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Mendel & Inheritance Sherry Flint-Garcia USDA ARS MU Division of Plant Sciences.
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Transcript of Mendel & Inheritance Sherry Flint-Garcia USDA ARS MU Division of Plant Sciences.
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Mendel & Inheritance
Sherry Flint-Garcia
USDA ARS
MU Division of Plant Sciences
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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
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Trait
• A broad term encompassing a distribution of phenotypic variation.
• Two types of traits:
Qualitative or DiscreteTrait: Kernel Color
Phenotype: Purple vs. Yellow
Governed by one or few genes
Quantitative or ContinuousTrait: Corn Rootworm Resistance
Phenotype: 1 (no damage) to 6 (severe damage)
Usually governed by many genes
1 2 3 4 5 6Source: USDA
Source: MGDB
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Advantages of Plants• Can make controlled crosses.
– Including making inbreds (in many plant species) without the severe effects that are typical in animals.
• 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.
Source: MGDB
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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!!!!
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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.
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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
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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.
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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.
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Mendel’s Experiment
Parental (Inbred) Lines Round Wrinkled×
ALL Round
F1 progeny
3 Round : 1 Wrinkled
Self-pollinate
Round5474
Wrinkled1850
F2 progeny
Since the F1 progeny are all round, we know that the
round phenotype is dominant and the wrinkled phenotype
is recessive.
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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.
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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.
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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.
• Law of Dominance– Some inherited factors are dominant and can
mask the other, recessive factor.
A a
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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.
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First Law Meiosis
• Cellular division resulting in four gametes, each possessing half the number of chromosomes of the original cell.
A Ba b AA BBaa bb
AA bb
aa BB
A b
a B
a B
A b
“Pair of Factors”
“Factor”
“Segregation of Factors”
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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 non-functional 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.’
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Punnett Square
• Grid used to determine the results of simple genetics crosses.
WwWww
WwWww
WWMale
Female
WW ww×
Ww
Round Wrinkled
Round
Female Male
All Round (Ww)
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Punnett Square
WwWww
WwWww
WWMale
Female
WwWww
WwWww
WWMale
Female
WW ww×
Ww
Round Wrinkled
Round
Female Male
WW wwWw
Round WrinkledRound
1 2 1
3 round : 1 wrinkled (phenotypic ratio)
Self-pollinate
1 WW : 2 Ww : 1 ww (genotypic ratio)
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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 Expected Expected obs - exp (obs – exp)2
Data Ratio Data
Round 5474 3 5493 19 361Wrinkled 1850 1 1831 19 361
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.
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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
WW or Wwww
×
ALL round Ww
Wrinkled
Round
Ww
Round
Round Two possible outcomes:
ww
Wrinkled
Unknown was WW
Unknown was Ww
1 : 1
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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.
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Independent Assortment During Meiosis-1
A Ba b AA BBaa bb
AA bb
aa BB
A
a
b
B
AA bb
aa BB
A
a
B
b
OR
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W = round; w = wrinkled; G = yellow; g = green
Di-hybrid (2-Gene) Cross
RoundYellow
WrinkledGreen
All F1 are Round, Yellow
RoundYellow
315
RoundGreen
108
WrinkledYellow
101
WrinkledGreen
32
×
Self-Pollinate
W is dominant to w; G is dominant to g.
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Take It One Gene at a Time
RoundYellow
315
RoundGreen
108
WrinkledYellow
101
WrinkledGreen
32
Round = 423Wrinkled = 133
Yellow = 416Green = 140
Each gene has a 3 : 1 ratio.
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Put Two Genes Together
WwGg
WG¼
Wg¼
wG¼
wg¼
F1
Gametes &Frequencies
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).
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The BIG Punnett SquareWG 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
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Trait-Based Punnett SquareYellow
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
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Violations of Mendel’s First Law
• Non-Dominant Gene Action– Partial or incomplete dominance
• Flower color in snapdragon
– Co-dominance • ABO blood groups
• Sex-linked inheritance – Human red-green colorblindness
• Polygenic Inheritance– Human skin color
"Ishihara Color Vision Test," developed by Dr. Shinobu Ishihara
Copyright © 2006 Pearson Prentice Hall Inc
ABC ABc AbC Abc aBC aBc abC abc
ABC 6 5 5 4 5 4 4 3
ABc 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
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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
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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
A
C
A
C
BB
a
c
a
c
bb
L M N O P
l m n o p
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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.