Mendelian Genetics

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Mendelian Genetics

• Reading Chap. 14
• I. Intro
• A. Motivating question
• B. Mendel
• II. Mendels findings
• A. Law of segregation
• B. Law of independent assortment
• III. Complications
• IV. Examples from human genetics

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Terms and Concepts

• - character, trait, alleles
• - P, F1, F2
• - dominant/recessive
• - law of segregation
• - law of independent assortment
• - homozygous/heterozygous
• - phenotype/genotype
• - testcross

Rules of probability Complications - complete,
incomplete and co- dominance - multiple alleles -
pleiotropy - Epistasis - quantitative characters
polygenic inheritance
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Motivating question Radiation of the Galápagos
finches

• Beak sizes
• Food availability
• Range overlap
• Probable ancestors

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Galápagos Islands
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What Darwin knew (and inferred)

• Patterns of distribution
• Mechanism of natural selection
• heritable traits
• struggle for existence
• higher fitness --gt more offspring
• Shift in average traits in population

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What he didnt know

• How did heritability work?
• What exactly was passed down from parents to
  offspring?
• Blending vs. particulate?
• No idea about Genes, chromosomes, DNA, mitosis
  and meiosis

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Gregor Mendel
Austrian contemporary of Darwin Published shortly
after Darwin - but work was buried
Fig 22.1
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Who was Mendel?

• - Austrian monk
• - Background in agriculture (grew up on a farm)
• - Failed his teachers exam
• - University of Vienna math, causes of variation
  in plants
• - Teaching at the Brünn Modern School

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What did he do?

• Pea breeding
• Testing mechanisms of inheritance
• Used many different characters
• Published results in 1865

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Why did his experiments succeed?
Control over fertilization
Multiple generations P, F1, F2
True breeding parents
Either/or characters
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II. What did Mendel find?

• A. Law of segregation (of alleles)
• B. Law of independent assortment (of traits)

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A1. Mendels experiments Simple cross
P - true breeding parents with different traits
for same character.
F1 - Cross two of same generation
F2 - evaluate resulting traits 3 to 1
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Mendel tested many traits
3 to 1!!!
Did Mendel fudge?
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A2. Mendels interpretation

• - one factor from each parent
• - dominant vs. recessive
• - particulate inheritance can get pure traits
  back

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Genotype vs. phenotype

• homozygous vs. heterozygous

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3. Law of segregation

• When hybrid plants produce gametes, the two
  parental factors segregate half the gametes get
  one type, half get the other type.

All possible combinations, random combinations
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4. Rules of probability

• - multiplicity
• - additivity

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OK, prove it! The testcross

• Dominant phenotype what genotype?
• Predictions follow from particulate inheritance

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5. What do we know now?
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Chromosomes, genes, and alleles
P
p
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How does the law of segregation relate to meiosis?
Homologous chromosomes separate after doubling
Sister chromatids separate
Fig. 13.6
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B. Law of independent assortment

• What about two or more characters? Are they
  inherited together or independently?

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1. Two traits an example
Together
Independent
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Law of independent assortment (of characters)

• Independent segregation of each pair of alleles
  (i.e., genes coding for each character) during
  gamete formation.

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Rules of probability
From YyRr x YyRr
Yellow round YYRR YYRr YyRR
YyRr (1/41/4) (1/21/4)(1/21/4)(1/21/2)
9/16
Green round yyRR yyRr (1/41/4) (1/41/2)
3/16
Yellow wrinkled YYrr Yyrr (1/41/4) (1/21/4)
3/16
Green wrinkled yyrr (1/41/4) 1/16
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2. What we know now

• Mendels independent assortment referred to
  characters.

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What if genes for two traits are on the same
chromosome?

• Independent or linked?
• Linked, except for?
• Crossing over
• Depends how close they are genes further apart
  are more likely to behave as indpendent.

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Mendel got luckytwice

• 1. Genes for traits he studied were either on
  separate chromosomes, or
• 2. Far enough apart on the same chromosome that
  they assorted independently

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III. Complications

• A. Dominance, Incomplete dominance and Codominance

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A1. Incomplete dominance in snapdragon

• - Phenotype is intermediate
• - NOT blending

Fig. 14.9
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A2. Codominance - M, N, MN blood groups
MM
MN
BOTH traits expressed
NN
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B. Complications Multiple alleles

• ABO blood groups

Dominant
Dominant
Codominant
Recessive
fig. 14.10
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C. Complications Pleiotropy

• - One gene affects many characters
• - Sickling allele of hemoglobin

fig. 14.15
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D. Complications Polygenic Inheritance and
Quantitative Characters

• - One trait determined by multiple genes
• - Converse of pleiotropy
• - e.g., skin color at least 3 genes

fig. 14.12
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E. Complications Epistasis

• - Expression of one gene depends on another
• - Mouse coat color
• B - black coat
• b - brown coat
• C - pigment
• c - no pigment

fig. 14.11
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IV. Examples from human genetics

• Several excellent examples in the book.
• - Simple traits, geneologies
• - Genetic disorders (Tay-Sachs disease,
  Huntingtons disease, cystic fibrosis, etc.)
• Understand how they work, but dont need to
  memorize the details of each.
• Why might mating between close offspring lead to
  increased incidence of genetic disorders?

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Where do we go from here?

• Have
• Mechanism for natural selection
• Mechanism for heritability
• Not yet
• Understanding of meiosis, maintenance of genetic
  variability
• Molecular carrier of heritable information

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The modern synthesis
Darwin Mendel Population genetics DNA
Fig 22.1