Mendel’s
Breakthrough
Patterns, Particles, and Principles of
Heredity
r. gilbert
Outline of Mendelian Genetics
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The historical puzzle of inheritance and how
Mendel’s experimental approach helped solve it
Mendel’s approach to genetic analysis including
his experiments and related analytic tools
A comprehensive example of Mendelian
inheritance in humans
Gregor Mendel (1822-1844)
Fig. 2.2
Themes of Mendel’s work
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Variation is widespread in nature
Observable variation is essential for following
genes
Variation is inherited according to genetic laws
and not solely by chance
Mendel’s laws apply to all sexually reproducing
organisms.
The historical puzzle of inheritance
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Artificial selection has been an important practice
since before recorded history
Domestication of animals
 Selective breeding of plants
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19th century – precise techniques for controlled
matings in plants and animals to produce desired
traits in many of offspring
Breeders could not explain why traits would
sometimes disappear and then reappear in
subsequent generations.
State of genetics in early 1800’s
What is inherited?
How is it inherited?
What is the role of chance in
heredity?
Mendel’s workplace
Fig. 2.5
Historical theories of inheritance
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One parent contributes
most features (e.g.,
homunculus, N.
Hartsoiker, 1694)
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Blending inheritance –
parental traits become
mixed and forever
changed in offspring
Fig.2.6
Keys to Mendel’s experiments
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The garden pea was an ideal organism
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Mendel analyzed traits with discrete alternative forms
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Vigorous growth
Self fertilization
Easy to cross fertilize
Produced large number of offspring each generation
purple vs. white flowers
yellow vs. green peas
round vs. wrinkled seeds
long vs. short stem length
Mendel established pure breeding lines to conduct his
experiments
Monohybrid crosses reveal units of
inheritance and Law of Segregation
Fig.2.9
Traits have dominant and recessive
forms
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Disappearance of traits in F1 generation and
reappearance in the F2 generation disproves the
hypothesis that traits blend
Trait must have two forms that can each breed
true
One form must be hidden when plants with
each trait are interbred
Trait that appears in F1 is dominant
Trait that is hidden in F1 is recessive
Alternative forms of traits are alleles
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Each trait carries two copies of a unit of
inheritance, one inherited from the mother and
the other from the father
Alternative forms of traits are called alleles
Law of Segregation
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Two alleles for each
trait separate
(segregate) during
gamete formation,
and then unite at
random, one from
each parent, at
fertilization
Fig. 2.10
The Punnet Square
Fig. 2.11
Rules of Probability
Independent events - probability of two events occurring together
What is the probability that both A and B will occur?
Solution = determine probability of each and multiply
them together.
Mutually exclusive events - probability of one or another event
occurring.
What is the probability of A or B occurring?
Solution = determine the probability of each and add
them together.
Probability and Mendel’s Results
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Cross Yy xYy pea plants.
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Chance of Y sperm uniting with a Y egg
½ chance of sperm with Y allele
 ½ chance of egg with Y allele
 Chance of Y and Y uniting = ½ x ½ = ¼
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Chance of Yy offpsring
½ chance of sperm with y allele and egg with Y allele
 ½ chance of sperm with Y allele and egg with y allele
 Chance of Yy – (½ x ½) + (½ x ½) = 2/4, or 1/2
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Further crosses confirm predicted
ratios
Fig. 2.12
Genotypes and Phenotypes
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Phenotype – observable characteristic of an
organism
Genotype – pair of alleles present in and
individual
Homozygous – two alleles of trait are the same
(YY or yy)
Heterozygous – two alleles of trait are different
(Yy)
Genotypes versus phenotpyes
Yy  Yy
1:2:1
YY:Yy:yy
3:1
yellow: green
Fig. 2.13
Test cross reveals unkown genotpye
Fig. 2.14
Dihybrid crosses reveal the law of
independent assortment
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A dihybrid is an individual that is heterozygous
at two genes
Mendel designed experiments to determine if
two genes segregate independently of one
another in dihybrids
First constructed true breeding lines for both
traits, crossed them to produce dihybrid
offspring, and examined the F2 for parental or
recombinant types (new combinations not
present in the parents)
Results of Mendels dihybrid crosses
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F2 generation contained both parental types and
recombinant types
Alleles of genes assort independently, and can
thus appear in any combination in the offspring
Dihybrid cross shows parental and
recombinant types
Fig. 2.15 top
Dihybrid cross produces a
predictable ratio of phenotypes
Fig. 2.15 bottom
The law of independent assortment
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During gamete formation different pairs of alleles
segregate independently of each other
Fig. 2.16
Summary of Mendel's work
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Inheritance is particulate - not blending
There are two copies of each trait in a germ cell
Gametes contain one copy of the trait
Alleles (different forms of the trait) segregate
randomly
Alleles are dominant or recessive - thus the
difference between genotype and phenotype
Different traits assort independently
Laws of probability for
multiple genes
P
RRYYTTSS X rryyttss
gametes
F1
RYTS
ryts
RrYyTtSs
X RrYyTtSs
RYTS
RYTs
RYtS
RYts
RYTS
RYTs
RYtS
RYts
RyTS
RyTs
RytS
Ryts
RyTS
RyTs
RytS
Ryts
rYTS
rYTs
rYts
rYTS
rYTS
rYTs
rYts
rYTS
ryTs
rYtS
rYts
ryts
ryTs
rYtS
rYts
ryts
gametes
F2 What is the ratio of different genotypes
and phenotypes?
