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Genetics is everywhere these
days – and it will continue as
a dominant force in biology
and society for decades to
come.
Wouldn’t it be nice if people
understood it better?
GREGOR
MENDEL
Biography
monk in Austria in
mid-1800s
studied statistics
Teacher
Gardener
MENDEL’S GARDEN PEAS
heredity: transmission of traits from
parents to offspring
traits: category in which one or more
variations an be found (ex: flower color)
Allele: value of a trait (purple flowers)
Molecular Genetics: study of
chromosomes and genes
MENDEL’S TRAITS
TRAIT
VARIANTS
TRAIT
VARIANTS
MENDEL’S METHODS
mated known traits together methodically
cross-pollination: involves flowers of two
separate plants
self-pollination: involves flowers of only one
plant
peas usually self pollinate, but Mendel was able
to force cross-pollination
Mendel’s Model
Organism –
the Garden Pea
A Method to the
Madness – How
Mendel Crossed
Strains of Pea
MENDEL’S EXPERIMENTS
Began with “pure” plants
Cross-pollinated P generation to create
F1 generation
Self-pollinated F1 generations to create F2
generation
Mendel was able to record the results of
every cross
MENDEL’S RESULTS AND
CONCLUSIONS
Recessive and Dominant Traits
All members of F1 generations had similar
traits, but F2 generation showed variation
The Reality of “Round and Wrinkled” – Two Alternative Traits of
the Seed Shape Character
It’s worth
noting that
each of these
seeds is a new
individual of a
different
generation
from the plant
that bears the
peas.
MENDEL’S RESULTS AND
CONCLUSIONS
Law of Dominance
when present, a dominant allele will be expressed
DOMINANT
AND
RECESSIVE
TRAITS
Phenotypes when dealing with different genotypes
MENDEL’S RESULTS AND
CONCLUSIONS
Law of segregation
a pair of factors is segregated, or separated, during
the formation of gametes
homologous pairs split during anaphase I
Monohybrid Crosses and the Principle of Segregation
A cross between individuals differing
in single character is a monohybrid
cross.
The analysis of monohybrid crosses
allowed Mendel to deduce the
Principle of Segregation ....
Genes come in pairs that separate
in the formation of sex cells (and
these sex cells unite randomly at
fertilization).
MENDEL’S RESULTS AND
CONCLUSIONS
Law of independent assortment
factors for different characteristics are
distributed to gametes independently
the orientation of one homologous pair does
not affect the orientation of other
homologous pairs
Independent Assortment in Meiosis
Linkage
Each individual has
exactly two alleles for
each trait.
They may be the same or
different.
Each gamete has
exactly one allele for
each trait
GENOTYPE AND PHENOTYPE
Terms you need to know:
Genotype
Phenotype
Homozygous
Heterozygous
PROBABILITY: LIKELIHOOD THAT
A SPECIFIC EVENT WILL OCCUR
numberof positiveoutcomes
Probability 
numberof possibleoutcomes
B. PROBABILITY OF A RECESSIVE
TRAIT IN F2 GENERATION: 1/4
Mendel’s Monohybrid Cross –
P to F1
A Punnett
square,
something we’ll
cover in a
moment.
Homozygous
x
homozygous
PP
P
P
Pp
Pp
Pp
Pp
p
pp
p
Staying the Course –
Mendel Continued
Crosses to the F2 (the
grandchildren)
What was learned?
The green trait was not lost or
altered, even though it disappeared
in the F1.
One trait is dominant to the other
in its expression.
The reappearance of the recessive trait in
¼ of the F2, suggests genes come in pairs
that separate in the formation of sex cells.
Pp x Pp
Pp
P
Heterozygous x
heterozygous
p
PP
Pp
Pp
pp
P
Pp
p
PP x Pp
or
pp x Pp
Pp
P
p
Pp
pp
Pp
pp
p
Homozygous
x
heterozygous
Test crosses
pp
p
Test Cross results for
unknown hybrid
Pp
P
p
Pp
pp
Pp
pp
p
pp
p
Test Cross results for
unknown pure bred
PP
P
P
Pp
Pp
Pp
Pp
p
pp
p
PREDICTING RESULTS OF
DIHYBRID CROSSES
Dihybrid: cross between
individuals that involves
comparing two traits
Homozygous x homozygous to produce dihybrids
RRYY
RY
rryy
Predict the gametes
ry
ry
ry
ry
Predict
RY the gametes
RY
RY
RrYy
RrYy
RrYy
RrYy
RrYy
RrYy
RrYy
RrYy
Predict the F1 generation
RrYy
RrYy
RrYy
RrYy
RrYy
RrYy
RrYy
RrYy
MAKING THE GAMETES
RRYY
RY
Each gamete has
exactly one allele for
each trait
All possibilities are equally likely
RrYy
RY Ry
rY
FOIL
(R+r)(Y+y)
ry
independent
assortment in
action
Heterozygous x heterozygous to produce
dihybrids
RrYy
RrYy
Predict the gametes
RY
Ry
rY
ry
RY
Predict
Ry the gametes
rY
ry
RRYY
RRYy
RrYY
RrYy
RRYy
RRyy
RrYy
Rryy
Predict
Predict
thethe
F1 offspring
generation
RrYY
RrYy
rrYY
rrYy
RrYy
rryy
Rryy
rrYy
With Complete Dominance, Different Genotypes Can
Produce the Same Phenotype
Mendel Truly Stayed the
Course, Creating an F3
(great-grandchildren) to
Test the Hypothesis of
Segregation
Are Different Characters Like
Color and Shape Inherited
Together or Inherited
Independently?
