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Mendel & Inheritance
Sherry Flint-Garcia
USDA ARS
MU Division of Plant Sciences
Genetic variation
• In the beginning, geneticists studied
differences they could see (i.e.
morphological differences).
• Individual variants are referred to as
phenotypes.
– tall vs. short corn plants
– red vs. green grapes
Source: USDA
Source: MGDB
Trait
• A broad term encompassing a distribution
of phenotypic variation.
• Two types of traits:
Qualitative or Discrete
Quantitative or Continuous
Trait: Kernel Color
Trait: Corn Rootworm Resistance
Phenotype: Purple vs. Yellow
Phenotype: 1 (no damage) to 6
(severe damage)
Governed by one or few genes
Usually governed by many genes
Source: MGDB
1
2
3
4
5
6
Source: USDA
Advantages of Plants
• Can make controlled crosses.
– Including making inbreds (in many
plant species) without the severe
effects that are typical in animals.
Source: MGDB
• One plant can produce tens to hundreds of
progeny, and we can store the progeny for
long periods of time.
• Less costly and time consuming to
maintain than animals.
– Generation time is often much shorter.
Pre-Mendel
• Blending Theory
– Traits present in the parents are mixed (or
blended) in offspring.
• Inheritance of Acquired Characteristics
– Traits present in parents are modified, through
use, and passed on to the offspring in their
modified form
• They didn’t know about genes or DNA!!!!
Gregor Mendel
• Performed several experiments
between 1856 and 1863 that
are the basis for what we know about
heredity today.
• Published his work in 1866.
• His papers were largely ignored for more
than 30 years until other researchers
appreciated the significance of his work.
• Described the behavior of particulate
(discrete) bodies = genes.
Garden Pea
Pisum sativum
• Many morphological differences.
• Diploid with short generation time.
• Ability to make controlled pollinations.
Each phenotype
Mendel studied
was controlled
by a single gene.
http://staff.tuhsd.k12.az.us/gfoster/standard/pea.gif
Terms
• Wild-type is the phenotype that would normally be
expected.
• Mutant is the phenotype that deviates from the norm: it is
unexpected but heritable.
– Notice that this definition does not imply that all mutants are bad. In
fact, many beneficial mutations have been selected by plant breeders.
• Phenotype is the observable or measurable
characteristic of an individual.
• Genotype is the genetic constitution of an individual.
Terms continued…
• An allele is a particular form of a gene.
– The “tall” gene has two alleles: T (tall) & t (short)
• A homozygous individual (homozygote) has two identical
alleles, e.g. TT or tt.
• A heterozygous individual (heterozygote) has two
different alleles, e.g. Tt.
• The dominant phenotype is expressed in either a
homozygous or heterozygous genotype.
– TT and Tt condition tall plants. Tall is dominant over short.
• The recessive phenotype is only expressed in a
homozygous genotype.
– tt conditions short plants. Short is recessive to tall.
Mendel’s Experiment
Parental
(Inbred) Lines Round × Wrinkled
F1
progeny
ALL Round
Self-pollinate
F2
progeny
Round
Wrinkled
5474
1850
3 Round : 1 Wrinkled
Since the F1 progeny are all
round, we know that the
round phenotype is dominant
and the wrinkled phenotype
is recessive.
Mendel’s Results
Parental Cross
F1 Phenotype
F2 data
Round x wrinkled seed
Round
5474 : 1850
Yellow x green seed
Yellow
6022 : 2001
Purple x white flower
Purple
705 : 224
Inflated x constricted pod
Inflated
882 : 299
Green x yellow pod
Green
428 : 152
Axial x terminal flower
Axial
651 : 207
Long x short stem
Long
787 : 277
All approximately 3 : 1
The 3 : 1 ratio is the key to interpreting Mendel’s data.
Important Observations
• F1 progeny are heterozygous, but express only
one phenotype, the dominant one.
• In the F2 generation, plants with both
phenotypes are observed: i.e. some plants have
recovered the recessive phenotype.
• In the F2 generation, there are approximately
three times as many of one phenotype as the
other.
Mendel’s Conclusions
• Law of Unit Character
– Characteristics of an individual are controlled
by hereditary factors (genes) that maintain
their characteristics when passed down.
These factors occur in pairs.
A a
• Law of Dominance
– Some inherited factors are dominant and can
mask the other, recessive factor.
Mendel’s First Law:
Law of Segregation
• Each gamete contains only one factor
from each pair of factors (from the parent).
• The pair of factors from a parent separate
(or segregate) during reproduction, such
that an offspring receives one factor from
each parent. Each gamete is equally likely
to contain either factor of the pair.
• Fertilization restores the paired condition
in the offspring.
First Law  Meiosis
• Cellular division resulting in four gametes,
each possessing half the number of
chromosomes of the original cell.
A
b
A
A a
b
B b
“Segregation of Factors”
a
B
a
B
“Pair of Factors”
“Factor”
Round vs. Wrinkled
• The wild-type allele (W) is round, and the mutant
allele (w) is wrinkled. W is dominant to w.
• The enzyme SBEI (starch-branching enzyme)
makes branched starch (amylopectin).
• Round peas (W-) have a functional SBEI, resulting
in amylopectin. Wrinkled peas (ww) have a nonfunctional SBEI and, therefore, do not make
amylopectin.
• Amylopectin causes the seed to shrink uniformly.
As a result, round peas shrink uniformly and remain
round. Wrinkled peas shrink non-uniformly and
become ‘wrinkled.’
Punnett Square
• Grid used to determine the results of
simple genetics crosses.
