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Genetics
&
The Work of Mendel
AP Biology
2006-2007
Do you know these terms?
 alleles
 Homozygous
 Heterozygous
 Phenotype
 Genotype
 Test cross
 Back cross
 Mendel’s Laws
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Genetics
 The study of inheritance
 how genes are passed on through


generations
What is a gene?
A section of DNA that codes for a
protein
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Gregor Mendel
 Modern genetics began in the
mid-1800s in an abbey garden,
where a monk named Gregor
Mendel documented inheritance
in peas
used experimental method
 used quantitative analysis

 collected data & counted them

AP Biology
excellent example of scientific
method
Mendel’s work
 Bred pea plants

Pollen transferred from white
flower to stigma of purple flower
P
cross-pollinate
true breeding parents (P)
 Pure or homozygous
raised seed & then
observed traits (F1)
 Hybrid or heterozygous
 allowed offspring
to self-pollinate
& observed next
generation (F2)

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anthers
removed
all purple flowers result
F1
self-pollinate
F2
Generations
 P - parents. Usually pure breeding –
ex. BB or bb
 F1 - are hybrid – Bb since they get a B
from one parent and b from the other
due to meiosis
 F2 - the result of crossing two Bb. The
ratio is usually 3:1 dominant:recessive
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Mendel collected data for 7 pea traits
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Looking closer at Mendel’s work
P
F1
true-breeding
true-breeding
X
purple-flower peas
white-flower peas
100%
purple-flower peas
Where did
the white
flowers go?
100%
generation
(hybrids)
self-pollinate
F2
generation
AP Biology
75%
purple-flower peas
White
flowers came
back!
25%
white-flower peas
3:1
What did Mendel’s findings mean?
 Traits come in alternative versions
purple vs. white flower color
 Alleles are genes for the same trait

 different alleles vary in the sequence of
nucleotides at the specific locus of a gene
 some difference in sequence of A, T, C, G
purple-flower allele &
white-flower allele are two DNA
variations at flower-color locus
different versions of gene at
same location on homologous
chromosomes
AP
Biology
Traits are inherited as discrete units
 For each characteristic, an organism
inherits 2 alleles, 1 from each parent

diploid organism
 inherits 2 sets of chromosomes,
1 from each parent
AP Biology
What did Mendel’s findings mean?
 Some traits mask others

purple & white flower colors are
separate traits that do not blend
 purple x white ≠ light purple
 purple masked white

_Wild type(Dominant)
 functional protein
 masks other alleles

mutant
allele producing
malfunctioning
protein
_Mutatant(recessive)
 allele makes a
malfunctioning protein
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wild type
allele producing
functional protein
homologous
chromosomes
Genotype vs. phenotype
 Difference between how an organism
“looks” & its genetics

Phenotype
 description of an organism’s trait
 the “physical”

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
Looking closer at Mendel’s work
P
true-breeding
true-breeding
X
purple-flower peas
white-flower peas phenotype
PP
pp
100%
purple-flower peas
F1
100%
generation
(hybrids)
Pp
self-pollinate
F2
75%
purple-flower peas
25%
white-flower peas
generation
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?
?
?
?
3:1
Punnett squares
Pp x Pp
F1
Aaaaah,
phenotype & genotype
can have different
ratios
generation
(hybrids)
%
genotype
male / sperm
female / eggs
P
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P
p
PP
%
phenotype
25%
75%
Pp
50%
Pp
p
pp
25% 25%
1:2:1
3:1
Genotypes
 Homozygous = same alleles = PP, pp
 Heterozygous = different alleles = Pp
homozygous
dominant
heterozygous
homozygous
recessive
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Phenotype vs. genotype
 2 organisms can have the same
phenotype but have different genotypes
purple
PP
purple
Pp heterozygous
homozygous dominant
How do you determine the
genotype of an individual with
with a dominant phenotype?
AP Biology
Test cross
 Cross the dominant phenotype —
the unknown genotype — with a
homozygous recessive (pp) to
determine the identity of the unknown
allele
How does
that work?
x
is it
PP or Pp?
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pp
How does a Test cross work?
Am I
this?
Or am I
this?
x
PP
P
pp
Pp
pp
P
P
p
p
p
p
p
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x
100% purple
50% purple:50% white or 1:1
RESULTS
 If any recessive offspring result, the
parents were hybrid
 If all (of many offspring) show the
dominant trait, the parents probably are
pure dominant for that trait, but you
can’t prove it.
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Back Cross
 If you know the parents, you can
determine the offspring genotypes
(look back at the parents)
 If any parents have the recessive trait,
the offspring (with the dominant trait)
MUST BE HYBRID.
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Pedigree chart
B-Brown allele
b-blue allele
We know that
The blues must be
pure
What is the
genotype
bb B_
here?
2
1
3
bb
5
7
bb
6
bb
Black shows recessive trait
AP Biology
4
8
Bb
Mendel’s laws of heredity
 Segregation
Tt
single trait
T
t
 each allele segregates into separate
gametes

