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Genetics
&
The Work of Mendel
Regents Biology
2007-2008
Gregor Mendel
 Modern genetics began in the mid1800s in an abbey garden, where a
monk named Gregor Mendel
documented inheritance in peas
used good experimental design
 used mathematical analysis

 collected data & counted them

excellent example of scientific
method
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Mendel’s work
Pollen transferred from white
flower to stigma of purple flower
 Bred pea plants
cross-pollinate
true breeding parents
 raised seed & then
observed traits
 allowed offspring
to self-pollinate
& observed next
generation

anthers
removed
all purple flowers result
self-pollinate
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?
Mendel collected data for 7 pea traits
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Looking closer at Mendel’s work
Parents
true-breeding
true-breeding
X
purple-flower peas
white-flower peas
100%
purple-flower peas
1st
generation
(hybrids)
100%
self-pollinate
2nd
75%
purple-flower peas
generation
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25%
white-flower peas
3:1
What did Mendel’s findings mean?
 Some traits mask others

purple & white flower colors are
separate traits that do not blend
I’ll speak for
both of us!
 purple x white ≠ light purple
 purple masked white

dominant allele
 functional protein
 affects characteristic
 masks other alleles

recessive allele
 no noticeable effect
 allele makes a
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non-functioning protein
allele producing
functional protein
mutant allele
malfunctioning
protein
homologous
chromosomes
Genotype vs. phenotype
 Difference between how an organism
“looks” & its genetics

phenotype
 description of an organism’s trait

genotype
 description of an organism’s genetic
makeup
X
P
Explain Mendel’s results using
…dominant & recessive
…phenotype & genotype
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purple
white
F1
all purple
Making crosses
 Can represent alleles as letters
flower color alleles  P or p
 true-breeding purple-flower peas  PP
 true-breeding white-flower peas  pp

PP x pp
X
P
purple
white
F1
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all purple
Pp
Punnett squares
Pp x Pp
1st
Aaaaah,
phenotype & genotype
can have different
ratios
generation
(hybrids)
%
genotype
male / sperm
female / eggs
P
p
PP
25%
75%
Pp
P
PP
%
phenotype
50%
Pp
Pp
p
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Pp
pp
pp
25% 25%
1:2:1
3:1
Any Questions??
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2007-2008
Beyond Mendel’s Laws
of Inheritance
Regents Biology
2007-2008
Extending Mendelian genetics
 Mendel worked with a simple system
peas are genetically simple
 most traits are controlled by single gene
 each gene has only 2 version

 1 completely dominant (A)
 1 recessive (a)
 But its usually not that simple!
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Incomplete dominance
 Hybrids have “in-between” appearance
RR = red flowers RR
 rr = white flowers WW
 Rr = pink flowers
RW

 make 50% less color
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RR
Rr
rr
Incomplete dominance
P
X
true-breeding
red flowers
true-breeding
white flowers
100% pink flowers
1st
100%
generation
(hybrids)
self-pollinate
25%
red
2nd
generation
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50%
pink
25%
white
1:2:1
Incomplete dominance
RW x RW
%
genotype
male / sperm
female / eggs
R
R
W
W
RR
RW
25% 25%
50% 50%
RW
WW
25% 25%
1:2:1
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%
phenotype
1:2:1
Codominance
 Equal dominance
human ABO blood groups
 3 version

 A, B, i
 A & B alleles are codominant
 both A & B alleles are
dominant over i allele

the genes code for different
sugars on the surface of
red blood cells
 “name tag” of red blood cell
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Genetics of Blood type
phenogenotype
type
A
B
antigen
on RBC
antibodies
in blood
donation
status
A A or A i
type A antigens
on surface
of RBC
anti-B antibodies
__
BB or B i
type B antigens
on surface
of RBC
anti-A antibodies
__
AB
both type A &
type B antigens
on surface
of RBC
no antibodies
universal
recipient
ii
no antigens
on surface
of RBC
anti-A & anti-B
antibodies
universal
donor
AB
O
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Blood donation
clotting clotting
clotting
clotting
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clotting
clotting
clotting
One gene: many effects
 The genes that we have covered so far
affect only one trait
 But most genes are affect many traits

1 gene affects more than 1 trait
 dwarfism (achondroplasia)
 gigantism (acromegaly)
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Acromegaly: André the Giant
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Inheritance pattern of Achondroplasia
Aa
x aa
a
a
A
Aa
Aa
A
a
aa
aa
a
50%Biology
dwarf:50%
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normal or 1:1
Aa
x Aa
A

a
AA
Aa
Aa
aa
67% dwarf:33% normal or 2:1
Many genes: one trait
 Polygenic inheritance
additive effects of many genes
 humans

 skin color
 height
 weight
 eye color
 intelligence
 behaviors
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Human skin color
 AaBbCc x AaBbCc
can produce a wide
range of shades
 most children =
intermediate skin
color
 some can be very
light & very dark

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Johnny & Edgar Winter
Albinism
albino
Africans
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melanin
= universal brown color
OCA1 albino
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Bianca Knowlton
Coat color in other animals
 2 genes: E,e and B,b


color (E) or no color (e)
how dark color will be: black (B) or brown (b)
eebb
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eeB–
E–bb
E–B–
Environment effect on genes
 Phenotype is controlled by
both
environment & genes
Human skin color is
influenced by both genetics
& environmental conditions
Color of Hydrangea flowers
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Biologyby soil pH
is
influenced
Coat color in arctic
fox influenced by
heat sensitive alleles
Genetics of sex
 Women & men are very different, but just a
few genes create that difference
 In mammals = 2 sex chromosomes

X&Y

2 X chromosomes = female: XX

X & Y chromosome = male: XY
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X
X
X
Y
Sex chromosomes
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Sex-linked traits
 Sex chromosomes have other genes on
them, too
especially the X chromosome
 hemophilia in humans

 blood doesn’t clot

Duchenne muscular dystrophy in X
humans
X
 loss of muscle control

red-green color blindness
 see green & red as shades of grey
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X
Y
sex-linked recessive
Sex-linked traits
2 normal parents,
but mother is carrier
HY x XHh
H Xh
XHH
male / sperm
XH
XH
Y
XH
XH XH
XH Y
Xh
XH Xh
XhY
Y
XH
XH Xh
Xh
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female / eggs
XH Y
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Dominant ≠ most common allele
 Because an allele is dominant
does not mean…
it is better, or
 it is more common

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Polydactyly
dominant allele
Polydactyly
individuals are born with
extra fingers or toes
the allele for >5 fingers/toes
is DOMINANT & the allele for
5 digits is recessive
recessive allele far more
common than dominant
 only 1 individual out of 500
has more than 5 fingers/toes
 so 499 out of 500 people are
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homozygous recessive (aa)
Hound Dog Taylor
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Any Questions?
Regents Biology
2007-2008