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Introduction to Genetics

Introduction to Genetics. Purebreds and Mutts–A Difference of Heredity Purebred dogs are very similar. Mutts, or mixed breed dogs show considerably more genetic variation. Early Ideas about Heredity. Sperm and eggs transmitted information Blending theory Problem: Variation would disappear.

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Introduction to Genetics

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  1. Introduction to Genetics

  2. Purebreds and Mutts–A Difference of Heredity • Purebred dogs are very similar

  3. Mutts, or mixed breed dogs show considerably more genetic variation

  4. Early Ideas about Heredity • Sperm and eggs transmitted information • Blending theory • Problem: • Variation would disappear

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

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

  7. 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 Gregor Mendel • Mendel hypothesized that there are alternative forms of genes • The units that determine heritable traits Figure 9.2 D

  8. Genes • Units of information about specific traits • Passed from parents to offspring • Each has a specific location (locus) on a chromosome

  9. Alleles • Different molecular forms of a gene • Arise by mutation • Dominant allele masks a recessive allele

  10. Allele Combinations • Homozygous • having two identical alleles at a locus • AA or aa • Heterozygous • having two different alleles at a locus • Aa

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

  12. Genotype & Phenotype • Genotype refers genes an individual carries • Phenotype refers to an individual’s observable traits • Cannot always determine genotype by observing phenotype

  13. Tracking Generations • Parental generation P mates to produce • First-generation offspring F1 mate to produce • Second-generation offspring F2

  14. 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 Theory of Segregation • Allele pairs separate from each other during the production of gametes Figure 9.3 B

  15. Mendel’s Theory of Segregation • An individual inherits a unit of information (allele) about a trait from each parent

  16. 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 Independent Assortment • Alleles of a pair segregate independently of other allele pairs during gamete formation Figure 9.5 A

  17. Independent Assortment Metaphase I: OR A A a a A A a a B B b b b b B B Metaphase II: A A a a A A a a B B b b b b B B Gametes: B B b b b b B B A A a a A A a a 1/4 AB 1/4 ab 1/4 Ab 1/4 aB

  18. 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 Independent Assortment

  19. Monohybrid Crosses Experimental intercross between two F1 heterozygotes AA X aa Aa (F1 monohybrids) Aa X Aa ?

  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 705 purple 224 white 651 long stem 207 at tip 787 tall 277 dwarf

  21. True-breeding homozygous recessive parent plant F1PHENOTYPES aa True-breeding homozygous dominant parent plant Aa Aa a a Aa Aa A AA A Aa Aa Aa Aa An F1 plant self-fertilizes and produces gametes: F2PHENOTYPES Aa AA Aa A a A AA Aa a Aa aa Aa aa Monohybrid CrossIllustrated

  22. Test Cross • Individual with dominant phenotype is crossed with individual with recessive phenotype • Examining offspring determines the genotype

  23. 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 Test Cross

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

  25. Dihybrid Cross Cross between individuals that are homozygous for different versions of two traits

  26. 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

  27. Dihybrid Cross: F2 Results X AaBb AaBb 1/4 AB 1/4 Ab 1/4 aB 1/4 ab 9/16 purple-flowered, tall 1/4 AB 1/16 AABB 1/16 AABb 1/16 AaBB 1/16 AaBb 3/16 purple-flowered, dwarf 3/16 white-flowered, tall 1/16 AaBb 1/16 AAbb 1/16 Aabb 1/4 Ab 1/16 AABb 1/16 white-flowered, dwarf 1/16 AaBB 1/16 aaBB 1/16 aaBb 1/4 aB 1/16 AaBb 1/16 Aabb 1/16 aaBb 1/16 aabb 1/16 AaBb 1/4 ab

  28. Probability The chance that each outcome of a given event will occur is proportional to the number of ways that event can be reached

  29. 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 Probability • The rule of multiplication • Probabilty of aa = ? • Probabilty of aa and bb? • Independent events Figure 9.7

  30. Table 9.9 Inherited Disorders • Many are controlled by a single gene

  31. Dominance Relations Complete dominance Incomplete dominance Codominance

  32. Incomplete Dominance X Homozygous parent Homozygous parent All F1 are heterozygous X F2 shows three phenotypes in 1:2:1 ratio

