CHAPTER 14
Mendel and the Gene Idea
Big Idea 3: Living systems store,
retrieve, transmit and respond to
information essential to life processes.
• Mendel was able to describe a model of inheritance of
traits, and his work represents an application of
mathematical reasoning to a biological problem.
• However, most traits result from interactions of many
genes and do not follow Mendelian patterns of
inheritance (ex. hair color, eye color.)
• Understanding the genetic basis of specific phenotypes
(physical outcomes due to proteins assembled) and their
transmission in humans can raise social and ethical
issues.
What is INHERITANCE?
The study of how genes
and traits are passed from
generation to generation.
Genetic information provides for
continuity of life and, in most cases,
this information is passed from
parent to offspring via DNA.
Review meiosis w/ your partner.
1. What are the 3 sources of genetic variation
for sexually reproducing organisms?
2. Why is it beneficial to be a “diploid”
organism?
1) Crossing over makes recombinant
chromosomes that are unique.
• Independent assortment makes new
combinations of chromosomes (one of each
kind) in the gametes by “shuffling” the pairs
of chromosomes.
• Fertilization joins gametes from two
separate gene lineages (hopefully) to make
a diploid zygote with new gene pairs.
2) Gene pairs = “back up copy” in case one
gene is mutated/nonfunctional.
GREGOR MENDEL:
The “father of genetics”
Austrian Monk
First to explain patterns of
inheritance.
Analyzed sweet pea plants
over 12 years.
Figure 14.x2 Round and wrinkled peas
Characteristic comes
in 2 contrasting traits:
Ex. PEA SHAPE
Round
Wrinkled
Ex. FLOWER COLOR
Purple
White
Mendel’s Experiments:
Parent Generation (P generation):
• Pure for one of two contrasting traits
• Result of many generations of self-pollination
Chose two plants pure for the two contrasting traits for
the characteristic flower color & crossed them.
purple X white 
Mendel’s
Experimental
RESULTS:
(F1) First Generation :
• Offspring of parent
generation
• All F1 plants looked like
only one of the parents (that
was the dominant trait)
• The recessive trait
“disappeared”
• Hybrid offspring
Table 14.1 The Results of Mendel’s F1 Crosses for Seven Characters in Pea Plants
Mendel’s 2nd
Experiment:
Mendel then crossed two F1
plants (“Ewww incest!” jk)
(F2) Second Filial
Generation :
• The traits from both
parents reappear in these
offspring
Mendel found the ratio of
expression to be 3:1
(3 dominant:1 recessive)
* note, not EXACTLY 3:1
IMPORTANT CONCLUSIONS:
1. If a characteristic exists in two contrasting forms one
is dominant, one is recessive.
2. Factors controlling traits occur in pairs.
3. The dominant factor prevents the recessive factor
from being expressed.
4. Recessive factors are only expressed when both
factors in the pair are recessive.
5. When pea plants reproduce, a factor pair is
segregated (split) and each factor ends up in a
separate gamete. Mendel’s law of segregation.
In modern terms:
1. Mendel’s factors are
called genes. Some genes
are dominant, others
recessive.
2. Different forms of a
single gene are called
alleles.
3. Genes occur in pairs, at
the same position on two
chromosomes.
4. The gene position is
called the locus.
5) The two chromosomes that contain the same genes are
called: homologous.
6) When solving inheritance problems, use capital letters to
represent dominant genes. T
7) When solving inheritance problems, use lowercase letters
to represent recessive genes. t
8) If both alleles in a gene pair are the same, we say the pair
is: homozygous (dominant or recessive) TT, tt
9) If both alleles in a gene pair are different, we say the pair
is: heterozygous. Tt
10) A genotype is the combination of genes an individual
possesses. (for one trait shown w/ pair of letters) TT
11) A phenotype is the appearance of an individual as the
genes are being expressed & proteins are being made
or not made.
