Mendelelian
Genetics
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Jumbo Genetics Combo Powerpoint
for Regular Biology
See Genetics Powerpoint for AP Biology to Borrow Some good slides.
Gregor Mendel
(1822-1884)
Discovered the
Laws governing
Inheritance of
Traits
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Gregor Johann Mendel
Austrian monk
Studied the
inheritance of
traits in pea plants
Developed the laws
of inheritance
Mendel's work was
not recognized until
the turn of the
20th century
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Gregor Johann Mendel
Between 1856 and
1863, Mendel
cultivated and
tested some 28,000
pea plants
He found that the
plants' offspring
retained traits of
the parents
Called the “Father
of Genetics"
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Site of
Gregor
Mendel’s
experimental
garden in the
Czech
Republic
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The Blending Hypothesis
In the early 1800’s the blending hypothesis was popular.
What would happen if this was the case?
Particulate Inheritance
Mendel stated that physical traits
are inherited due to“particles”
Mendel did not know what the
“particles” were at the time.
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Chromosome Theory of Inheritance
Improved microscopy techniques, and a better understanding of
cellular processes converged with Mendel’s work during the late
1800’s and early 1900’s.
It was discovered that Mendelian inheritance has its physical basis in the
behavior of chromosomes during sexual life cycles.
Walter S. Sutton
Theodor Boveri
Hugo de Vries
Chromosomes and Classical Genetics
Walter Sutton in 1902 proposed that chromosomes
were the physical carriers of Mendel's “particles”.
Terminology – See written vocabulary
 Genes – portions of DNA that
code for one protein
 Genetics - study of heredity
 Heredity - passing of traits from
parent to offspring
 Alleles – different variations of a
gene, control the same trait, but
in a different way
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Terminology
 Dominant - stronger of two alleles
expressed in the hybrid; represented
by a capital letter (R)
 Recessive – the weaker of two alleles
which is not expressed; represented
by a lowercase letter (r)
 Trait (Character) – a physical feature
that can be passed from parent to
offspring
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More Terminology
 Genotype - gene or allele
combination for a trait (e.g.
RR, Rr, rr)
 Phenotype - the physical
feature resulting from a
genotype (e.g. red, white)
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More Terminology
Purebreeding - containing only one type of
allele for a trait in the reproductive line of
the organisms, can be dominant or
recessive (AA or aa)
Hybrid – another name for heterozygous,
containing two or more types of alleles for
a trait in the reproductive line of the
organisms (Aa)
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More Terminology
Homozygous – a genotype in which both
alleles are identical or pure (AA or aa)
Heterozygous (Hybrid) – a genotype in which
the two alleles for a trait are different
(Aa)
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Genotype & Phenotype in Flowers
Genotype of alleles:
R = red flower
r = yellow flower
All genes occur in pairs, so 2
alleles affect a characteristic
Possible combinations are:
Genotypes
RR
Rr
rr
Phenotypes
RED
RED
YELLOW
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Mendel’s Pea Plant
Experiments
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Why peas, Pisum sativum?
Can be grown in a
small area
Produce lots of
offspring
Produce pure plants
when allowed to
self-pollinate
several generations
Can be artificially
cross-pollinated
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Reproduction in Flowering Plants
Pollen contains sperm
Produced by the
stamen
Ovary contains eggs
Found inside the
flower
Pollen carries sperm to the
eggs for fertilization
Self-fertilization can
occur in the same flower
Cross-fertilization can
occur between flowers
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Mendel studied seven characteristics in the garden pea
:
Mendel’s Experimental Results
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Flower Color - Mendel cross-fertilized true-breeding plants that were
different in just one character—in this case, flower color. The Law of
Dominance was evident.
P generation (parental generation)
F1 generation (first filial generation, the word filial from
the Latin word for "son") are the hybrid offspring.
F2 generation (second filial generation, produced from
self-pollination of the F1 generation)
Height – regardless of which of the seven characteristics
were studied… the results were the same.
