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Mendelian Genetics

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Title: Mendelian Genetics


1
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

2
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
3
Motivating question Radiation of the Galápagos
finches
  • Beak sizes
  • Food availability
  • Range overlap
  • Probable ancestors

4
Galápagos Islands
5
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

6
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

7
Gregor Mendel
Austrian contemporary of Darwin Published shortly
after Darwin - but work was buried
Fig 22.1
8
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

9
What did he do?
  • Pea breeding
  • Testing mechanisms of inheritance
  • Used many different characters
  • Published results in 1865

10
Why did his experiments succeed?
Control over fertilization
Multiple generations P, F1, F2
True breeding parents
Either/or characters
11
II. What did Mendel find?
  • A. Law of segregation (of alleles)
  • B. Law of independent assortment (of traits)

12
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
13
Mendel tested many traits
3 to 1!!!
Did Mendel fudge?
14
A2. Mendels interpretation
  • - one factor from each parent
  • - dominant vs. recessive
  • - particulate inheritance can get pure traits
    back

15
Genotype vs. phenotype
  • homozygous vs. heterozygous

16
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
17
4. Rules of probability
  • - multiplicity
  • - additivity

18
OK, prove it! The testcross
  • Dominant phenotype what genotype?
  • Predictions follow from particulate inheritance

19
5. What do we know now?
20
Chromosomes, genes, and alleles
P
p
21
How does the law of segregation relate to meiosis?
Homologous chromosomes separate after doubling
Sister chromatids separate
Fig. 13.6
22
B. Law of independent assortment
  • What about two or more characters? Are they
    inherited together or independently?

23
1. Two traits an example
Together
Independent
24
Law of independent assortment (of characters)
  • Independent segregation of each pair of alleles
    (i.e., genes coding for each character) during
    gamete formation.

25
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
26
2. What we know now
  • Mendels independent assortment referred to
    characters.

27
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.

28
Mendel got luckytwice
  • (not that way - he was a monk!)
  • 1. Genes for traits he studied were either on
    separate chromosomes, or
  • 2. Far enough apart on the same chromosome that
    they assorted independently

29
III. Complications
  • A. Dominance, Incomplete dominance and Codominance

30
A1. Incomplete dominance in snapdragon
  • - Phenotype is intermediate
  • - NOT blending

Fig. 14.9
31
A2. Codominance - M, N, MN blood groups
MM
MN
BOTH traits expressed
NN
32
B. Complications Multiple alleles
  • ABO blood groups

Dominant
Dominant
Codominant
Recessive
fig. 14.10
33
C. Complications Pleiotropy
  • - One gene affects many characters
  • - Sickling allele of hemoglobin

fig. 14.15
34
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
35
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
36
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?

37
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

38
The modern synthesis
Darwin Mendel Population genetics DNA
Fig 22.1
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