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Patterns Of Inheritance Modified



Slightly modified version of original powerpoint presentation on slide view to cater for a lower secondary science class. Certain slides deleted.

Slightly modified version of original powerpoint presentation on slide view to cater for a lower secondary science class. Certain slides deleted.



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  • Biology: Life on Earth (Audesirk) Chapter 12

Patterns Of Inheritance Modified Presentation Transcript

  • 1. Patterns of Inheritance
  • 2. Inheritance
    • Inheritance is the process by which the characteristics of individuals are passed to their offspring
    • Genes encode these characteristics
    • A gene is a unit of heredity that encodes information for the form of a particular characteristic
    • The location of a gene on a chromosome is called its locus
    Chapter 12
  • 3. Alleles
    • Homologous chromosomes carry the same kinds of genes for the same characteristics
    • Genes for the same characteristic are found at the same loci on both homologous chromosomes
    Chapter 12
  • 4. Alleles
    • Genes for a characteristic found on homologous chromosomes may not be identical
    • Alternate versions or forms of genes found at the same gene locus are called alleles
    Chapter 12
  • 5. Alleles
    • Each cell carries two alleles per characteristic, one on each of the two homologous chromosomes
    • If both homologous chromosomes carry the same allele (gene form) at a given gene locus, the organism is homozygous at that locus
    • If two homologous chromosomes carry different alleles at a given locus, the organism is heterozygous at that locus (a hybrid )
    Chapter 12
  • 6. Genes, Alleles, Loci, and Chromosomes Chapter 12 Chromosome from One Parent Homologous Chromosome from Other Parent M locus has gene that controls leaf color . Plant homozygous for this gene D locus has gene that controls plant height . Plant homozygous for this gene Bk locus has gene that controls fruit shape . Plant heterozygous for this gene 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 Loci: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 Loci:
  • 7. Definitions 1
    • Must know these!!!
    • Trait —A variable characteristic of organism
    • Gene —A segment of chromosomal DNA controlling a specific trait
    • Locus —Chromosomal position where DNA for a specific gene lives
    • Genome —Refers to all standard loci for a species
    Chapter 12
  • 8. Definitions 2
    • Must know these!!!
    • Alleles —Different forms of a gene
      • “ Flower color” is a gene;
      • “ Purple” is one flower-color allele
      • “ White” is another flower-color allele
    • Genotype —List of alleles for an individual at specific genes
      • Familiar organisms are diploid
      • One or two alleles per individual
    Chapter 12
  • 9. Definitions 3
    • Homozygous —Maternal & paternal alleles same
      • Father donates purple-flower allele
      • Mother donates purple-flower allele
    • Heterozygous —Maternal & paternal alleles differ
      • Father donates purple-flower allele
      • Mom donates white-flower allele
    Chapter 12
  • 10. Definitions 4
    • Phenotype :
      • List of traits exhibited by individual
      • Doesn’t always represent genotype
    • Dominant —Allele that is expressed 100% in heterozygote
    • Recessive —Allele is not expressed in heterozygote
    • Incomplete dominance —heterozygote displays intermediate trait
    Chapter 12
  • 11. Genetic Symbolism
    • Often use initial letter of dominant allele
      • Capital letter represents dominant
      • Lower case of same letter represents recessive
    • If purple flower dominant to white…
      • “ P” represents allele for purple
      • “ p” represents allele for white
    Chapter 12
  • 12. Cross Fertilization of Parents Chapter 12 True-breeding Purple-flowered Parent True-breeding White-flowered Parent Cross-Fertilize All Purple-flowered Offspring Pollen Pollen P P F 1
  • 13. Self-fertilization of F 2 Chapter 12 F 1 Self-Fertilize F 2 F 2 F 2 F 2 75% Purple 25% White
  • 14. Genotype vs Phenotype
    • Phenotype is how we look/behave
      • Purple flowers
      • White flowers
    • Genotype is what our genes say
      • White Flowers / White Flowers
      • White Flowers / Purple Flowers
      • Purple Flowers / Purple Flowers
    Chapter 12
  • 15. Genotype vs Phenotype 2
    • Genotypes
      • PP = homozygous for purple flower
      • pp = homozygous for white flower
      • Pp = heterozygous for flower color
    • Phenotype from genotype:
      • PP = purple flower
      • Pp = purple flower
      • pP = purple flower
      • pp = White flower
    Chapter 12
  • 16. How Meiosis Separates Genes
