Seed color Seed shape Round Wrinkled Pod shape Inflated Constricted Green Yellow Pod color Tall Stem length Dwarf • Mendel hypothesized that there are alternative forms of genes • The units that determine heritable traits Figure 9.2 D

  9. Mendel’s Principles of Genetics • Mendel refuted the “blending theory” of heredity and provided an explanation of how inheritance works without knowing anything about chromosomes or genes. • He figured that traits must be coded for by some kind of inheritable particle which he called “factors” and now we call “genes”. • He said that those genes were transmitted as independent entities from one generation to the next.

  10. Mendel’s insight continued…3. He figured that there must be different versions of these “genes”( we call them now “alleles”)and that every individual has two genes for each trait. (Or we can say that: For each characteristican organism inherits two alleles, one from each parent)He identified one as dominant, the other as recessive.

  11. 4. He figured that the two alleles a parent has are separated into different cells when gametes (sex cells) are formed. This actuallyhappens during metaphase of meiosisI ( no one knew about meiosis in those days). This is known as the Law of SegregationWhat are alleles?Different versions of the same gene

  12. Mendel’s Theory of Segregation • An individual inherits a unit of information (allele) about a trait from each parent • During gamete formation, the alleles segregate from each other

  13. P plants Genetic makeup (alleles) pp PP Gametes All p All P F1 plants (hybrids) All Pp 1 2 1 2 P p Gametes Sperm p P F2 plants Phenotypic ratio 3 purple : 1 white Pp P PP Eggs Genotypic ratio 1 PP: 2 Pp: 1 pp Pp p pp • Mendel’s law of segregation • Predicts that allele pairs separate from each other during the production of gametes Figure 9.3 B

  14. P generation (true-breedingparents)  Purple flowers White flowers F1 generation All plants havepurple flowers Fertilizationamong F1 plants(F1 F1) F2 generation 3 4 1 4 of plantshave white flowers of plantshave purple flowers • Mendel’s law of segregation describes the inheritance of a single characteristic • From his experimental data • Mendel deduced that an organism has two genes (alleles) for each inherited characteristic Figure 9.3 A

  15. What is a dominant trait?The trait that shows, the allele that is fullyexpressedWhat is a recessive trait?The alleles that is masked, the gene is there but it doesn’t showWhat is the phenotype?The observable traitsWhat is the genotype?The genetic make up

  16. If the two alleles of an inherited pair differ • Then one determines the organism’s appearance and is called the dominant allele ( use capital letters) • The other allele • Has no noticeable effect on the organism’s appearance and is called the recessive allele

  17. Vocabulary • When you mate two contrasting true breeding plants you get a Hybrid. • The true breeding parents are called the “P” (parent) generation • The hybrid offspring of the P generation are called the F1 generation • When two F1 individuals self pollinate you get the F2 generation

  18. F1 Results of One Monohybrid Cross

  19. F2 Results of Monohybrid Cross

  20. Mendel’s Monohybrid Cross Results 5,474 round 1,850 wrinkled 6,022 yellow 2,001 green 882 inflated 299 wrinkled 428 green 152 yellow F2 plants showed dominant-to-recessive ratio that averaged 3:1 705 purple 224 white 651 long stem 207 at tip 787 tall 277 dwarf

  21. Female gametes A a A AA Aa Male gametes a Aa aa Punnett Square of a Monohybrid Cross Dominant phenotype can arise 3 ways, recessive only one

  22. A Test cross • In a pea plant with purple flowers the genotype is not obvious. Could be homozygous or heterozygous • Why do a test cross? It allows us to determine the genotype of an organism with a dominant phenotype but unknown genotype

  23. Test Cross • You cross an individual that shows the dominant phenotype with an individual with recessive phenotype ( one who is homozygous recessive for that trait) • Examining offspring allows you to determine the genotype of the dominant individual

  24. Punnett Squares of Test Crosses Homozygous recessive Homozygous recessive a a a a A A Aa Aa Aa Aa A a aa Aa aa Aa Two phenotypes All dominant phenotype