The Punnett Square
• Used to solve inheritance problems
• Predicts the possible gene combinations inherited by
the offspring
• For example, a monohybrid cross: involves one
character/trait.
ex. Heterozygous X Heterozygous (on board)
Figure 14.8 Segregation of alleles and fertilization as chance events
POSSIBLE GENE
COMBINATIONS FOR
OFFSPRING ARE A
RESULT OF 3
CHANCE EVENTS:
1. Segregation of
alleles into eggs
2. Segregation of
alleles into sperm
3. Fertilization of
WHICH egg by
WHICH sperm
Figure 14.4 Mendel’s law of segregation (Layer 2)
Figure 14.6 A testcross
A test-cross: used to
determine the genotype
of an individual with a
dominant phenotype…
Not sure if they are
Homo dom: PP
Or
Hetero: Pp
cross the individual in
question with an
individual with a
recessive phenotype
____ X homo rec
-> offspring phenotype
If all dominant -> PP
If ½ recessive -> Pp
For another example, a DIHYBRID cross:
a two character/trait cross, with two gene pairs.
ex. AaBb X AaBb
(see board)
Attractive, Boring X Attractive, Boring
Man
Woman
Both are Heterozygous for both genes…
AaBb
X
AaBb
1. What gametes can each make?
2. What are their chances of having:
Attractive Boring kids? List the genotypes possible. #/16
Attractive Exciting kids? List the genotypes possible. #/16
Unattractive Boring kids? List the genotypes possible. #/16
Unattractive Exciting kids? List the genotypes… #/16
Mendel’s Law of Independent Assortment
states when the two gene pairs are located
on non-homologous chromosomes they
segregate independently of each other.
Results = 4 possible gamete
combinations. (use FOIL)
first: AB
outer: Ab
inner: aB
last: ab
Attractive, Boring X Attractive, Boring
Man
Woman
Both are Heterozygous for both genes…
AaBb
X
AaBb
1. What gametes can each make? AB, Ab, aB, ab
2. What are their chances of having:
Attractive Boring kids? List the genotypes possible. #/16
AABB, AaBB, AaBb, AABb
Attractive Exciting kids? List the genotypes possible. #/16
Aabb, AAbb
Unattractive Boring kids? List the genotypes possible. #/16
aaBB, aaBb
Unattractive Exciting kids? List the genotypes… #/16
aabb
AaBb X AaBb
AaBb X AaBb
AB
AB
Ab
aB
ab
Ab
aB
ab
AaBb X AaBb
AB
Ab
aB
ab
AB
AABB
AABb
AaBB
AaBb
Ab
AABb
AAbb
AaBb
Aabb
AaBB
AaBb
aaBB
aaBb
AaBb
Aabb
aaBb
aabb
aB
ab
AaBb X AaBb
AB
Ab
aB
ab
AB
AABB
AABb
AaBB
AaBb
Ab
AABb
AAbb
AaBb
Aabb
AaBB
AaBb
aaBB
aaBb
AaBb
Aabb
aaBb
aabb
aB
ab
Phenotype Ratio of dihybrid cross
9:3:3:1
9/16 Attractive, Boring
3/16 Attractive, Exciting
3/16 Unattractive, Boring
1/16 Unattractive, Exciting
Figure 14.7 Testing two hypotheses for segregation in a dihybrid cross
Now, it can’t be that easy &
boring can it?
No it can’t.
1)Incomplete Dominance:
Both alleles are the same strength
• Heterozygous
individuals…express
BOTH alleles, The
resulting phenotype is a
mix (blend) of the two.
• 3 phenotypes possible.
• For example: Carnation
flower color
• The alleles are also written
differently:
AA’ or CR CW
CRCR
CR CW
CWCW
2) Codominant Genes:
• In a heterozygous individual
both alleles affect the
phenotype in separate
distinguishable ways.
• For example: Roan Horses
have red (or black) and
white hairs
• Another example: A, B, and
AB blood groups in
humans.
3) Multiple Alleles:
When more than
2 forms of the
gene exist
• For example: Human Blood Types (ABO blood groups)
• Blood cells have a carbohydrate marker that is found on
the surface of red blood cells.
• These markers are called: A substance or B substance.
• They are recognized by antibodies present in the blood
serum of individuals for foreign carbohydrate markers.
• There are two types of RBC antibody: Anti-A & Anti-B
• The genes for creating these antibodies are determined by
the genes for creating the A or B substance.
Write the possible genotypes for the blood types A, B, AB and O
Blood Type
A
B
AB
O
Genotype
Antigens
on surface
of RBC
Plasma
Antibodies
Donate
Receive
What are the actual antigens (surface molecules) found on the RBCs?
Blood Type
A
B
AB
O
Genotype
IA IA
I Ai
IB IB
IBi
IA IB
ii
Antigens
Plasma
Antibodies
Donate
Receive
What kinds of antibodies does Blood Type A make? Who can he/she
Donate blood to? Receive it from?
