Chapter 14: Mendel and the Gene Idea

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Chapter 14Mendel andthe Gene Idea
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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!
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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?

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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?

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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)
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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
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Crossing Peas
Crossing is mating
Controlled breeding, with specific characters
scored for specific traits (e.g., character
flower color, trait purple vs. white)
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First Generation Offspring
First filial generation
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Pea Characters Traits
Note difference between character and trait
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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
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Even more
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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
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Monohybrid Cross
Dominant phenotype
Recessive phenotype
Heterozygote
Genotype unknown (homo- vs. heterozygote)
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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)

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Monohybrid Cross
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Monohybrid Cross
Homozygous recessive
Homozygous dominant
Heterozygote
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Genotype vs. Phenotype
Dominant phenotype
Recessive phenotype
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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

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Following Genotype
Segregation of alleles occurs here
31 ratios
Punnett square
Example of complete dominance, a.k.a., dominance
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Test Cross
1 phenotype, 2 possible genotypes
Blank slate
Homozygous recessive
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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
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Segregation of Alleles
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Dihybrid Cross
Dihybrids
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Many Loci, Many Alleles
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Probability Theory

• Statistical Independence
• Range of Probabilities (0..1)
• Law of Multiplication
• Calculation for Events not Happening
• The Law of Addition

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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?
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Incomplete Dominance
Note 11 correspondence between genotype
phenotype!
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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

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Molecular Codominance
Note codominant at molecular level
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Codominance, etc.
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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

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Pleitropy
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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
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Polygenic Inheritance
Many loci, quantitatively contributing to single
continuously varying character, e.g., hair color
or height
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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

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Norms of Reaction

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

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Pedigree Analysis
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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

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Dominant vs. Recessive
Note that lethal dominant traits tend to be very
rare because affected individuals tend to die
before mating
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Autosomal Dominant Inheritance
No silent carriers
Generations are not skipped
Typically about half the offspring are affected,
but dont count on this!!!
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Autosomal Dominant Inheritance
Generations are not skipped
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Autosomal Dominant Inheritance
Generations are not skipped
Huntingtons disease
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Pedigree Analysis (dominant)
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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

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Autosomal Recessive Inheritance
Generation skipped
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Autosomal Recessive Inheritance
Sickle-cell disease
Often both parents are silent carriers
Cystic Fibrosis
Generations skipped
Typical is 1/4th affected
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Consanguineous Mating
Inbreeding unmasks otherwise rare recessive
traits because genotypes of parents are not
independent
Consanguineous mating ()
With blood
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Autosomal Recessive Inheritance
Generations skipped
More likely early onset lethal than if dominant
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Pedigree Analysis (recessive)
Generation skipped
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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)

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The End