Mendel performed dihybrid
crosses to find out.
Mendel’s conclusion: Different
characters are inherited
independently.
What Works for Peas Also Works for Humans
In the cross Aa x Aa, where A is a
dominant allele for wild type
(standard) pigmentation and a is a
recessive allele for no pigmentation
(albinism), ¾ of offspring will be
wild type and ¼ will be albino.
An albino woman
RULES FOR WRITING GENOTYPES
1. ALWAYS KEEP ALLELES FOR EACH
TRAIT TOGETHER
2. ALWAYS PLACE DOMINANT ALLELES
BEFORE RECESSIVE ALLELES
Some Alleles Are Related
Through Incomplete Dominance
Incomplete dominance is a partial
expression of both alleles
Dominance relationships may differ,
but the Principle of Segregation
remains the same.
Pleiotropy – When One Allele Influences Many Traits
EXAMPLES OF PLEIOTROPY
Marfan Syndrome
PKU
can cause weakness of the aorta and
extremely long thin bones
can cause mental retardation and
reduced hair and skin pigmentation
Polygenic Inheritance – When a Single Trait is Influenced by
Many Genes
Height is a
polygenic trait
This slide
took too
long to
make to
remove it,
even it
doesn't
make sense
Skin Color is
polygenic
SAsaSBsbSCsc
SAsaSBsbSCsc
20
––
64
Fraction of population
15
––
64
6
––
64
1
––
64
Skin color
SUMMARY
Pleiotropy
Polygenism
Trait
Gene
Trait
Trait
Gene
Trait
Gene
Trait
Trait
Trait
Gene
Gene
Gene
Gene
Multiple Alleles
Many genes are present in 3 or more
versions (alleles) – this is known as
multiple alleles.
The human ABO blood group is
determined by three alleles (IA, IB, and i)
of a single gene.
Codominance
The human ABO blood group illustrates
another genetic phenomenon –
codominance.
Codominance occurs when the
phenotype associated with each allele is
expressed in the heterozygote.
The AB phenotype (genotype IA IB) is an
example of codominance
Chromosomes and Inheritance
Since genes are carried
on chromosomes,
knowledge of
chromosome number
and structure has farreaching implications for
basic genetics and
human health.
A normal human male karyotype.
X-linked Inheritance
X-linked Inheritance – When Men and Woman Play by
Different Rules
Behind the 8-ball? Colorblindness is an X-linked recessive trait.
X-linked Inheritance – When Men and Woman Play by
Different Rules
What a color
blind individual
sees.
What a
normal
person
sees.
Behind the 8-ball? Colorblindness is an X-linked recessive trait.
Many Genetic Diseases are Autosomal Recessive Traits
What’s an autosome?
Sickle cell anemia is a recessive autosomal disease common in
areas where malaria is endemic.
What Works in Peas Works (genetically
speaking) Works in People
¼ of offspring of
two carriers of a
recessive allele
are expected to
show the recessive
trait; ½ of offspring
are expected to be
carriers.
Many Human Traits are Autosomal Dominant Traits
For disease
traits,
autosomal
dominant
inheritance
is less
common
than
autosomal
recessive
inheritance.
The Fundamental Question
What is the relationship
between genes (genotype)
and observable
characteristics (phenotype)?
or
Nature versus nurture?
The answer?
Genotype
+ Environment
Phenotype
Genes and Environment Determine Characters
Genetically identical hydrangeas growing in soils of different acidity
(different environments).
The phenotype = genotype + environment principle applies equally to
human traits.
CONCEPT CHECK
The figure diagrams one of the
genetic crosses that helped Mendel
form his theories of inheritance.
Which process distributes “P” alleles
to approximately one half of the F1
gametes and the “p” allele to the
other half?
Independent assortment.
Mitosis.
Meiosis.
Chromosome crossover.
ANSWER
The figure diagrams one of the
genetic crosses that helped Mendel
form his theories of inheritance.
Which process distributes “P” alleles
to approximately one half of the F1
gametes and the “p” allele to the
other half?
3)
Meiosis.
CONCEPT CHECK
Mendel’s principle of independent assortment holds true only for traits
with genes that
are on homologous chromosome (linked).
are on separate chromosomes (unlinked).
have loci far apart on homologous chromosomes.
are both “1” and “2”.
ANSWER
Mendel’s principle of independent assortment holds true only for traits
with genes that
4)
are both “1” and “2”.
CONCEPT CHECK
The traits studied by Mendel were mostly discrete traits with two phenotypes.
Many traits such as human height vary continuously. Which of the following
describe the inheritance of continuously varying traits using Mendel’s rules of
inheritance?
1) Two or more genes interact to
produce the variation in the trait
apparent in the population of the
organism.
2)
Two or more alleles interact to
produce the variation in the trait
apparent in the population of the
organism.
3)
Mendel’s principles do not apply to
continuously varying traits.
4)
Traits with continuous variability are
not influenced by heredity.
ANSWER
The traits studied by Mendel were mostly discrete traits with two phenotypes.
Many traits such as human height vary continuously. Which of the following
describe the inheritance of continuously varying traits using Mendel’s rules of
inheritance?
1) Two or more genes interact to
produce the variation in the
trait apparent in the population
of the organism.

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