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Biology 2250 Principles of Genetics

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Title: Biology 2250 Principles of Genetics


1
Biology 2250Principles of Genetics
  • Announcements
  • Lab 4 Information B2250 (Innes) webpage
  • download and print before lab.
  • Virtual fly log in and practice
  • http//biologylab.awlonline.com/

2
Quiz 3 answers
  • http//webct.mun.ca8900/
  • All quizzes on WebCT for Review
  • Office Hours 130 230 Tue, Wed., Thr
  • or by appointment 737-4754, dinnes_at_mun.ca

3
Mendelian Genetics
  • Topics
  • -Transmission of DNA during cell division
  • Mitosis and Meiosis
  • - Segregation
  • - Sex linkage (problem how to get a
    white-eyed female)
  • - Inheritance and probability
  • - Independent Assortment
  • - Mendelian genetics in humans
  • - Linkage
  • - Gene mapping

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?
?
?
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  • Gene mapping in other organisms
  • (fungi, bacteria)
  • - Extensions to Mendelian Genetics
  • - Gene mutation
  • - Chromosome mutation
  • (- Quantitative and population genetics)

?
?
?
?
4
Linkage Summary
  • Recombination generates new combinations
  • (inter and intrachromosomal)
  • Genetic maps
  • - genes linked on the same chromosome
  • - location of new genes relative to genes
  • already mapped

5
Linkage Summary
  • Hunting for genes (Human Diseases)
  • - genetic markers DNA variation
  • - co-inheritance with diseases using
    pedigree
  • information
  • - recombinants used to estimate linkage

6
Extensions to Mendelian Genetics Ch. 14 From
Gene to Phenotype
  • Readings Ch. 14 p. 454 473
  • Problems Ch. 14 2, 3, 4, 5, 6, 7

7
Chapter 1 Genes, environment, organism
  • Phenotype
  • gene env. gene x env. gene x gene
  • Mendelian Genetics
  • Genotype Phenotype
  • Dominance ?

8
G x E interaction
9
Extensions to Mendelian Genetics (Gene ?
Phenotype)
  • 1. Dominance
  • 2. Multiple alleles
  • 3. Pleiotropy
  • 4. Epistasis (gene interaction)
  • 5. Penetrance and expressivity

10
Gene interaction
  • Alleles at one gene Dominance
  • Different genes Epistasis

11
1. Dominance
  • Location of heterozygote between
  • two homozygotes
  • 1. Complete
  • 2. No dominance
  • 3. Incomplete
    (partial)
  • 4. Codominance

12
Homozygotes A1A1 A2A2
Heterozygote A1A2
13
Incomplete Dominance
red
white
pink
14
Codominance
  • Human Blood Groups
  • Genotype Phenotype
  • AA A
  • AB AB co-dominance
  • BB B
  • antigen protein on RBC

15
Codominance
  • Molecular Markers

Allele
A B
AB AA BB BB
Heterozygote distinguished from homozygotes
16
2. Multiple Alleles(ABO Blood groups - 3 alleles)
  • Genotype Phenotype
  • (6) (4)
  • ---------------------------------------------
  • OO O
    recessive
  • AA, AO A dominant
  • BB, BO B dominant
  • AB AB
    co-dominant
  • ---------------------------------------------

17
Multiple alleles in clover
18
Test for Allelism
  • Possibilities
  • 1. alleles for the same gene - all crosses show
  • Mendelian ratios (11 31 121)
  • 2. more complex inheritance (gt 1 gene)

or
19
Example white, yellow, pink
Cross F1
F2 white x yellow yellow
31 yellow white white x pink
pink 31 pink white yellow x pink
pink 31 pink yellow
3 alleles w y p 6 genotypes w
w y y p p p w y w y p
20
3. Pleiotropy(one gene affects gt 1 trait)
  • Example Mouse
  • Gene affects
  • 1. coat colour ( , yellow)
  • 2. survival

dark
AA Homozygous wildtype
21
Yellow Parents
zzz
22
Crosses
  • A. x -----gt all
  • B. x ---gt 1/2
    1/2
  • C. x ----gt 2/3
    1/3

23
Explanation
  • A. AA x AA all AA
  • B. AA x AYA ½ AYA , ½
    AA
  • C. AYA x AYA ¼ AA ½ AYA ¼
    AYAY

1 2 1/3
2/3
dies
24
Interpretation
  • Gene affects both coat colour and
  • survival
  • 1. AY dominant to A for coat colour
  • 2. AY recessive lethal for survival

