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I. Introduction
A. Evolution is driven by natural selection.
B. Questions
1. What drives natural selection?
and/or
2. How are traits passed on to offspring?
C. Possible Answers
1. Fluid Hypothesis
a. A blending of materials
--in the blood?
2. Particle Hypothesis
b. Combination of "factors"
--in the cell?
II. Johann Gregor Mendel (1822-1884)
A. Brief Biography
1. Raised on a farm
2. Became a monk
3. Eventually becomes a math and
science teacher
B. Pea Plant Crosses
1. Pure Tall x Pure Short --> All Tall
2. All Tall x All Tall --> Tall & Short
(75%) (25%)
3 : 1
C. Explanation
1. Traits must be carried as discrete units
("particles", "factors" -- or "genes").
2. Two of these units interact with each
other to produce a particular trait in
the offspring.
3. Some units must be "dominant" (A) over
the other, "recessive" (a) units.
4. Possible combinations are therefore:
AA or Aa or aA or aa
III. Some Definitions
A. Phenotype
B. Genotype
C. Gene
D. Chromosome
E. Homologous Chromosomes
F. Locus
G. Alleles
H. Dominant Trait (Dominant Allele)
I. Recessive Trait (Recessive Allele)
J. Homozygous Condition
a. Homozygous Dominant (DD)
b. Homozygous Recessive (dd)
K. Heterozygous Condition (Dd or dD)
L. Monohybrid Cross
M. Dihybrid Cross
IV. Punnett Squares
[after Reginald Crundall Punnett (1875-1967)]
A. Basic Setup (for Aa x Aa)
1. Figure out the possible gamete genotypes.
(Separate the alleles.)
2. Place the possible gamete genotypes on the
outside of a square.
3. Simulate fertilization by moving the gametes
into each of the internal boxes (which
simulate possible offspring); interpret the
results.
V. Monohybrid Cross (Mendel Revisited)
A. Pure Tall (TT) x Pure Short (tt)
B. Tall (Tt) x Tall (Tt)
C. Mendel's First Law
1. The Law of Segregation
a. Each "particle" [gene] is a separate
entity not connected to any others.
[Today we know this is not entirely
correct.]
VI. Dihybrid Cross
A. Tall Round (TTRR) x Short Wrinkled (ttrr)
B. Tall Round (TtRr) x Tall Round (TtRr)
C. Mendel's Second Law
1. The Law of Independent Assortment
a. Each trait [gene] is on a separate
"particle" [chromosome] and each
"particle" moves separately
(independently) from any other particle.
VII. Beyond the Basics
A. Mendel was lucky.
VIII. Incomplete Dominance
A. Snapdragon Crosses
1. Pure Red (RR) x Pure White (WW)
2. Pink (RW) x Pink (RW)
IX. Codominance / Multiple Alleles
A. Blood Types
Type Antigens Alleles Genotypes
A A only IA IAIA or IAi
B B only IB IBIB or IBi
AB A and B IAIB
O Neither A/B i ii
B. Things To Remember
1. Multiple Alleles (IA, IB, i)
2. Codominance (IA and IB over i)
3. The 6 genotypes needed to produce the
4 phenotypes.
C. A Problem
1. If two people have type A blood, what
blood type will any of their offspring have?
D. A Solution
1. Figure out the genotypes of the parents
and the possible crosses.
a. Male (A): IAIA or IAi
b. Female (A): IAIA or IAi
IAIA x IAIA IAIA x IAi IAi x IAi
2. Do the Punnett Squares
E. Other Problems
X. Linked Traits
A. Linked Traits (on the same chromosome)
B. Chromosome Sets (2 per cell):
1. 22 autosomes
2. 1 sex chromosome
C. Sex Chromosomes: X & Y
1. Female: XX
2. Male: XY
D. Sex Linked Traits
1. Color Blindness: Xc
2. Hemophilia: Xh
normal male: XY
hemophiliac male: XhY
--------------------------
normal female: XX
normal female, carrier: XhX
hemophiliac female: XhXh
E. A Problem Concerning Hemophilia
1. If a normal male mates with a female
carrier, what types of offspring could
be produced?
XY x XXh
2. Solution:
XI. Polygenic Traits
A. Skin Color
1. Genes: A B C
2. AABBCC x aabbcc --> AaBbCc
3. AaBbCc x AaBbCc -->
XII. Summary
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