  33. Reaction When Blood from Groups Below Is Mixed with Antibodies from Groups at Left Blood Group (Phenotype) Antibodies Present in Blood Genotypes O A B AB Anti-A Anti-B ii O IAIA or IAi Anti-B A IBIB or IBi Anti-A B — AB IAIB Codominance: ABO Blood Types • Gene that controls ABO type codes for a glycolipid on blood cells • Two alleles (IA and IB) are codominant when paired • Third allele (i) is recessive to others

  34. Individual homozygous for sickle-cell allele Sickle-cell (abnormal) hemoglobin Abnormal hemoglobin crystallizes, causing red blood cells to become sickle-shaped Sickle cells 5,555 Clumping of cells and clogging of small blood vessels Breakdown of red blood cells Accumulation of sickled cells in spleen Brain damage Pain and fever Spleen damage Damage to other organs Physical weakness Heart failure Anemia Impaired mental function Pneumonia and other infections Kidney failure Paralysis Rheumatism Pleiotropy • One gene may have effects on two or more traits • Sickle-cell disease

  35. Polygenic Inheritance • A range of small differences in a given trait among individuals • Effected by the number of genes and env. Factors

  36. P generation aabbcc (very light) AABBCC (very dark)  F1 generation AaBbCc AaBbCc 20 64 15 64 15 64 1 64 6 64 1 64 6 64 Sperm 1 8 1 8 1 8 1 8 1 8 1 8 1 8 1 8 20 64 1 8 F2 generation 1 8 15 64 1 8 Fraction of population 1 8 Eggs 1 8 6 64 1 8 1 8 1 64 1 8 Skin color

  37. Temperature Effects on Phenotype • Rabbit is homozygous heat-sensitive version of an enzyme • Melanin is produced in cooler areas of body

  38. Environmental Effects on Plant Phenotype • Hydrangea macrophylla • Flower color ranges from pink to blue

  39. Experiment Purple flower PpLI PpLI Long pollen • Observed Prediction • Phenotypes offspring (9:3:3:1) Purple long Purple round Red long Red round 215 71 71 24 284 21 21 55 Explanation: linked genes P L Parental diploid cell PpLI P I Meiosis Most gametes P L P I Fertilization Sperm P I P L P L P L P L Most offspring P L P I Eggs P I P I P I P I P L 3 purple long : 1 red round Not accounted for: purple round and red long Linkages • Certain genes are linked • Inheritedtogether because they are close together onthe same chromosome Figure 9.19

  40. A B a b A B a b A b a B Crossing over Tetrad Gametes Crossing Over • Crossing over can separate linked alleles • Producing gametes with recombinant chromosomes Figure 9.20 A

  41. Crossover Frequency A B C D

  42. A A a B B b A a B b a b Full Linkage x Parents: AB ab F1 offspring: All AaBb meiosis, gamete formation Equal ratios of two types of gametes: Figure 12.8aPage 201 50% AB 50% ab

  43. A a a c c C A C Incomplete Linkage AC ac x Parents: F1 offspring: All AaCc meiosis, gamete formation a a A A Unequal ratios of four types of gametes: C c C c parental genotypes recombinant genotypes Figure 12.8bPage 201

  44. homozygous dominant female recessive male Discovering Linkage x Gametes: X X X Y All F1 have red eyes x Gametes: X X X Y 1/2 1/2 1/4 1/2 1/2 1/4 1/4 F2 generation: 1/4

  45. Mutant phenotypes Black body (g) Cinnabar eyes (c) Vestigial wings (l) Brown eyes Short aristae Chromosome g l c 17% 9% 9.5% Recombination frequencies Normal wings (L) Red eyes Long aristae (appendages on head) Gray body (G) Red eyes (C) Wild-type phenotypes Discovering Linkage • Recombination frequencies • Used to map the relative positions of genes on chromosomes. Figure 9.21 B Figure 9.21 C

  46. X X x X Y x Y X X X XX XX XY XY female (XX) male (XY) Sex Determination eggs sperm Human sex determination interaction.

  47. appearance of structures that will give rise to external genitalia appearance of “uncommitted” duct system of embryo at 7 weeks Effect of YChromosome 7 weeks Y present Y absent Y present Y absent testes ovaries 10 weeks ovary testis birth approaching

  48. The X Chromosome • Carries more than 2,300 genes • Most genes deal with nonsexual traits • Genes on X chromosome can be expressed in both males and females

  49. 22 + X 22 + XX 76 + ZW 76 + ZZ 32 16 Sex Determination Figure 9.22 B Figure 9.22 C Figure 9.22 D

  50. X-Linked Recessive Inheritance • Mutant gene on X chromosome • Males affected more often

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