25
Pleiotropy

  • Phenotype
  • Genotype coat colour survival
  • A A dark
    alive
  • A AY yellow
    alive
  • AY AY ?
    dead

dark
26
G E P
Trait 1
Pleiotropy
Gene A
Trait 2
Gene A
Epistasis
Trait
Gene B
Gene interaction
27
4. Epistasis(gene interaction)
  • More than one gene affects a character
  • One gene pair masks or modifies the
  • expression of another gene pair
  • AABB x aabb ----gt AaBb x AaBb ---gt
    F2

F1 Dihybrid
28
F2
Epistasis
  • AaBb x AaBb
  • A- B- 9/16
  • A- bb 3/16
  • aa B- 3/16
  • aa bb 1/16

Gene A and B unlinked
4 distinct phenotypes (2 traits) (peas shape,
colour)
Epistasis Gene A and Gene B interact ?
phenotype of 1 trait
29
Epistasis(BbEe X BbEe)
1.
  • Labrador retriever Coat Colour (B and E genes)
  • F2 Ratio Genotype Phenotype
    Ratio
  • 9/16 B- E- black
    9/16
  • 3/16 B- ee gold
    4/16
  • 3/16 bb E- brown
    3/16
  • 1/16 bb ee gold
  • Gene E allows colour deposition

30
Epistasis
  • Allele E Allele
    B
  • Golden brown
    black
  • B- ee bb E-
    B- E-
  • bb ee

31
Epistasis(AaBb X AaBb)
2.
  • Example Flower petal colour
  • F2 Ratio Genotype Phenotype
    Ratio
  • 9/16 A- B- Purple
    9/16
  • 3/16 A- bb White
    7/16
  • 3/16 aa B- White
  • 1/16 aa bb White

32
Gene B
Gene A colourless colourless
purple (white)
(white)
A-bb aaB-
A- B- aabb
33
5. Penetrance and Expressivity
  • Phenotype genotype, genetic background,
  • and environment
  • Variable Expression Penetrance
  • Expressivity

34
  • Penetrance
  • percentage of individuals that show some
    degree of expression of a mutant genotype

35
Example Polydactyly (P) extra digits
  • pp Pp
    PP
  • normal 10 normal polydactyly
  • 90 polydactyly

36
Expressivity degree that a
given genotype is expressed
phenotypically
  • Example Pp individuals which do express
  • the extra digits can vary
  • (a) extra digit on each hand and foot
  • (b) extra digit on one hand only
  • (c) complete digit or vestige

37
Same genotype
38
Variable expressivity of piebald spotting in
beagles
39
Summary
  • - segregation and independent assortment
  • can explain a variety of patterns of
  • genetic variation
  • Phenotype Genotype Environment
  • Genetic interaction genotype, epistasis,
  • genetic
    background

40
Mutation
  • Source of genetic variation
  • Gene Mutation
  • - somatic, germinal
  • Chromosome mutations (Ch. 11 prob. 1, 2)
  • - structure
  • - number

41
Mutation
  • Gene Mutation
  • a------gta Forward mutation
  • a ------gta Reverse mutation
  • 1. Somatic mutation
  • - not transmitted to progeny
  • 2. Germinal Mutation
  • - transmitted to next generation

42
Somatic Mutations
Petal colour Rr red rr white
Plant genotype Rr mutation Rr
rr
43
(No Transcript)
44
Somatic mutations
45
Germinal mutations
AA (blue) Aa ? self ? aa(white)
46
Mutant Phenotypes
  • Morphological
  • Lethal
  • Biochemical
  • Resistance
  • Conditional - DTS (David T. Suzuki)
  • (permissive and restrictive conditions)

47
Mutation Frequency
Drosophila eye-colour w ? w 4 x 10-5 per
gamete Humans Hemophilia (X-linked
recessive) 4 x 10-5 per gamete

(1 in 25,000)
It is estimated that up to 30 of cases of
hemophilia have no known family history. Many of
these cases are the result of new mutations. This
means that hemophilia can affect any family.
48
Mutation Frequency
Drosophila eye-colour w ? w 4 x 10-5 per
gamete Mutation rate for a particular gene
very low (efficient repair) but, Large number of
genes in a genome mutations occur every
generation 4 x 10-5 x 50,000 genes 2
mutations
49
Gene Mutation
  • Mutations are rare and random
  • Ultimate source of genetic variation
  • Cancer Proto-oncogene ?oncogene ? cancer
  • mutation

