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Chapter 14: Mendel and the Gene Idea

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Title: Chapter 14: Mendel and the Gene Idea


1
Chapter 14Mendel andthe Gene Idea
2
Important Point
If you are having trouble understanding lecture
material Try reading your text before
attending lectures. And take the time to read it
well!
3
Doing Well in 113!
  • Do you feel that you knew the material going in
    to the last exam?
  • At what level do feel you knew the material?
  • Familiar with it in a general way?
  • Able to recognize specifics if you saw them?
  • Able to regurgitate specifics if prompted (e.g.,
    via flash cards)?
  • Able to recite specifics without prompting?
  • Do you feel that you understood the material that
    you were studying?
  • Have you been reading your text book prior to
    attending lectures?
  • Do you understand your text book?
  • Have you been giving yourself the time to go
    through the text book leisurely, or are you
    rushing to get in your reading just prior to
    class?
  • What does studying for an exam mean to you?

4
Doing Well in 113!
  • What do you do after lectures to make sure that
    you understand and are familiar with material?
  • Do you copy over the notes?
  • Do you make flash cards?
  • Do you read through the material presented in
    lecture, along with your notes, to identify what
    material might be giving you trouble?
  • When do you start preparing for an exam?
  • How well do you take care of yourself during the
    24-hour run up toward the exam?
  • Do you get sufficient sleep?
  • How do you prove to yourself that you know the
    material when you reach the end of your studying
    for an exam?
  • Do you find that you still have questions about
    the material during the day of the exam?
  •  Do you find that you are anxious on the day of
    the exam? If yes, what could you do to minimize
    that anxiety?

5
Gregor Mendel
The best way to gain an understanding of
genetics is to work with it. The fundamental
principles discussed (below) will become clear to
you, and you will grasp them more surely, if you
carefully think through . . . problems which
illustrate the various patterns of inheritance
(Keeton, 1980, Biological Science third edition,
W.W. Norton Company, p. 621)
6
Alleles and Loci
Different alleles may or may not code for
different phenotypes
An allele is a gene variant (often differ only by
one or few nucleotides)
A gene is a discrete heritable unit
Gene location on chromosome
7
Crossing Peas
Crossing is mating
Controlled breeding, with specific characters
scored for specific traits (e.g., character
flower color, trait purple vs. white)
8
First Generation Offspring
First filial generation
9
Pea Characters Traits
Note difference between character and trait
10
More Pea Characters Traits
A trait is a variant of a character
The interaction between non-identical alleles
results in interesting non-correspondences
between genotype and phenotype
Note 31 ratios
11
Even more
12
True Breeding
True breeding results when both parents are
homozygous for the same trait, e.g., a purple
purple x purple purple cross can result only in
purple purple ? purple-flowered progeny
similarly ww x ww ? only ww progeny
13
Monohybrid Cross
Dominant phenotype
Recessive phenotype
Heterozygote
Genotype unknown (homo- vs. heterozygote)
14
Genetics Problem-Solving Secrets!
  • Known Genotype can be used to infer unknown
    Phenotype
  • (but not always, due to complications, e.g.,
    penetrance)
  • Known Phenotype can be used to infer unknown
    Genotype
  • (but not always due to lack of 11
    correspondence more than one genotype can give
    rise to a given phenotype)
  • Genotype (diploid) gives rise to Gametes
    (haploid) via Meiosis
  • Gametes (haploid) give rise to Progeny
    (diploid) via Fertilization
  • Fertilization (syngamy) always results in
    Diploidy (I.e., gtploidy than haploid)
  • Meiosis always results in Haploidy (I.e.,
    anaphase I reduction division from diploidy to
    haploidy)

15
Monohybrid Cross
16
Monohybrid Cross
Homozygous recessive
Homozygous dominant
Heterozygote
17
Genotype vs. Phenotype
Dominant phenotype
Recessive phenotype
18
Contrasting Genotype Phenotype
  • Genotype
  • DNA nucleotide sequence
  • Gene, Allele
  • Chromosomes
  • Diploidy, Haploidy
  • Homozygous
  • Heterozygous
  • Law of Segregation
  • Law of Independent Assortment
  • Multiple Alleles
  • Polygenic Inheritance
  • Phenotype
  • What an organisms looks like
  • Character, Trait
  • Dominant, Recessive
  • Incomplete Dominance
  • Complete Dominance
  • 31 9331 ratios
  • Codominance
  • Pleiotropy, Epistatsis
  • Quantitative Characters
  • Norm of Reaction
  • Nature vs. Nuture

19
Following Genotype
Segregation of alleles occurs here
31 ratios
Punnett square
Example of complete dominance, a.k.a., dominance
20
Test Cross
1 phenotype, 2 possible genotypes
Blank slate
Homozygous recessive
21
Dihybrid Cross (2 loci, 2 alleles)
  • 9331 ratio that is dependent on
  • Two loci, two alleles per locus
  • Independent assortment between loci (genotypic
    independence)
  • Dominance-recessive relationships between the
    alleles found at each locus
  • One locus does not affect the phenotype of the
    other locus (phenotypic independence)

