Today: Mendelian Genetics

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Today Mendelian Genetics
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Mendelian Genetics Consider this.
8 million possible chromosome combinations in
each egg, and each sperm gt70 trillion
possibilities!
How are we able to predict ANYTHING about
inheritance??
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With all these possibilities, how can we predict
anything about inheritance?

• Gregor Mendel
• 1857- this monk (with extensive training in
  physics and botany) begins studying genetics
• Current dogma??

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• Mendels Technique
• Studies peas-
• Typically Self- Fertilizing
• Multiple distinct CHARACTERS, with easy to
  identify TRAITS
• Several TRUE-BREEDING varieties available

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What Mendel Observes
What does this data suggest about blended
inheritance?
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For many traits, we can predict the genotypic
frequencies of the offspring of two individuals
using a PUNNETT SQUARE
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The PUNNETT Square constructed for Mendels
experiments predicts a 31 ratio. The data
support Mendels Hypothesis!
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Simple Mendelian Inheritance- A Practice
Problem Cystic Fibrosis is a Recessive Trait with
Unusual Gene Frequencies

1. If two carriers of the cystic fibrosis trait
   marry, what is the probability that their first
   child will be affected?
2. It they eventually have three children, what is
   the probability that all three will be affected?

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Complication 1 (Mendel was lucky!)
INCOMPLETE DOMINANCE Heterozygotes have a unique
phenotype, between that of the homozygous
dominant or recessive parents. Note This is not
blended inheritance!
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Complication 2 PLEIOTROPY (multiple
effects) Example Sickle-Cell Disease
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Complication 3 Codominance Multiple
Alleles Example Human Blood Types
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 Example Paternity testing       Scenario
Suppose mother is Type A, baby is Type
B. Consider these three putative fathers can
any be the actual father?        1 (Type
A)       2 (Type B)        3 (Type O)
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Complication 4 EPISTASIS Example The color
gene, C, allows pigment to be deposited in
hair. When lacking, a mouse is albino,
regardless of its genotype at the other locus.
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Other Issues Individuals may display a range of
small differences in traits, known as CONTINUOUS
VARIATION
This usually indicates POLYGENIC INHERITANCE,
where two or more genes create a single
phenotypic character
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Other Issues Environmental Effects on Phenotype
Many factors, both genetic and environmental,
influence the phenotype.
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A More Recent Complication

• If you see the number 74, then you do not have
  red-green color blindness. If you see the number
  21, you are color blind to some extent. A totally
  color-blind person will not be able to see any of
  the numbers.

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Human males are more likely to be color-blind
than human females.
Why???
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Chromosomes!
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Is this what Mendel would expect?
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Is this what Mendel would expect?
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Practice Question Chromosomal Inheritance

• If a color blind man marries a wild-type woman,
  what are the chances that a daughter of theirs
  will be colorblind?
• What are the chances that their son will be
  colorblind?
• Can females be colorblind? What would the
  genotype of the parents have to be?

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Connecting Mendel to Lab
Reminder One of the restriction enzymes
(molecular scissors) we used can cut the normal
hemoglobin gene, but not the mutant sickle
version.
Sickle cell allele
normal
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Connecting Mendel to Lab
Reminder One of the restriction enzymes
(molecular scissors) we used can cut the normal
hemoglobin gene, but not the mutant sickle
version.
Sickle cell allele
normal