Cross 2
Pure
Plants
TT x tt
Results
in all
Hybrids
Tt
Cross 2 Hybrids
get
3 Tall & 1 Short
TT, Tt, tt
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Experimental
Crosses and Punnett
Squares
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Types of Genetic Crosses
 Purebreed cross – cross involving two
homozygous genotypes
 Monohybrid cross - cross involving a single
trait
 Dihybrid cross - cross involving two traits
 Test cross – crossing an unknown genotype
with a homozygous recessive genotype
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Meiosis Review
An understanding of meiosis is necessary to
fully understand genetics, so lets review
meiosis now. Advance slide.
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Metaphase I
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Punnett Square
Used to help
solve genetics
problems
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Karyotype
Purebreed Cross
Trait: Seed Shape
Alleles: R – Round
r – Wrinkled
Cross: Round seeds
x Wrinkled seeds
RR
x
rr
r
r
R
Rr
Rr
R
Rr
Rr
Genotype: Rr
Phenotype: Round
Genotypic
Ratio: 4/4 Rr
Phenotypic
Ratio: 4/4 Round
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Monohybrid Cross
Trait: Seed Shape
Alleles: R – Round
r – Wrinkled
Cross: Round seeds (Rr) x Round seeds (Rr)
R
r
R
RR
Rr
r
Rr
rr
Genotypes:
Phenotypes:
Ratios:
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Monohybrid Cross Results
 F2 Offspring:
25% Homozygous dominant RR
50% Heterozygous Rr
25% Homozygous Recessive rr
 Genotypic ratio is 1:2:1
 Phenotypic Ratio is 3:1
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What Do the Peas Look Like?
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Pedigree – another tool to analyze patterns of inheritance
Squares symbolize males and circles represent females. A horizontal line
connecting a male and female (--) indicates a mating, with offspring listed
below in their order of birth, from left to right. Shaded symbols stand for
individuals with the trait being traced.
Dihybrid Cross
A breeding experiment that tracks the
inheritance of two traits and is the
proof behind the “Law of Independent
Assortment”
Each pair of alleles (homologous
chromosomes in metaphase I)
segregates independently of the
other pairs during gamete formation
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Dihybrid Cross
Traits: Seed shape & Seed color
Alleles: R round
r wrinkled
Y yellow
y green
RrYy
x
RrYy
RY Ry rY ry
RY Ry rY ry
All possible gamete combinations
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Dihybrid Cross
Round/Yellow: 9
Round/green:
3
wrinkled/Yellow: 3
wrinkled/green: 1
9:3:3:1
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Test Cross (Back Cross)
Used to determine whether an individual is
homozygous dominant or heterozygous.
Method:
Always cross unknown genotype with a homozygous
recessive.
Observe (count) large numbers of offspring to insure
accuracy in determining the unknown genotype.
Then the genotype of the unknown should be evident
Testcross
What if…
B = Brown eyes
E = Curly hair
b = Blue eyes
e = Straight hair
An unknown genotype is crossed with bbee and the result is:
1) All offspring with blue eyes and straight hair
2) All offspring with brown eyes and curly hair
3) Some with blue eyes and straight hair
Some with brown eyes and curly hair
Some with blue eyes and curly hair
Some with brown eyes and straight hair
Mendel’s
Discoveries and
Laws
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Dominance:
Law of Segregation:
Law of Independent Assortment
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Dominance
(Some alleles mask the effects of weaker alleles)
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Law of Segregation
During the formation of gametes the
two alleles responsible for a trait
separate from each other
(Anaphase I).
Alleles for a trait are then "recombined" at
fertilization, producing the genotype for the
traits of the offspring.
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Applying the Law of Segregation
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Law of Independent Assortment
Alleles (actually chromosomes) for
different traits are distributed to
sex cells (& offspring) independently
of one another.
Mendel illustrated this law by using
dihybrid crosses.