31 ratios are all over this
22
Segregation of Alleles
23
Dihybrid Cross
Dihybrids
24
Many Loci, Many Alleles
25
Probability Theory
  • Statistical Independence
  • Range of Probabilities (0..1)
  • Law of Multiplication
  • Calculation for Events not Happening
  • The Law of Addition

26
Genotype Probabilities
AaBbCcDdEe x AABbCcDDEc
pA 0.5
pAX 0.5 0.5 1.0
pa 0.5
pA 0.0
pA 0.5
X
pA 0.5
pAa 0.5 x 1.0 0.5
pXa 0.0 0.5 0.5
What Fraction AaBbCcDcEe?
27
Incomplete Dominance
Note 11 correspondence between genotype
phenotype!
28
Codominance
  • In codominance the phenotype consists of the
    phenotypes normally associated with both alleles,
    i.e., not a watered down version of one (as one
    sees with incomplete dominance)
  • Generally, at the molecular level to the extent
    that proteins are made at all, most alleles are
    codominant
  • In the heterozygote more than one type of protein
    product is produced per locus per chromosome
  • Aa and AA (actually Ia IA) have different
    molecular phenotypes even if A is dominant to a
    at the organismal level
  • Example is ABO blood group where A and B are
    codominant to each other (whereas both A and B
    are fully dominant to O)
  • Note (again) 11 correspondence between genotype
    phenotype

29
Molecular Codominance
Note codominant at molecular level
30
Codominance, etc.
31
Pleitropy
  • Genes that exert effects on multiple aspects of
    physiology or anatomy are pleiotropic
  • This is a common feature of human (etc.) genes
  • Marfan syndrome Affects the eye, the skeleton
    and the cardiovascular system
  • Albinism Affects skin, eyes, and even hearing
  • White eye in Drosophila flight muscles also
    affected
  • What all of the this means is that individual
    genes typically are active within numerous
    tissues, and that a character often may be
    modified via different pathways and routes
  • e.g., more than one gene may be involved in a
    characters expression, some with more-generally
    acting and others with more-specific effects

32
Pleitropy
33
Epistasis
C ? Color c ? colorless Cx ? Color cc ? colorless
B ? Black b ? brown Bx ? Black bb ? brown
Lack of phenotypic independence between loci!
Note not 9331 ratios
34
Polygenic Inheritance
Many loci, quantitatively contributing to single
continuously varying character, e.g., hair color
or height
35
Nature vs. Nurture
  • Nature Genetics (Genotype)
  • Nurture the Environment
  • Phenotype Genotype Environment (the
    Interaction of Genotype Environment)
  • Nature vs. Nurture is a shorthand for asking
    whether or not a Reaction Norm (phenotype as a
    function of environment) is a Horizontal Line
  • Often Nature vs. Nurture debates center around
    phenomenon for which we dont have a strong
    mechanistic understanding, e.g., human psychology

36
Norms of Reaction
  • Reaction norms Trait varies with environment
  • Reaction norms are quantitative measures of how
    genotypes respond, phenotypically, to environments

37
Pedigree Analysis
38
Human Traits
Table is from http//207.233.44.253/wms/reynolmj/l
ifesciences/lecturenote/bio3/Chap09.ppt
  • Most genetic diseases are recessive traits
  • In other words, there is an absence of a protein
    function

39
Dominant vs. Recessive
Note that lethal dominant traits tend to be very
rare because affected individuals tend to die
before mating
40
Autosomal Dominant Inheritance
No silent carriers
Generations are not skipped
Typically about half the offspring are affected,
but dont count on this!!!
41
Autosomal Dominant Inheritance
Generations are not skipped
42
Autosomal Dominant Inheritance
Generations are not skipped
Huntingtons disease
43
Pedigree Analysis (dominant)
44
Autosomal Recessive Inheritance
  • Heterozygotes carry the recessive allele but
    exhibit the wildtype phenotype
  • Males and females are equally affected and may
    transmit the trait
  • May skip generations
  • Note that with rare recessive traits we usually
    assume that people from outside of a family do
    not possess the affecting allele

45
Autosomal Recessive Inheritance
Generation skipped
46
Autosomal Recessive Inheritance
Sickle-cell disease
Often both parents are silent carriers
Cystic Fibrosis
Generations skipped
Typical is 1/4th affected
47
Consanguineous Mating
Inbreeding unmasks otherwise rare recessive
traits because genotypes of parents are not
independent
Consanguineous mating ()
With blood
48
Autosomal Recessive Inheritance
Generations skipped
More likely early onset lethal than if dominant
49
Pedigree Analysis (recessive)
Generation skipped
50
Genetics Problem-Solving Secrets!
  • Known Genotype can be used to infer unknown
    Phenotype
  • (but not always, due to complications, e.g.,
    penetrance)
  • Known Phenotype can be used to infer unknown
    Genotype
  • (but not always due to lack of 11
    correspondence more than one genotype can give
    rise to a given phenotype)
  • Genotype (diploid) gives rise to Gametes
    (haploid) via Meiosis
  • Gametes (haploid) give rise to Progeny
    (diploid) via Fertilization
  • Fertilization (syngamy) always results in
    Diploidy (I.e., gtploidy than haploid)
  • Meiosis always results in Haploidy (I.e.,
    anaphase I reduction division from diploidy to
    haploidy)

51
The End
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