50
Chromosome Mutations
  • Gene mutation detected
    genetically
  • Chromosome Mutations detected genetically and

  • cytologically
  • 1. Structure
  • 2. Number

51
Chromosome Mutations
  • 1. Structure Ch. 11 363 372
  • 2. Number Ch. 11 p. 350 - 363

52
1. Chromosome Structure
  • Karyotype
  • 1. size and number
  • 2. centromere position
  • telocentric
  • acrocentric
  • metacentric
  • submetacentric
  • acentric

(lost)
53
Chromosome Structure
  • 3. Heterochromatin pattern
  • - heterochromatin (dark)
  • - euchromatin (light)
  • 4. Banding patterns
  • a) staining Giemsa bands
  • b) polytene chromosomes (flies)

54
G-bands
55
Paint of Chr-22
56
Paint
57
Structural Abnormalities
  • Normal a b c d e f
  • 1. Deletion a c d e f
  • 2. Duplication a b b c d e f
  • 3. Inversion a e d c b f
  • 4. Translocation
  • a b c d j k g h i
    e f

58
Structural Abnormalities
  • 1. Deletions
  • deletion homozygote----gtusually lethal
  • deletion heterozygote----gt viable
  • deletion loop b
  • (pairing of a c d
  • homologues) a c d
  • deletion

59
Deletion heterozygote
deletion loop
60
Pseudodominance
  • Deletion Heterozygote
  • deletion loop b
  • (pairing of a c d
  • homologues)
  • deletion

Phenotype b
61
Deletion Mapping
Prune pn
62
Structural Abnormalities
  • Deletion notch-wing (Drosophila)
  • Phenotype
  • Genotype wing survival
  • N N normal alive
  • N N notch alive
  • N N - dead

  • (recessive lethal)

63
Genetics of Deletions
  • Reduced map distance ( chromosome shortened)
  • Recessive lethal
  • Deletion loop (detected during meiosis)

64
Structural Abnormalities
  • 2. Duplications
  • tandem duplication
  • a b b c d
  • maintain original evolve new
  • function function

65
Unequal crossing over
deletion
Tandem duplication
66
Bar Eye Mutation (Dominant)
67
Gene Duplication and Evolution
  • Gene duplication - Evolution of new function
  • Example Hemoglobin genes - duplication
  • Express in different stages
  • embryo fetus adult

68
Hemoglobin Alpha Beta Gamma ..
69
Structural Abnormalities
  • 3. Inversions - different gene order
  • - usually viable
  • a b c d e f a b e d c f a b e d
    c f
  • a b c d e f a b c d e f a b e d
    c f
  • homozygote heterozygote homozygote
  • N N N I
    I I
  • normal (N) inversion (I)

70
Cytological consequences of an Inversion Heterozyg
ote Inversion Loop
a b c d e
a d c b e
Fig. 11-21
crossover
X
Inversion Loop
71
  • Cytological consequences of an Inversion
  • Heterozygote Inversion Loop
  • Cross-over within an inversion
  • dicentric bridge (broken)
  • acentric fragment (lost)
  • deletions

72
Inversion heterozygotewith crossing over
Fig. 11-22
73
Inversion Heterozygote
  • Reduced recombination frequency
  • (suppression of crossing over)
  • Semisterile

74
4. Translocation
  • a b c d j k g h i e
    f

Translocation Heterozygote (meiosis)
N2
N1
T2
T1
75
Translocation
76
Fig. 11-24
Translocation heterozygote
77
Translocation heterozygoteAdjacent segregation
T1
N2
N1
T2
inviable
78
Translocation heterozygoteAlternate segregation
N1
N2
T1
T2
viable
79
Translocation
  • Change linkage relationships
  • (position effects)
  • Change chromosome size
  • Semisterile - unbalanced meiotic products

normal
Corn Pollen
aborted
aborted ??
80
Structural Abnormalities
  • Normal a b c d e f
  • 1. Deletion a c d e f
  • 2. Duplication a b b c d e f
  • 3. Inversion a e d c b f
  • 4. Translocation
  • a b c d j k g h i
    e f

81
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