 established by Metaphase 1
 Independent Assortment
 2 or more traits

genes on separate chromosomes
assort into gametes independently
 established by Metaphase 1
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EXCEPTION
 linked genes
metaphase1
Law of Dominance
 When parents with different
expressions of a trait (brown/blue,
red/white) are crossed, the trait that is
shown in the F1 is the dominant trait,
and the hidden trait is recessive
 NOTE: This is not always true but it was
for the traits Mendel studied.
AP Biology
Law of Segregation-explanation

during meiosis, alleles segregate
PP
 (homologous chromosomes separate)

P
P
each allele for a trait is packaged into
a separate gamete
p
pp
p
P
Pp
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p
Law of Segregation
 Which stage of
meiosis creates the
law of segregation?
Whoa!
And Mendel
didn’t even know
DNA or genes
existed!
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Monohybrid cross
 Some of Mendel’s experiments followed
the inheritance of single characters
flower color
 seed color
 Monohybrids

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Dihybrid cross
 Other of Mendel’s
experiments followed
the inheritance of 2
different characters
seed color and
seed shape
 Dihybrids

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Mendel
was working out
many of the
genetic rules!
Yellow/green, Round/wrinkled
Dihybrid cross
P
true-breeding
yellow, round peas
Y = yellow
R = round
true-breeding
green, wrinkled peas
x
YYRR
yyrr
y = green
r = wrinkled
yellow, round peas
F1
100%
generation
(hybrids)
YyRr
self-pollinate
F2
generation
AP Biology
9:3:3:1
9/16
yellow
round
peas
3/16
green
round
peas
3/16
yellow
wrinkled
peas
1/16
green
wrinkled
peas
What’s going on here?
 If genes are on different chromosomes…
how do they assort in the gametes?
 together or independently?

YyRr
YR
AP Biology
yr
Is it this?
Or this?
YR
YyRr
Yr
Which system
explains the
data?
yR
yr
YyRr
Dihybrid cross
YR
YyRr x YyRr
YR
YR
Yr
yR
yr
AP Biology
Yr
yR
yr
yr
or
YyRr
YR Yr
yR
yr
9/16
yellow
round
3/16
green
round
3/16
yellow
wrinkled
1/16
green
wrinkled
Law of Independent Assortment
 Law of Independent Assortment

different loci (genes) separate into gametes
independently
 non-homologous chromosomes align independently
yellow
 classes of gametes produced in equal amounts
 YR = Yr = yR = yr
green
 only true for genes on separate chromosomes
round
wrinkled
Yr
YyRr
Yr
1
AP Biology
yR
:
yR
1
YR
:
YR
1
yr
:
yr
1
Law of Independent Assortment
 Which stage of meiosis
creates the law of
independent assortment?
Remember
Mendel didn’t
even know DNA
—or genes—
existed!
AP Biology
EXCEPTION
 If genes are on same
chromosome
 will usually be inherited
together
Linkage
The
chromosomal
basis of Mendel’s
laws…
Trace the genetic
events through
meiosis, gamete
formation &
fertilization to
offspring
AP Biology
Linkage – Morgan used fruit flies
 Genes on the same chromosome are
usually inherited together. Ex. red hair
and freckles in humans
 Linked genes are changed by crossing
over
 More crossing over occurs between
genes that are far apart on the
chromosome
 Chromosome maps are based on
percentage of crossing over.
AP Biology
Chromosome maps
 Problem: What is the relative position
of the genes ABCD on the chromosome
if cross-over experiments show the
following: (% of cross-over)
A-B 40%
----20----- -----20----C-B 10%
A
D--10-- C--10-- B
D-C 10%
------------40-------------D-B 20%
A-D 20%
Answer: ADCB or BCDA
AP Biology
Linkage and Crossing Over
 Example Tall, Yellow hybrids are crossed TtYy X
TtYy
 If Linkage occurs (genes are on the same
chromosome), you expect 75% Tall Yellow and
25% short green in the F2
 If crossing over occurs, you expect most
offspring to look like either parent, and a small
number to have a different combination than the
parents.
AP Biology
Mendel chose peas wisely
 Pea plants are good for genetic research

available in many varieties with distinct
heritable features with different variations
 flower color, seed color, seed shape, etc.

Mendel had strict control over
which plants mated with which
 each pea plant has male & female
AP Biology
structures
 pea plants can self-fertilize
 Mendel could also cross-pollinate
plants: moving pollen from one plant
to another
Mendel chose peas luckily
 Pea plants are good for genetic research

relatively simple genetically
 most characters are controlled by a single gene
with each gene having only 2 alleles,
 one completely dominant over
the other
AP Biology
Any Questions??
AP Biology
2006-2007