Female
Round
Male
×
WW
Wrinkled
ww
Female
W
W
w
Ww
Ww
w
Ww
Ww
Male
Round
Ww
All Round (Ww)
Punnett Square
Female
Male
Female
Round
Wrinkled
×
WW
ww
Round
W
W
w
Ww
Ww
w
Ww
Ww
W
W
w
Ww
Ww
w
Ww
Ww
Male
Ww
Self-pollinate
Female
Male
Round
Round
Wrinkled
WW
Ww
ww
1
2
1
3 round : 1 wrinkled
(phenotypic ratio)
1 WW : 2 Ww : 1 ww
(genotypic ratio)
Chi-Squared (2) Analysis
• Tests if your observations are statistically
different from your expectation.
• For example, does the Mendel data (round
vs wrinkled) fit the 3:1 hypothesis?
• 2 = [(observed-expected)2/expected]
Observed
Data
Expected
Ratio
Expected
Data
obs - exp
(obs – exp)2
Round
Wrinkled
5474
1850
3
1
5493
1831
19
19
361
361
Total
7324
4
2= 722
Look up in 2 tables…The difference between observed and expected is NOT significant.
Therefore, the observed data fit the 3:1 hypothesis.
Testcross / Backcross
• Used to determine whether a dominant
individual is homozygous or heterozygous.
– Cross the dominant individual to a
homozygous recessive individual.
UNKNOWN
could be Round
WW or Ww
×
Wrinkled
ww
Two possible outcomes:
Round
ALL round
Ww
Unknown
was WW
Round
Wrinkled
1:1
Ww
ww
Unknown
was Ww
Mendel’s Second Law:
Law of Independent Assortment
• Each pair of factors (genes) segregates
(assorts) independently of the other pairs
in a di-hybrid (tri-hybrid, etc.) cross.
• Different pairs of alleles are passed to
offspring independently so that new
combinations are possible.
• Example: yellow seed color in pea plants
can appear in combination with either the
round or wrinkled phenotype.
Independent Assortment During
Meiosis-1
A
b
a B
A a
B b
OR
A
B
a b
Di-hybrid (2-Gene) Cross
W = round; w = wrinkled; G = yellow; g = green
Round
Yellow
Wrinkled
Green
×
W is dominant to w;
G is dominant to g.
All F1 are Round, Yellow
Self-Pollinate
Round
Yellow
315
Round
Green
108
Wrinkled Wrinkled
Yellow
Green
101
32
Take It One Gene at a Time
Round
Green
108
Round
Yellow
315
Round = 423
Wrinkled = 133
Wrinkled
Yellow
101
Wrinkled
Green
32
Yellow = 416
Green = 140
Each gene has a 3 : 1 ratio.
Put Two Genes Together
F1
Gametes &
Frequencies
WG
¼
WwGg
Wg
¼
wG
¼
wg
¼
If a gamete contains W, the probability that it contains G
is equal to the probability that it contains g (i.e. W/w and
G/g are independent of each other).
The BIG Punnett Square
WG
Wg
wG
wg
WG
WWGG
WWGg
WwGG
WwGg
Wg
WWGg
WWgg
WwGg
Wwgg
wG
WwGG
WwGg
wwGG
wwGg
wg
WwGg
Wwgg
wwGg
wwgg
9
3
3
1
Trait-Based Punnett Square
Yellow
¾
Green
¼
Round
¾
Round, Yellow
¾ x ¾ = 9/16
Round, Green
¾ x ¼ = 3/16
Wrinkled
¼
Wrinkled, Yellow
¼ x ¾ = 3/16
Wrinkled, Green
¼ x ¼ = 1/16
9
3
3
1
Violations of Mendel’s First Law
• Non-Dominant Gene Action
– Partial or incomplete dominance
• Flower color in snapdragon
Copyright © 2006 Pearson Prentice Hall Inc
– Co-dominance
• ABO blood groups
• Sex-linked inheritance
"Ishihara Color
Vision Test,"
developed by
Dr. Shinobu Ishihara
– Human red-green colorblindness
• Polygenic Inheritance
– Human skin color
ABC ABc AbC Abc aBC aBc abC abc
ABC
ABc
6
5
5
4
5
4
4
3
5
4
4
3
4
3
3
2
AbC
5
4
4
3
4
3
3
2
Abc
4
3
3
2
3
2
2
1
aBC
5
4
4
3
4
3
3
2
aBc
4
3
3
2
3
2
2
1
abC
4
3
3
2
3
2
2
1
Abc
3
2
2
1
2
1
1
0
Violations of Mendel’s First Law
• Organellar Inheritance (Maternal Effect)
– Mitochondria/chloroplast inherited from mother
• Epigenetic
– Imprinting, paramutation, gene silencing
• Penetrance, e.g. hemachromatosis
• Expressivity, e.g. timing of onset or severity
• Epistasis
– Interaction between different genes and alleles
• one gene masks the effect of another gene
• two gene pairs complement each other
Violations of Mendel’s Second Law
• Linkage
– Genes are physically linked to one
another on chromosomes
– Genes that are close together will
NOT assort independently
• Deviation from the 9:3:3:1 ratio
– Recombination (during meiosis)
may break these linkages
• Recombination is a function
of distance
L
M
N
O
P
l
m
n
o
p
Summary of Mendel
• Inherited traits are controlled by factors (genes)
present in the gametes.
• In a diploid, the pair of factors segregate during
gamete formation, and progeny inherit one
factor of each pair of factors (one allele) from the
mother and one from the father.
• Each pair of factors assorts independently, such
that new combinations are possible in progeny.
• With some exceptions (e.g. due to epigenetics
and linkage), Mendel established the foundation
of modern genetics.