  25. Testcross: Genotypes bb B_ Two possibilities for the black dog: BB or Bb Gametes B b B b Bb b bb Bb 1 black : 1 chocolate All black Offspring Geneticists use the testcross to determine unknown genotypes • The offspring of a testcross, a mating between an individual of unknown genotype and a homozygous recessive individual • Can reveal the unknown’s genotype Figure 9.6

  26. Dominantallele Gene loci a B P a b P Recessiveallele Genotype: PP aa Bb Heterozygous Homozygousfor thedominant allele Homozygousfor therecessive allele Homologous chromosomes bear the two alleles for each characteristic • Alternative forms of a gene • Reside at the same locus on homologous chromosomes Figure 9.4

  27. Web sites to check • http://gslc.genetics.utah.edu/units/basics/tour/inheritance.swf • http://science.nhmccd.edu/biol/genetics.html • http://library.thinkquest.org/20465/games.html

  28. Mendel’s two Laws • 1. Law of segregation The two alleles for a trait segregate during gameteformation and only one allele for a trait is carriedin a gamete. The gametes combine at random (In other words:A cell contains two copies of a particular gene, they separate when a gamete is made). • 2. Law of Independent Assortment Alleles from one trait behave independently fromalleles for another trait. Traits are inherited independently from one another

  29. Independent Assortment • Mendel concluded that the two “units” for the first trait were to be assorted into gametes independently of the two “units” for the other trait • Members of each pair of homologous chromosomes are sorted into gametes at random during meiosis

  30. The law of independent assortment is revealed by tracking two characteristics at once • By looking at two characteristics at once • Mendel tried to determine how two characteristics were inherited

  31. Hypothesis: Independent assortment Hypothesis: Dependent assortment RRYY P generation rryy RRYY rryy ry ry Gametes Gametes RY  RY RrYy RrYy F1 generation Sperm Sperm 1 4 1 4 1 4 1 4 ry ry RY RY 1 2 1 2 ry RY 1 4 RY 1 2 RY RrYY RRYY RRYy RrYy F2 generation Eggs 1 4 ry 1 2 ry rrYY rrYy RrYy RrYY Eggs Yellowround 9 16 1 4 Ry RrYy RRyy RRYy Rryy Greenround 3 16 1 4 ry Yellowwrinkled Actual resultscontradict hypothesis 3 16 rryy RrYy rrYy Rryy Greenwrinkled 1 16 Actual resultssupport hypothesis • Mendel’s law of independent assortment • States that alleles of a pair segregate independently of other allele pairs during gamete formation Figure 9.5 A

  32. Blind Blind Phenotypes Genotypes Black coat, normal vision B_N_ Black coat, blind (PRA) B_nn Chocolate coat, normal vision bbN_ Chocolate coat, blind (PRA) bbnn Mating of heterozygotes (black, normal vision) BbNn  BbNn 9 black coat, normal vision 3 black coat, blind (PRA) 1 chocolate coat, blind (PRA) 3 chocolate coat, normal vision Phenotypic ratio of offspring Figure 9.5 B • An example of independent assortment

  33. A Dihybrid Cross - F1 Results purple flowers, tall white flowers, dwarf TRUE- BREEDING PARENTS: AABB x aabb GAMETES: AB AB ab ab AaBb F1 HYBRID OFFSPRING: All purple-flowered, tall

  34. ab ab aB AB AB Ab Ab aB 16 Allele Combinations in F2 1/4 1/4 1/4 1/4 1/4 1/16 1/16 1/16 1/16 AABB AABb AaBB AaBb 1/4 1/16 1/16 1/16 1/16 AABb AAbb AaBb Aabb 1/4 1/16 1/16 1/16 1/16 AaBB AaBb aaBB aaBb 1/4 1/16 1/16 1/16 1/16 AaBb Aabb aaBb aabb