    • The two alleles for a characteristic separate during gamete formation (meiosis)
      • Homologous chromosomes separate in meiosis anaphase I
      • Each gamete receives one of each pair of homologous chromosomes and thus one of the two alleles per characteristic
      • The separation of alleles in meiosis is known as Mendel’s Law of Segregation
    Chapter 12
  • 17. Gametes of Homozygotes Chapter 12 A A Homozygous Parent Gametes All gametes identical regarding this gene A A
  • 18. Gametes of Heterozygotes Chapter 12 A a Heterozygous Parent Gametes Gametes 50/50 regarding this gene A a
  • 19. Homozygous Dominant X Homozygous Recessive Chapter 12 pp homozygous recessive P p P p Purple Parent PP homozygous dominant White Parent sperm nuclei egg nuclei sperm nuclei egg nuclei
  • 20. P Sperm + p Eggs same as p Sperm + P Eggs Chapter 12 Pp pP Purple F 1 Purple F 1 P p sperm nucleus egg nucleus + p P egg nucleus sperm nucleus +
  • 21. Pp X Pp Cross Chapter 12 Purple homozygous dominant (PP) Purple heterozygous (Pp) Purple heterozygous (pP) White homozygous recessive (pp) P p p P p P P p + + + + F 1 Sperm F 1 Eggs F 2 Offspring
  • 22. Using Punnett Squares in Genetic Crosses
    • Named after geneticist Reginald Punnett
    • Figured using Punnett squares
      • Considers only genes of interest
      • List sperm genotypes across top
      • List egg genotypes down side
      • Fill in boxes with zygote genotypes
    Chapter 12
  • 23. Consider Flower Color
    • Pretend flower color affected by only one gene ( monohybrid cross )
    • Assume all alleles are purple or white
    • Purple (P) is dominant to white (p)
    • Heterozygotes will have flowers as purple as homozygous dominants
    Chapter 12
  • 24. Making a Punnett Square: Heterozygous X Heterozygous Chapter 12 P p 1(25%) White 3 (75%) Purple Eggs of Heterozygous Plant Pollen of Heterozygous Plant 1 1 P p p P P p P P p p Frequencies Phenotypes Genotypes Frequencies 2 PP p p p P P p
  • 25. Practical Application: The Test Cross
    • A test cross is used to deduce the actual genotype of an organism with a dominant phenotype (i.e., is the organism PP or Pp ?)
      • Cross the unknown dominant-phenotype organism ( P _) with a homozygous recessive organism ( pp )…
    Chapter 12
  • 26. Practical Application: The Test Cross
      • 2. If the dominant-phenotype organism is homozygous dominant ( PP ), only dominant-phenotype offspring will be produced ( Pp )
      • If the dominant-phenotype organism is heterozygous ( Pp ), approximately half of the offspring will be of recessive phenotype ( pp )
    Chapter 12
  • 27. Test Cross: Heterozygous X Homozygous Recessive Chapter 12 p p (50%) White (50%) Purple Eggs of Homozygous Recessive Pollen of unknown plant with dominant phenotype (Heterozygous) P p p p P p P P p p Frequencies Phenotypes Genotypes Frequencies 2 Pp p p p P p p 2
  • 28. Test Cross: Homozygous X Homozygous Recessive Chapter 12 p p (100%) Purple Eggs of Homozygous Recessive Pollen of unknown plant with dominant phenotype (Homozygous) P P p P p P p P p P Frequencies Phenotypes Genotypes Frequencies Pp P p P p P p 4
  • 29. Traits of Peas Studied by Mendel Chapter 12 Plant size Flower location Flower color Pod color Pod shape Seed shape Seed color
  • 30. Traits Are Inherited Independently
    • Seed color (yellow vs. green peas) and seed shape (smooth vs. wrinkled peas) were the characteristics studied
    • The allele symbols were assigned:
      • Y = yellow (dominant), y = green (recessive)
      • S = smooth (dominant), s = wrinkled (recessive)
    • Two trait cross was between two true breeding varieties for each characteristic
      • P: SSYY x ssyy
    Chapter 12
  • 31. Recombination
    • Genes on the same chromosome do not always sort together
    • Crossing over in Prophase I of meiosis creates new gene combinations
    • Crossing over involves the exchange of DNA between chromatids of paired homologous chromosomes in synapsis
    Chapter 12
  • 32. Crossing Over Chapter 12 red red Purple Purple round round Long Long Sister Chromatids Sister Chromatids old combination new combination new combination old combination P P p p L L l l P p p L L l l P L p L l l P p P L p L l l P p L L l l P P p p P P p p L L l l P p p L L l l Duplicated Chromosome Duplicated Chromosome L L l l P P p p Homologous Chromosomes P P p p L L l l P p p L L l l p L P l L P l p Flower Color Pollen Shape
  • 33. Chapter 12
  • 34. Chapter 12
  • 35. Sex Chromosomes and Autosomes
    • Mammals and many insect species have a set of sex chromosomes that dictate gender
      • Females have two X chromosomes
      • Males have an X chromosome and a Y chromosome
      • Sex chromosomes segregate during meiosis
      • [The rest of the (non-sex) chromosomes are called autosomes]
