Lab Exam

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Lab Exam
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Genotype Phenotype
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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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Characters Traits
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Mendels Protocol
Controlled breeding, with specific characters
scored for specific traits (e.g., character
flower color, trait purple vs. white)
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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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Dominant Recessive Alleles
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Homo- Heterozygotic
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Segregation of Alleles
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Meiosis and Segregation
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Generations
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And Dont Forget
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121 Genotypic Ratio
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31 Phenotypic Ratio
dominant
recessive
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Monohybrid Cross
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Monohybrid Cross
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Test Cross

• What is the genotype of the purple-flowered
  plant, PP or Pp?
• To find out we can cross the plant with a Blank
  Slate, I.e., a homozygous recessive plant (pp)
• In that case, the phenotypes of all progeny will
  (in a sense) be dependent only on the genotype of
  the purple-flowered parent
• We call this kind of test of genotype a Test
  Cross

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Test Cross
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Test Cross
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Beyond 1 locus, 2 alleles, Complete Dominance
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Dihybrid Cross (2 loci, 2 alleles)
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Dihybrid Cross (2 loci, 2 alleles)

• Note that 9331 ratio is dependent on
• Two loci, two alleles per locus
• Independent assortment between loci (genotypic
  independence)
• Dominance-recessive relationships betwee the
  alleles found at each locus
• One locus does not affect the phenotype of the
  other locus (phenotypic independence)

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Dihybrid Cross (2 loci, 2 alleles)
Dominant-Dominant
Recessive-Dominant
Dominant-Recessive
Recessive-Recessive
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Dihybrid Cross (2 loci, 2 alleles)
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Independent Assortment
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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 correspondance between genotype
phenotype!
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Incomplete Dominance
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Incomplete Dominance
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1 Locus, gt2 Alleles
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1 Locus, gt2 Alleles
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1 Locus, gt2 Alleles
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Codominance
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Codominance

• 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 have different molecular phenotypes
  even if A is dominant to a at the organismal level

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Codominance
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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 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
(but, in fact, probably did not suffer from
Marfan Syndrome)
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Pleitropy
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Epistasis (interacting loci)
B ? Black b ? brown Bx ? Black bb ? brown
C ? color c ? no color CX ? color cc ? no color

• Lack of 9331 ratio is due to lack of
  phenotypic indepence
• In the next chapter (15) well see the
  consequence of lack of genotypic independence,
  a.k.a., linkage

Note not 9331 ratios
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Polygenic Inheritance

• Means Many Genes, One Character
• Influenced by alleles at two or more loci, with
  each making a contribution to the phenotype
• Polygenic traits account for most of the
  observable variations seen in humans
• Example skin color, hair color, eye color,
  stature, weight, height, shape of face, behavior,
  etc.

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Norms of Reaction (Reaction Norms)
Many Characters, One Gene Reaction Norms are the
measure of how organisms (or genoyptes) respond,
phenotypically, to their environments
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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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Human Genetics (Pedigree Analysis)
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Human Dominant Recessive 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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Pedigree Analysis
This is Skipping of Generations
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Pedigree Analysis
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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
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Autosomal Dominant Inheritance
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Dominant vs. Recessive
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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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Link to Next Presentation
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Acknowledgements
http//www.bbchs.k12.il.us/Teacher_Pages/Hammond/P
owerpoint/Chapter_14.ppt http//www.westga.edu/wb
arger/mendel_gene_idea.ppt http//nsm1.utdallas.ed
u/bio/Yasbin/HumanGenetics/HG4W.ppt http//www.ucl
.ac.uk/ucbhjow/medicine/RGD_lectures/modes_of_inh
eritance.ppt http//cim.usuhs.mil/genetics/present
ations/Ada1r/ada1r.ppt http//biology.ucf.edu/co
urses/pcb3063/0620Gene20Interactions20Present-0
2.ppt http//www.lec.edu/facstaff/dmcdermot/Biolog
y/Genetics.ppt http//staff.stir.ac.uk/steve.pater
son/30EG/lectures/lecture201020rxn20norms.ppt h
ttp//207.233.44.253/wms/reynolmj/lifesciences/lec
turenote/bio3/Chap09.ppt
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