Punnet Square method - 24 = 16 possible gamete
combinations for each parent
Thus, a 16  16 Punnet Square with 256 genotypes
That’s one big Punnet Square!
Loci Assort Independently - So we can look at each locus
independently to get the answer.
P
F1
RRYYTTSS  rryyttss
RrYyTtSs  RrYyTtSs
What is the probability of obtaining the genotype RrYyTtss?
Rr  Rr
Yy X Yy
Tt  Tt
Ss  Ss
1RR:2Rr:1rr 1YY:2Yy:1yy 1TT:2Tt:1tt
1SS:2Ss:1ss
2/4 Rr
1/4 ss
2/4 Yy
2/4 Tt
Probability of obtaining individual with Rr and Yy and Tt and ss.
2/4  2/4  2/4  1/4 = 8/256 (or 1/32)
P
F1
RRYYTTSS  rryyttss
RrYyTtSs  RrYyTtSs
What is the probability of obtaining a completely homozygous
genotype?
Genotype could be RRYYTTSS or rryyttss
Rr  Rr
Yy  Yy
Tt  Tt
Ss  Ss
1RR:2Rr:1rr 1YY:2Yy:1yy 1TT:2Tt:1tt
1SS:2Ss:1ss
1/4 RR
1/4 rr
1/4 SS
1/4 ss
1/4 YY
1/4 yy
1/4 TT
1/4 tt
(1/4  1/4  1/4  1/4) + (1/4  1/4  1/4  1/4) = 2/256
Rediscovery of Mendel
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Mendel’s work was unappreciated and remained
dormant for 34 years
Even Darwin’s theories were viewed with
skepticism in the late 1800’s because he could
not explain the mode of inheritance of variation
In 1900, 16 years after Mendel died, four
scientists rediscovered and acknowledged
Mendel’s work, giving birth to the science of
genetics
1900 - Carl Correns, Hugo deVries,
and Erich von Tschermak rediscover
and confirm Mendel’s laws
Fig. 2.19
Mendelian inheritance in humans
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Most traits in humans are due to the interaction
of multiple genes and do not show a simple
Mendelian pattern of inheritance.
A few traits represent single-genes. Examples
include sickle-cell anemia, cystic fibrosis, TaySachs disease, and Huntington’s disease (see
Table 2.1 in text)
Because we can not do breeding experiments on
humans, we use model organisms.
In humans we must use pedigrees to
study inheritance
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Pedigrees are an orderly diagram of a families
relevant genetic features extending through
multiple generations
Pedigrees help us infer if a trait is from a single
gene and if the trait is dominant or recessive
Anatomy of a pedigree
Fig. 2.20
A vertical pattern of inheritance
indicates a rare dominant trait
Fig. 2.20
Hunitington’s disease: A rare dominant trait
Assign the genotypes by working backward through the pedigree
A horizontal pattern of inheritance
indicates a rare recessive trait
Fig.2.21
Cystic fibrosis: a recessive condition
Assign the genotypes for each pedigree