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Summary of Mendel’s laws
LAW
DOMINANCE
SEGREGATION
INDEPENDENT
ASSORTMENT
PARENT
CROSS
OFFSPRING
TT x tt
tall x short
100% Tt
tall
Tt x Tt
tall x tall
75% tall
25% short
RrGg x RrGg
round & green
x
round & green
9/16 round seeds & green
pods
3/16 round seeds & yellow
pods
3/16 wrinkled seeds & green
pods
1/16 wrinkled seeds & yellow
pods
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Probability in Genetics
In doing the punnett squares you have
already begun to see probabilities in
expected genotype and phenotype
ratios.
Math collides with science again!
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Probability in Genetics
In humans there are 23 pairs of chromosomes and there
are 2 ways they can segregate in anaphase I.
Each of the 23 pairs can segregate in 2 ways during
anaphase I and the pairs assort independently of the
other homologous pairs.
This property of independent assortment alone leads to a
great deal of genetic variety in sexually reproducing
organisms.
223 = 8,308,688
The Rule: 2n (n = # of pairs of chromosomes in organism)
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Probability in Genetics
And that is just the number of chromosome combinations
in one parent organism.
If you calculate the possible chromosome combinations in
both parents, the number of possible combinations
increases as follows:
8,308,688 X 8,308,688 = Almost 70 trillion !
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Probability and Punnett Squares
Punnett square: diagram showing the
probabilities of the possible outcomes of a
genetic cross
Product rule: (Rule of Multiplication)
the probability of two independent events
occurring together can be calculated by
multiplying the individual probabilities of each
event occurring alone
Sum rule: (Rule of Addition)
the probability of the occurrence of one event or
the other event, of two mutually exclusive
events, is the sum of their individual
probabilities
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Calculating Dihybrid Probability
Rule of multiplication application with Dihybrid crosses:
heterozygous parents — YyRr
probability of producing yyrr?
probability of producing y gamete = 1/2
probability of producing r gamete = 1/2
probability of producing yr gamete
= 1/2 x 1/2 = 1/4
probability of producing a yyrr offspring
= 1/4 x 1/4 = 1/16
Non-Mendelian Genetics
(Modern Genetics)
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Incomplete Dominance
Incomplete Dominance
F1 hybrids have an appearance somewhat
in between the phenotypes of the two
parental varieties.
Example: snapdragons (flower)
red (RR) x white (rr)
r
r
RR = red flower
rr = white flower
R
R
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Incomplete Dominance
r
r
R Rr
Rr
R Rr
Rr
produces the
F1 generation
All Rr = pink
(heterozygous pink)
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Incomplete Dominance
(Traits appear to blend)
Incomplete Dominance
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RADISHES!!!
PURPLE = RW or WR
Red = RR
White = WW
Incomplete Dominance
In Incomplete Dominance, every genotype has its own
phenotype. (One allele not completely dominant over the
other.) Third phenotype that is a blending of the
parental traits. (2 alleles produce 3 phenotypes.)
Result: Heterozygous phenotype somewhere in between
homozygous phenotype.
Codominance
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Codominance
Neither allele is dominant; both are
expressed. A cross between organisms
with two different phenotypes produces
offspring that have both phenotypes.
2. Codominance
Both alleles contribute to the phenotype.
Example: In some chickens
Black Chicken x White  Speckled Chicken
Codominance
B = Black Feathers
W = White Feathers
Father = BB
Mother = WW
Phenotypes?
B
B
W
BW
BW
W
BW
BW
Codominance
B = Black Feathers
W = White Feathers
Father = BW
Mother = BW
Phenotypes?
B
W
B
BB
BW
W
BW
WW
Incomplete Dominance
B = Black Feathers
W = White Feathers
Father = BB
Mother = WW
Phenotypes?
B
B
W
BW
BW
W
BW
BW
Incomplete Dominance
B = Black Feathers
W = White Feathers
Father = BW
Mother = BW
Phenotypes?
