Genetics
Mendel and the Gene Idea
Multifactorial Inheritance
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Genotype does not rigidly define the phenotype,
but a range of possibilities over which the
environment can influence
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genes + environment & diet
phenotype
Phenotype depends on environment
and genes
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Tree: leaves that vary in size, shape, and greenness,
depending on exposure to wind and sun.
Humans: nutrition influences height, exercise alters
build, sun-tanning darkens the skin, and experience
improves performance on intelligence tests.
Identical twins: genetic equals, accumulate
phenotypic differences as a result of their unique
experiences.
Environment contributes to the
phenotype
acidic soil
basic soil
Early Ideas about Heredity
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Before Mendel
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basic facts of heredity
studied garden pea plant
formed hybrids
found recessive traits in some offspring
Gregor Mendel
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father of genetics
mentors - physics, botany
quantified results
subject - garden pea plant
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1843 – entered monastery
1851-53 – studied at U of Vienna
1857 – began breeding pea plants
Advantages of peas
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Mendel had strict control over
which plants mated with which
Each pea plant has male
(stamens) and female
(carpal) sexual organs.
Mendel could allow selfpollination or also move
pollen from one plant to
another to cross-pollinate
Mendel’s
Seven
Characters
and Alleles
Mendel’s Experimental Design
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self-pollinate
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crossed alternative traits to produce hybrids
(hybrids expressed only dominant traits)
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hybrids were self-pollinated
(produced 3:1 ratio)
Reginald Crundall Punnett
Punnett square
Symbols
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P = parental generation
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F1 = first filial generation
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F2 = second filial generation
Terminology
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homozygous
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homozygous dominant = RR
homozygous recessive= rr
heterozygous = Rr
genotype and phenotype
complete dominance
Monohybrid Cross
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parental varieties differ in a single character
character = flower color (using letter P)
allelles
purple, dominant (PP, Pp)
white, recessive (pp)
Flower Color (Pea plant)
topveg.com/wp-content/uploads/2009/03/pea-flower-ambience-300x286.jpg
Monohybrid Cross
F1
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purple (PP) X white (pp)
dominant x recessive
F1 are all purple (Pp)
all Pp = hybrid
Monohybrid Cross
Ratio in F2
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phenotypic ratio 3:1
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3 dominant: 1 recessive
PP & Pp are
indistinguishable in
complete dominance
genotypic ratio 1:2:1
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1 homozygous dominant (PP)
2 heterozygous (Pp)
1 homozygous recessive (pp)
Testcross (Pedigree Analysis)
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to determine unknown genotype
utilizes a homozygous recessive
phenotypic ratio of offspring determines genotype
F2
F1
P
Example
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F2
75% are black (B_)
25% are brown (bb)
B_
B_
B_
bb
B
B
b
bb
B_
b
BB
Bb
Bb
bb
F1
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all black (Bb)
B
B
b
Bb
Bb
b
Bb
Bb
Parent generation (P)
Black Labrador (BB)
Brown Labrador (bb)
Mendel’s First Law
"Law of Segregation"
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allele pairs separate during gametogenesis then
randomly reform pairs at fertilization
Dihybrid Cross
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parental varieties differ in two characters
characters:
 seed color (Y)
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seed shape (R)
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seed color alleles
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yellow: dominant (YY, Yy)
green: recessive (yy)
seed shape alleles
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round: dominant (RR, Rr)
wrinkled: recessive (rr)
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P: yellow/round X green/wrinkled
F1 : dihybrid (YyRr)
P1
F1
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F2 : offspring (9:3:3:1)
9 Y_R_
3 Y_rr
3 yyR_
1 yyrr
F2
How do I know my Punnett is done
correctly? all four are homozygous on both genes
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look at the diagonals!
all four are heterozygous on both genes (YyRr)
Mendel’s Second Law
"Law of Independent Assortment"
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each allele pair assorts independently from
one another during gamete formation
Patterns of Inheritance
not described by Mendel
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still follow Mendel’s laws
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Incomplete dominance
Codominance
Pleiotropy
Epistasis
Polygenic inheritance
Incomplete Dominance
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Blends are formed
 examples:
 Snapdragons
 Tay-Sachs disease
Snapdragon
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Intermediate phenotype
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Red = CRCR
Pink = CRCW
White = CWCW
Codominance
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Both alleles are expressed
examples:
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Landsteiner blood groups
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shorthorn cattle
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(A x B = AB)
(white x red = roan)
calico cat
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(orange-yellow x black)
Blood groups: Codominance & multiple alleles
http://health.stateuniversity.com/article_images/gem_01_img0116.jpg
Blood type Test
coagulates when it has the
antibody for that type of blood
same happens with the
Rh group
Blood Genotypes
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IAIA
IAi
IBIB
IB i
IAIB
ii
homozygous
heterozygous
homozygous
heterozygous
Rh group functions independently by
complete dominance rules:
 DD or Dd is Rh positive
 dd is Rh negative
Blood type A
Blood type B
Blood type AB
Blood type O
Examples : IAiDd
IBIBdd
iiDD
A+
BO+
Class Activity
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A X B = O is it possible?
What are the genotypes of the parents?
AB X O = O is it possible?
Give the genotypes of the possible children
Shorthorn Cattle Codominance
Red
White
Roan
Calico Cat Codominance
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B = black
R = red
XBXR
black
XB
red
XR
Barr body
Barr body is an inactive strand of DNA
Pleiotropy
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single gene has multiple effects
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example:
in Siamese cats, one gene causes abnormal fur
pigmentation and esophoria (crossed-eyed)
Waardenburg syndrome
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white forelock
pale iris
deafness
http://dermatology.cdlib.org/123/case_presentations/waardenburg/1.jpg
Epistasis
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one gene alters the
expression of another
that is independently
inherited
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example:
albinism in animals (mice)
 9 B_ C_ = black
 3 B_ cc = albino
 3 bb C_ = brown
 1 bb cc = albino
 9:3:4 instead of 9:3:3:1
Polygenic Inheritance
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An additive effect of 2 or more
genes on one character
example
 skin pigmentation
 at least 3 genes
 hair color
 at least 4 genes
Rule of Addition
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The probability of an event that can occur in 2 or
more different ways is the sum of the separate
probabilities of those ways
Rule of multiplication
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Multiply individual probabilities to get overall
probability
Rules of Addition and Multiplication
Female
roll tongue (Rr)
long eyelashes (Ll)
cleft chin (Cc)
Probability of having a
Male
no roll tongue (rr)
short eyelashes (ll)
cleft chin (Cc)
daughter
2/4 (xx, xx, xy, xy)
 roll tongue
2/4 (Rr, Rr, rr, rr)
 long eyelashes
2/4 (Ll, Ll, ll, ll)
 cleft chin
3/4 (CC, Cc, Cc, cc)
2/4 x 2/4 x 2/4 x 3/4 = 24/256 = 9.375%
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Activity
Eye color
Tongue rolling
small finger
widow's peak
hand clasping
earlobe
Hitchhiker's thumb
PTC tasting
The End
dark/light
yes/no
crooked/straight
yes/no
right/left
free/attached
no/yes
yes/no
P_/pp
R_/rr
B_/bb
W_/ww
C_/cc
A_/aa
H_/hh
D_/dd