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Genetics &
Gregor Mendel
Gregor Mendel
• Modern genetics began in the
mid-1800’s in an abbey garden,
where a monk named Gregor Mendel
documented inheritance in peas
– Used experimental method
– Used quantitative analysis
• Collected & counted data
– Excellent example of scientific
method
Mendel’s work
• Bred pea plants
– Cross-pollinated
true breeding
parents (P)
Pollen transferred from white flower to
stigma of purple flower
P
• P = parental
– Raised seeds &
observed traits in
(F1)
anthers
removed
offspring
all purple flowers result
• F = filial
– Allowed offspring
to self-pollinate &
observed traits in
next generation (F2)
F1
self-pollinate
F2
Mendel collected data for 7 traits
Taking a closer look …
true-breeding
purple-flower peas
P
F1
X
true-breeding
white-flower peas
100%
purple-flower peas
Where did
the white
flowers go?
100%
generation
(hybrids)
self-pollinate
F2
generation
75%
purple-flower peas
White
flowers came
back!
25%
white-flower peas
3:1
What did Mendel’s findings mean?
• Traits come in alternative forms
– Purple vs white flower color
– Alleles
• Different alleles vary in sequence of nucleotides at the
specific locus of a gene
– Some difference in sequence of A, T, C, or 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
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
• Homologous chromosomes
• Like having 2 editions of a Biology book
– Campbell “Biology”
– Raven & Johnson “Biology”
What are the
advantages of
being diploid?
What did Mendel’s findings mean?
• Some traits mask others
– Purple & white flower colors are separate traits
that do not blend
• Purple X white ≠ lavender
• Purple masked white I’ll speak for
– Dominant allele
both of us!
• Functional protein
• Masks other allele
– Recessive allele
wild type
allele producing
functional protein
mutant
allele producing
malfunctioning
protein
• Allele makes a malfunctioning
protein
homologous
chromosomes
Genotype vs phenotype
• Difference between how an organism “looks”
& its genetic code
– Phenotype
• Description of an organism’s trait
• Physical expression
– Genotype
• Organism’s genetic makeup
X
P
Explain Mendel’s results using
…dominant & recessive
…phenotype & genotype
purple
F1
all purple
white
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
Pp
F1
all purple
Looking closer at Mendel’s work
true-breeding
purple-flower peas
P
X
true-breeding
white-flower peas
PP
pp
100%
purple-flower peas
F1
Pp
Pp
genotype
100%
generation
(hybrids)
Pp
phenotype
Pp
self-pollinate
F2
75%
purple-flower peas
25%
white-flower peas
generation
?
?
?
?
3:1
Punnett squares
Pp x Pp
F1
Aaaaah,
phenotype & genotype
can have different
ratios
generation
(hybrids)
%
genotype
male / sperm
P
p
PP
25%
75%
female / eggs
Pp
P
PP
%
phenotype
50%
Pp
Pp
p
Pp
pp
pp
25%
1:2:1
25%
3:1
Genotypes
• Homozygous = same alleles = PP or pp
• Heterozygous = different alleles = Pp
homozygous
dominant
heterozygous
homozygous
recessive
Phenotypes vs genotypes
• 2 organism can have the same phenotype but
different genotypes
purple PP
homozygous dominant
purple Pp
heterozygous
How do you determine the
genotype of an individual with
with a dominant phenotype?
Can’t tell
by lookin’
at ya!
Test cross
• Breed the dominant phenotype – the
unknown genotype – with a homozygous
recessive (pp) to determine the identity of the
unknown allele
x
is it
PP or Pp?
pp
How does
that work?
How does a test cross work?
Am I
this?
Or am I
this?
x
PP
p
pp
x
Pp
p
p
pp
p
P
Pp
Pp
P
Pp
Pp
P
Pp
Pp
p
pp
pp
100% purple
50% purple:50% white or 1:1
Mendel’s 2nd law of heredity
• Law of Segregation
– During meiosis, alleles segregate
P
PP
P
• Homologous chromosomes separate
– Each allele for a trait is packaged into a separate
p
gamete
pp
p
P
Pp
p
Law of Segregation
• Which stage of meiosis is
responsible for the
Law of Segregation?
Metaphase 1
Whoa!
And Mendel
didn’t even know
DNA or genes
existed!
Monohybrid cross
• Inheritance of a single characteristic
– Flower color
– Seed color
• Both parents are heterozygous (Pp x Pp)
X
Dihybrid cross
• Inheritance of two
different characteristics
– Seed color AND seed
shape
• Both parents are
heterozygous for both
traits
Dihybrid cross
true-breeding
yellow, round peas
P
Y = yellow
R = round
x
YYRR
generation
(hybrids)
100%
YyRr
Dihybrid X
generation
y = green
r = wrinkled
yellow, round peas
F1
F2
yyrr
true-breeding
green, wrinkled peas
self-pollinate
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 into the gametes?
– Together or independently??
Is it this?
YyRr
YR
yr
Or this?
YR
YyRr
Yr
Which system
explains the
data?
yR
yr
Is this the way it works?
YyRr
YR
YyRr x YyRr
yr
or
YR
YyRr
Yr
yR
yr
9/16
yellow
round
YR
YR
yr
yr
YYRR
YyRr
YyRr
yyrr
Well, that’s
NOT right!
3/16
green
round
3/16
yellow
wrinkled
1/16
green
wrinkled
Or this way?
YyRr
YR
YyRr x YyRr
YR
YR
YYRR
Yr
YYRr
yR
YyRR
yr
YYrr
YyRr
Yyrr
yR
YyRR
YyRr
yyRR
yyRr
Yyrr
yyRr
yyrr
Yr
yR
yr
9/16
yellow
round
3/16
green
round
YyRr
YYRr
YyRr
YR
yr
Yr
yr
or
YyRr
BINGO!
3/16
yellow
wrinkled
1/16
green
wrinkled
We will find an
exception
to this?
Mendel’s 3rd law of heredity
• Law of Independent Assortment
– Different loci (genes) separate into gametes independently
• Non-homologous chromosomes align independently
• Gamete types produced in equal amounts
yellow
– YR = Yr = yR = yr
green
• Only true for genes on separate chromosomes or on same
chromosome, but far enough apart for crossing-over to occur
frequently
round
wrinkled
YyRr
Yr
Yr
1
yR
:
yR
1
YR
:
YR
1
yr
:
yr
1
Law of Independent Assortment
• Which stage of meiosis is
responsible for the Law of
Independent Assortment?
Again Mendel didn’t
even know DNA
—or genes—
existed!
Metaphase 1
EXCEPTION
 If genes are on same
chromosome & close together
 will usually be inherited
together
 rarely crossover separately
 “linked”
• The
chromosomal
basis of
Mendel’s
Laws
Review: Mendel’s laws of heredity
• Law of Dominance
– One allele will be dominant over the other
• Law of Segregation
– Each allele segregates into separate gametes
• Established in Meiosis I
• Law of Independent Assortment
– Genes on separate chromosomes assort into
gametes independently
• Established in Meiosis I
EXCEPTION
 linked genes
metaphase1
Mendel was wise to chose peas
• Peas 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 structures
• Pea plants can self-fertilize
• Mendel could also cross-pollinate plants:
moving pollen from one plant to another
Mendel was wise & lucky to chose peas
• Peas plants are good for genetic research
– Relatively simple genetically
– most characteristics are controlled by a single
gene with each gene having only 2 alleles
• One completely dominant over the other
Any questions?