    Chapter 12
  • 36. Chapter 12
  • 37. Sex Determination in Mammals Chapter 12 X 1 X 2 EGGS Male Parent Y X m S P E R M Female Offspring Male Offspring Y X m X m X 1 X 2 X m Y Y X 1 X 2 X 1 X 2 Female Parent
  • 38. Sex-Linked Genes Are on the X or the Y
    • Genes carried on one sex chromosome are sex-linked
      • X chromosome is much larger than the Y and carries over 1000 genes
      • Y chromosome is smaller and carries only 78 genes
    • The X and the Y have very few genes in common
      • Females (XX) can be homozygous or heterozygous for a characteristic
      • Males (XY) have only one copy of the genes on the X or the Y
    Chapter 12
  • 39. How Sex-Linkage Affects Inheritance
    • Patterns of sex-linked inheritance were first discovered in fruit flies ( Drosophila ) in early 1900s
    • Eye color genes were found to be carried by the X chromosome
      • R = red eyes (dominant)
      • r = white eyes (recessive)
    Chapter 12
  • 40. How Sex-Linkage Affects Inheritance
    • Sex-linked (specifically X-linked ) recessive alleles displayed their phenotype more often in males
      • Males showed recessive white-eyed phenotype more often than females in an
      • X R X r x X r Y cross
    • Males do not have a second X-linked gene (as do females) which can mask a recessive gene if dominant
    Chapter 12
  • 41. Sex Linkage: Eye Color in Fruit Flies Chapter 12 25% Normal f Carrier f Normal m 25% 25% 25% White-e m Eggs of X R X r Female Sperm of X R Y Male 1 1 Y X R X R X r X R X R Y X r Female Female Male Male 1 1 Frequencies Phenotypes Genotypes Frequencies X R X R X r Y X R X r X R Y R r R
  • 42. Incomplete Dominance: Homozygous-X Homo Recessive Chapter 12 R R (100%) Pink (intermediate) Eggs of Homozygous RR Red Parent Pollen of Homozygous R ' R ' White Parent R' R' R' R R' R R' R R' R Pink Pink Pink Pink 1 Frequencies Phenotypes Genotypes Frequencies R'R R'R R'R R'R
  • 43. Codominance
    • Some alleles are always expressed even in combination with other alleles
    • Heterozygotes display phenotypes of both the homozygote phenotypes in codominance
    Chapter 12
  • 44. Codominance
    • Example: Human blood group alleles
      • Alleles A and B are codominant
      • Type AB blood is seen where individual has the genotype AB
    Chapter 12
  • 45. Human ABO Blood Group Chapter 12 10% 40% 46% 4% B or AB A or AB O,AB, A,B (universal) AB (universal) B or O A or O O AB, A, B, O (universal) A B Both Neither BB or BO AA or AO OO AB O AB B A Freq Donates Re- ceives Anti- bodies RBCs Genotype Type
  • 46. Polygenic Inheritance
    • Phenotypes produced by polygenic inheritance are governed by the interaction of more than two genes at multiple loci
    • Human skin color is controlled by at least 3 genes, each with pairs of incompletely dominant alleles
    Chapter 12
  • 47. Chapter 12
  • 48. Pedigree Analysis
    • Records of gene expression over several generations of a family can be diagrammed
    • Careful analysis of this diagram (a pedigree ) can reveal inheritance pattern of a trait
    • Pedigree analysis is often combined with molecular genetics technology to elucidate gene action and expression
    Chapter 12
  • 49. How to Read Pedigrees Chapter 12 = male = female = parents or = individual who shows the trait or = heterozygous carrier of autosomal trait = offspring 1 2 3 I, II, III, IV, or V = generation
  • 50. A Recessive Pedigree Chapter 12
  • 51. Pedigrees: Legacy of Queen Victoria Chapter 12
  • 52. Sickle-Cell Anemia Chapter 12
    • Hemoglobin is an oxygen-transporting protein found in red blood cells
    • A mutant hemoglobin gene causes hemoglobin molecules in blood cells to clump together
      • Red blood cells take on a sickle (crescent) shape and easily break
      • Blood clots can form, leading to oxygen starvation of tissues and paralysis
      • Condition is known as sickle-cell anemia
  • 53. Normal Red Blood Cells Chapter 12
  • 54. Sickled Cells Chapter 12
  • 55. Sex-Linked Genetic Disorders Chapter 12
    • Several defective alleles for characteristics encoded on the X chromosome are known
    • Sex-linked disorders appear more frequently in males and often skip generations
    • Examples of sex-linked (X-linked) disorders
      • Red-green color blindness
  • 56. Chapter 12
  • 57. Chapter 12
  • 58. Non-Disjunction Chapter 12 Incorrect separation of chromosomes or chromatids in meiosis known as non-disjunction Most embryos arising from gametes with abnormal chromosome numbers abort spontaneously (are miscarried) Some combinations of abnormal chromosome number survive to birth or beyond
  • 59. Chapter 12
  • 60. Chapter 12
  • 61. Incidence of Down Syndrome Chapter 12 Age of Mother (years) Number per 1000 Births 10 20 30 40 50 0 100 200 300 400
  • 62. The end Chapter 12