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Lecture 18

Lecture 18. Genetics. Outline. Recombination – crossing over Basic Genetic concepts Genetic terms (Genotype, Phenotype, F1…) Genetic Tools ( Punnett Squares, Probabilities, Pedigrees). Review. Review. Alleles – different versions of the same gene

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Lecture 18

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  1. Lecture 18 Genetics

  2. Outline • Recombination – crossing over • Basic Genetic concepts • Genetic terms (Genotype, Phenotype, F1…) • Genetic Tools (Punnett Squares, Probabilities, Pedigrees)

  3. Review

  4. Review • Alleles – different versions of the same gene • Maternal Allele – the version of the gene from your mother • Paternal Allele – the version of the gene from your father

  5. Independent Assortment • Homologous pairs of chromosomes orient randomly at metaphase I of meiosis • Each pair of chromosomes sorts maternal and paternal homologs into daughter cells independently of every other pair

  6. Independent Assortment • The number of combinations possible when chromosomes assort independently into gametes is 2n, where n is the haploid number • For humans (n = 23), there are more than 8 million (223) possible combinations of chromosomes

  7. Figure 13.10-1 Possibility 2 Possibility 1 Two equally probablearrangements ofchromosomes atmetaphase I

  8. Figure 13.10-2 Possibility 2 Possibility 1 Two equally probablearrangements ofchromosomes atmetaphase I Metaphase II

  9. Figure 13.10-3 Possibility 2 Possibility 1 Two equally probablearrangements ofchromosomes atmetaphase I Metaphase II Daughtercells Combination 1 Combination 2 Combination 3 Combination 4 Followed by Random Fertilization

  10. Crossing Over • Crossing over produces recombinant chromosomes, which combine DNA inherited from each parent • Crossing over begins very early in prophase I, as homologous chromosomes pair up gene by gene

  11. Crossing Over • In crossing over, homologous portions of two nonsisterchromatids trade places • Crossing over contributes to genetic variation by combining DNA from two parents into a single chromosome

  12. Figure 13.11-1 Prophase Iof meiosis Nonsister chromatidsheld togetherduring synapsis Pair of homologs

  13. Figure 13.11-2 Prophase Iof meiosis Nonsister chromatidsheld togetherduring synapsis Pair of homologs Chiasma Centromere TEM

  14. Figure 13.11-3 Prophase Iof meiosis Nonsister chromatidsheld togetherduring synapsis Pair of homologs Chiasma Centromere TEM Anaphase I

  15. Figure 13.11-4 Prophase Iof meiosis Nonsister chromatidsheld togetherduring synapsis Pair of homologs Chiasma Centromere TEM Anaphase I Anaphase II

  16. Figure 13.11-5 Prophase Iof meiosis Nonsister chromatidsheld togetherduring synapsis Pair of homologs Chiasma Centromere TEM Anaphase I Anaphase II Daughtercells Recombinant chromosomes

  17. Summary of genetic variation • Three mechanisms contribute to genetic variation • Independent assortment of chromosomes • Crossing over • Random fertilization

  18. Figure 13.7-3 2 1 Interphase Pair of homologouschromosomes indiploid parent cell Chromosomesduplicate Duplicated pairof homologouschromosomes Sisterchromatids Diploid cell withduplicatedchromosomes Meiosis I Homologouschromosomes separate Haploid cells withduplicated chromosomes Meiosis II Sister chromatidsseparate Haploid cells with unduplicated chromosomes

  19. Figure 14.2 3 2 1 4 5 TECHNIQUE Parentalgeneration(P) Stamens Carpel RESULTS First filialgenerationoffspring(F1)

  20. Figure 14.3-1 EXPERIMENT P Generation (true-breedingparents) Purpleflowers Whiteflowers

  21. Figure 14.3-2 EXPERIMENT P Generation (true-breedingparents) Purpleflowers Whiteflowers F1 Generation(hybrids) All plants had purple flowers Self- or cross-pollination

  22. Figure 14.3-3 EXPERIMENT P Generation (true-breedingparents) Purpleflowers Whiteflowers F1 Generation(hybrids) All plants had purple flowers Self- or cross-pollination F2 Generation 705 purple-floweredplants 224 whitefloweredplants

  23. Table 14.1

  24. Terms • Trait/Phenotype/Genotype • Generations: Parental, F1, F2 • Self pollination vs Cross pollination • True breeding • Hybrid

  25. Mendel’s Model • Mendel developed a hypothesis to explain the 3:1 inheritance pattern he observed in F2 offspring • Four related concepts make up this model • We now know the molecular explanation for this model

  26. 1st Concept To Explain 3:1 Pattern in F2 generation • First: alternative versions of genes account for variations in inherited characters • One Gene: Purple flower – White Flower • These alternative versions of a gene are alleles • Each gene resides at a specific locus on a specific chromosome

  27. Figure 14.4 Allele for purple flowers Pair ofhomologouschromosomes Locus for flower-color gene Allele for white flowers

  28. 2nd Concept To Explain 3:1 Pattern in F2 generation • Second: for each character (phenotype), an organism inherits two alleles, one from each parent • The two alleles at a particular locus may be identical, as in the true-breeding plants of Mendel’s P generation • Alternatively, the two alleles at a locus may differ, as in the F1 hybrids

  29. 3rd Concept To Explain 3:1 Pattern in F2 generation • Third: if the two alleles at a locus differ, then one (the dominant allele) determines the organism’s appearance, and the other (the recessive allele) has no noticeable effect on appearance • In the flower-color example, the F1 plants had purple flowers because the allele for that trait is dominant