  35. Phenotypic Ratios in F2 Four Phenotypes: • Tall, purple-flowered (9/16) • Tall, white-flowered (3/16) • Dwarf, purple-flowered (3/16) • Dwarf, white-flowered (1/16) AaBbX AaBb

  36. ab ab aB AB AB Ab Ab aB Explanation of Mendel’s Dihybrid Results If the two traits are coded for by genes on separate chromosomes, sixteen gamete combinations are possible 1/4 1/4 1/4 1/4 1/4 1/16 1/16 1/16 1/16 AABB AABb AaBB AaBb 1/4 1/16 1/16 1/16 1/16 AABb AAbb AaBb Aabb 1/4 1/16 1/16 1/16 1/16 AaBB AaBb aaBB aaBb 1/4 1/16 1/16 1/16 1/16 AaBb Aabb aaBb aabb

  37. Mendel’s laws reflect the rules of probability • Inheritance follows the rules of probability

  38. F1 genotypes Bbmale Formation of sperm Bbfemale Formation of eggs 1 2 1 2 b B b B B B 1 2 B 1 4 1 4 F2 genotypes 1 2 B b b b b 1 4 1 4 • The rule of multiplication • Calculates the probability of two independent events • The rule of addition • Calculates the probability of an event that can occur in alternate ways Figure 9.7

  39. Dominant Traits Recessive Traits Freckles No freckles Widow’s peak Straight hairline Free earlobe Attached earlobe Genetic traits in humans can be tracked through family pedigrees • The inheritance of many human traits • Follows Mendel’s laws Figure 9.8 A

  40. D ? John Eddy Dd Abigail Linnell D ? Hepzibah Daggett Dd Joshua Lambert dd Jonathan Lambert Dd Elizabeth Eddy D ? Abigail Lambert Dd Dd dd Dd Dd Dd dd Female Male Deaf Hearing • Family pedigrees • Can be used to determine individual genotypes Figure 9.8 B

  41. Parents Normal Dd Normal Dd  Sperm Dd Dd Normal (carrier) DD Normal D Offspring Eggs Dd Normal (carrier) dd Deaf d • Recessive Disorders • Most human genetic disorders are recessive Figure 9.9 A

  42. VARIATIONS ON MENDEL’S LAWS The relationship of genotype to phenotype is rarely simple • Mendel’s principles are valid for all sexually reproducing species • But genotype often does not dictate phenotype in the simple way his laws describe

  43. Genetics is not as simple as Gregor Mendel concluded, (one gene, one trait). • We know now that there is a range of dominance and • that genescan work togetherand interact. • Incomplete dominance: • When the F1 generation have an appearance in between the • phenotypes of the parents. • Ex: pink snapdragons offspring of red and white ones. • Another way to say it is • Inincomplete dominance • Heterozygote phenotype is somewhere between that of two • homozygotes

  44. Flower Color in Snapdragons: Incomplete Dominance Red-flowered plant X White-flowered plant Pink-flowered F1 plants (homozygote) (homozygote) (heterozygotes)

  45. Incomplete dominance in snapdragon color

  46. Flower Color in Snapdragons: Incomplete Dominance • Red flowers - two alleles allow them to make a red pigment • White flowers - two mutant alleles; can’t make red pigment • Pink flowers have one normal and one mutant allele; make a smaller amount of red pigment

  47. Flower Color in Snapdragons: Incomplete Dominance Pink-flowered plant X Pink-flowered plant White-, pink-, and red-flowered plants in a 1:2:1 ratio (heterozygote) (heterozygote)

  48. Incomplete dominance in carnations

  49. Co-Dominance or multiple alleles: • Codominance • Non-identical alleles specify two phenotypes that are both expressed in heterozygotes • Having more than 2 alleles for a given trait and both alleles show in the phenotype. No single one is dominant over the other. • Example: ABO blood types

  50. Genetics of ABO Blood Types: Three Alleles • Gene that controls ABO type codes for enzyme that dictates structure of a glycolipid on blood cells • Two alleles (IA and IB) are codominant when paired • Third allele (i) is recessive to others