B
W
B
BB
BW
W
BW
WW
Blood Types
Answer:
B
B
b
b
BB
ii
Parents:
genotypes = IAi and IBi
phenotypes = A and B
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Blood Types
1.
2.
3.
4.
type
type
type
type
A
B
AB
O
=
=
=
=
IAIA or IAi
IBIB or IBi
IAIB
ii
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Codominance Problem
Example: homozygous male Type B (IBIB)
x
heterozygous female Type A (IAi)
IA
i
IB
IAIB
IBi
IB
IAIB
IBi
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1/2 = IAIB
1/2 = IBi
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Another Codominance Problem
• Example: male Type O (ii)
x
female type AB (IAIB)
IA
IB
i
IAi
IBi
i
IAi
IBi
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1/2 = IAi
1/2 = IBi
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Codominance
Question:
If a boy has a blood type O and
his sister has blood type
AB,
what are the genotypes
and
phenotypes of their
parents?
boy - type O (ii)
AB (IAIB)
X
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girl - type
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Codominance
Answer:
IA
IB
i
i
IAIB
ii
Parents:
genotypes = IAi and IBi
phenotypes = A and B
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Sex-linked Traits
Traits determined by genes located on
the sex chromosomes
In humans 23rd pair = sex chromosomes
1 – 22 = Autosomes
Mammals:
XX genotype for females
XY genotype for males
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Thomas Hunt Morgan First Recognized
Sex linkage in Fruit Flies (Drosophila)
He found that white eyed
flies were very rare and
almost all of the white eyed
flies were male suggesting a
link to gender.
Sex Linked Traits
Any gene on either sex chromosome could be called a sex
linked gene, but the term is more often used in
reference to the X chromosome.
The X and Y chromosomes are not homologous and the Y
is much smaller and carries fewer genes.
Most, but not all of these genes code for proteins related
to gender specific functions.
In a male all alleles on the X and Y chromosomes are
expressed because there is not a corresponding allele
on an homologous chromosome.
Example: Eye color in fruit flies
Sex Chromosomes
fruit fly
eye color
XX chromosome - female
Xy chromosome - male
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(Red-Eyed Male) x (White-Eyed Female)
XRY
x
XrXr
R = Red
r = White
Xr
Xr
XR
Y
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Hemophilia is a sex-linked recessive trait defined by the absence of one or
more of the proteins required for blood clotting.
Color Blindness In Humans: An X-Linked Trait
Numbers That You Should See If You Are In One Of The Following
Four Categories: [Some Letter Choices Show No Visible Numbers]
Sex-Linked Traits:
1. Normal Color Vision:
A: 29, B: 45, C: --, D: 26
2. Red-Green Color-Blind:
A: 70, B: --, C: 5, D: -3. Red Color-blind:
A: 70, B: --, C: 5, D: 6
4. Green Color-Blind:
A: 70, B: --, C: 5, D: 2
Polygenic Traits:
Many traits may
have a wide
range of
continuous
values. Eg.
Human height
can vary
considerably.
There are not
just "tall" or
"short" humans
Multiple Alleles
Phenotypes are controlled by more than 1 allele. Eg. Blood types are
regulated by 3 separate genes.
ABO Blood typing
Humans have multiple types of surface antigens on RBC's
The nature of these surface proteins determines a person's
Blood Type.
There are 3 alleles which determine blood type IA, IB, or IO.
This is referred to as having multiple alleles
Human blood types are designated as A, B or O.
Type A denotes having the A surface antigen, and is denoted by IA
Type B denotes having the B surface antigen, and is denoted by IB
Type O denotes having neither A or B surface antigen, and is
denoted by IO
There are several blood type combinations possible
A
B
AB (Universal recipient)
O (Universal donor)
Genes and Environment
Determine Characteristics
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Environmental Influence on Phenotype
Hydrangea