  30. 4th Concept To Explain 3:1 Pattern in F2 generation • Fourth: Thelaw of independent segregation: the two alleles for a heritable characteristic (phenotype) separate (segregate) during gamete formation and end up in different gametes • An egg or a sperm get only one of the two alleles • Allele segregation is because homologous chromosomes segregate during meiosis

  31. TECHNIQUE Figure 14.7 Dominant phenotype,unknown genotype:PP or Pp? Recessive phenotype,known genotype:pp Predictions If purple-floweredparent is PP If purple-floweredparent is Pp or Sperm Sperm p p p p P P Pp Pp Pp Pp Eggs Eggs P p pp pp Pp Pp RESULTS or All offspring purple 1/2 offspring purple and1/2 offspring white

  32. Rr Rr  Figure 14.9 Segregation ofalleles into sperm Segregation ofalleles into eggs Sperm r 1/2 1/2 R R R r R R 1/2 1/4 1/4 Eggs r r r R r 1/2 1/4 1/4

  33. Figure 14.8 EXPERIMENT YYRR yyrr P Generation Gametes yr YR F1 Generation YyRr Hypothesis ofdependent assortment Predictions Hypothesis ofindependent assortment Sperm or Predictedoffspring ofF2 generation 1/4 1/4 1/4 1/4 yR yr Yr YR Sperm 1/2 YR 1/2 yr 1/4 YR YYRR YYRr YyRR YyRr 1/2 YR YyRr YYRR 1/4 Yr Eggs YYRr YYrr Yyrr YyRr Eggs 1/2 yr YyRr yyrr 1/4 yR YyRr yyRr YyRR yyRR 3/4 1/4 yr 1/4 Phenotypic ratio 3:1 Yyrr yyRr YyRr yyrr 3/16 3/16 1/16 9/16 Phenotypic ratio 9:3:3:1 RESULTS 108 101 315 Phenotypic ratio approximately 9:3:3:1 32

  34. Figure 14.UN02 1/4 (probability of pp)  1/2 (yy)  1/2 (Rr)  1/16 ppyyRr  1/16 ppYyrr 1/41/21/2  2/16 Ppyyrr 1/21/21/2  1/16 1/41/21/2 PPyyrr ppyyrr  1/16 1/41/21/2  6/16 or 3/8 Chance of at least two recessive traits

  35. The ability to curl your tongue up on the sides (T, tongue rolling) is dominant to not being able to roll your tongue. A woman who can roll her tongue marries a man who cannot. Their first child has his father's phenotype. What are the genotypes of the mother, father, and child? • What is the probability that a second child won't be a tongue roller? 

  36. Often inheritance patterns are more complicated • Many heritable characters are not determined by only one gene with two alleles • Basic principles of segregation and independent assortment apply even to more complex patterns of inheritance

  37. Examples of single gene not following Mendelian patterns • Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations: • When alleles are not completely dominant or recessive • When a gene has more than two alleles • When a gene produces multiple phenotypes

  38. Degrees of Dominance • Complete dominance: phenotypes of the heterozygote and dominant homozygote are identical • Incomplete dominance, the phenotype of F1 hybrids is in between the phenotypes of the two parental varieties • Codominance, two dominant alleles affect the phenotype in separate, distinguishable ways

  39. Figure 14.10-1 P Generation White Red CWCW CRCR Gametes CW CR

  40. Figure 14.10-2 P Generation White Red CWCW CRCR Gametes CW CR F1 Generation Pink CRCW 1/2 1/2 CR Gametes CW

  41. Figure 14.10-3 P Generation White Red CWCW CRCR Gametes CW CR F1 Generation Pink CRCW 1/2 1/2 CR CW Gametes Sperm F2 Generation 1/2 1/2 CW CR 1/2 CR CRCR CRCW Eggs 1/2 CW CRCW CWCW

  42. Tay-Sachsdisease is fatal; a dysfunctional enzyme causes an accumulation of lipids in the brain • At the organismal level, the allele is recessive • At the biochemical level, the phenotype (i.e., the enzyme activity level) is incompletely dominant • At the molecular level, the alleles are codominant

  43. Multiple Alleles • Most genes exist in populations in more than two allelic forms • The ABO blood group in humans are determined by three alleles • Single Gene codes for an enzyme that attaches a specific carbohydrate to the surface of the RBC • IA allele – The enzyme adds the A carbohydrate • IB allele – The enzyme adds the B carbohydrate • i allele – Adds neither

  44. Figure 14.11 (a) The three alleles for the ABO blood groups and their carbohydrates Allele IA IB i none Carbohydrate B A (b) Blood group genotypes and phenotypes Genotype ii IAIA or IAi IBIB or IBi IAIB Red blood cellappearance Phenotype(blood group) A AB O B

  45. Pleotrophy • Most genes have multiple phenotypic effects, a property called pleiotropy • Pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease • Some traits may be determined by two or more genes

  46. Epistasis • In epistasis, a gene at one locus alters the phenotypic expression of a gene at a second locus • Labrador retrievers and many other mammals, coat color depends on two genes • One gene determines the pigment color (with alleles B for black and b for brown) • The other gene (with alleles C for color and c for no color) determines whether the pigment will be deposited in the hair

  47. Figure 14.12 BbEe BbEe Sperm 1/4 1/4 1/4 1/4 Be BE be bE Eggs 1/4 BE BbEE BBEe BbEe BBEE 1/4 bE BbEE bbEe bbEE BbEe 1/4 Be BBEe BBee Bbee BbEe 1/4 be BbEe bbEe bbee Bbee : 3 9 : 4

  48. Polygenic Inheritance • Quantitative characters are those that vary in the population along a continuum • Quantitative variation usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotype • Skin color in humans is an example of polygenic inheritance

  49. Nature vs. Nurture

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