Blood Type Genotype Antigens on
surface
A
B
AB
O
IA IA
IAi
IB IB
IBi
IA IB
Substance
A
ii
none
Substance
B
Substance
A and B
Plasma
Antibodies
Donate
Receive
Fill in the rest… check your work. Which kind are you? Which kind
Which is called the “universal donor”? The “universal recipient?”
Blood Type Genotype
A
B
AB
O
Substance
Plasma
on surface of Antibodies
RBC
IA IA
IAi
IB IB
IBi
IA IB
Substance
A
ii
none
Substance
B
AB
Anti-B
Donate
Receive
Type A
& AB
Type A
&O
Type AB is the universal recipient, Type O is the universal donor.
Blood Type Genotype
A
B
AB
O
antigens
Plasma
Antibodies
IA IA
IAi
IB IB
IBi
IA IB
Substance
A
Anti-B
Substance
B
Anti-A
AB
none
ii
none
Anti-A
Anti-B
Donate
Type A
& AB
Type B
& AB
TYPE
AB
All
blood
types
Receive
Type A
&O
Type B
&O
A, B,
AB & O
Type O
Figure 14.10 Multiple alleles for the ABO blood groups
Figure 14.10x ABO blood types
• Remember, the IA & IB alleles are
CODOMINANT.
• With human blood type, another marker called the
Rh factor is denoted as + or -.
• So, type O -, is the worst to be in terms of getting
a blood transfusion… but is the best donor.
• “universal donor”
1) Pleiotropy:
One gene influences many traits.
• (i.e. disease genes)
• PKU (phenylketonuria). Mutation in a single gene
that codes for an enzyme results in: mental
retardation, reduced hair, and skin pigmentation.
• Enzyme phenylalanine hydroxylase converts the
amino acid phenylalanine to tyrosine.
• Absence of the enzyme causes phenylalanine to
accumulate in toxic levels.
Figure 14.15 Pleiotropic effects of the sickle-cell allele in a homozygote
2) Epistasis: When two genes control the
expression of a single trait. One gene pair
interferes with the expression of the other.
•
•
•
•
Common for expression of pigment
For example: Fur color in mice.
Gene 1 = Fur color (B = Black fur, b = brown fur)
Gene 2 = Depositing pigment in the hair
(C = deposits color, c = deposits no color (albino))
• What is the phenotypic ratio that results from the
cross a Black haired mouse, heterozygous both
gene pairs and a Brown Haired mouseheterozygous for gene 2?
Figure 14.11 An example of epistasis
9:3:4
3) Polygenic inheritance: When
one phenotype is under the
control of multiple gene pairs
• For example: Human Skin Color
• Alleles are… Dark and light
• At least… three genes control the color of your
skin
• The phenotype is a result of… the cumulative
effects of the dominant genes
• Human skin color exists… as a gradient
Figure 14.12 A simplified model for polygenic inheritance of skin color
Figure 14.13 The effect of environment of phenotype
Multifactorial characters are influenced by genetics and environment.
Ph differences control the color of the hydrangeas.
TAN=genes + Sun exposure / melanin production.
"temperature sensitive" or ts mutants
Conditional mutants that grow at a low
temperature but not at a high temperature
are are called "temperature sensitive" or
ts mutants.
Conditional mutants are not necessarily
associated with lethality.
The dark ear tips, nose and feet of a Siamese cat
are the phenotype of a temperature
sensitive mutation in the c locus
(determining fur color).
The enzyme encoded is not functional at higher
temperatures, but is functional at lower
temperatures, such as the extremities of the
cat.
Hence the fur on these parts of the Siamese cat’s
body is pigmented.
PEDIGREE PROJECT (Complete over Thanksgiving)
3 generations
Examples of TRUE mendelian traits (show dominance)
WET EARWAX vs. dry earwax (recessive)
PIGMENT vs. Albinism
Recessive: magenta urine after eating beats
Pedigree
• An analysis of the results of
mating that have already occurredespecially when studying a species
with a long generation time.
• As much information as possible
is collected about a family’s
history for a particular trait.
• Information is assembled into a
family tree describing the
interrelationships across the
generations.
• Helps us understand the past and
predict the future.
For those traits exhibiting
dominant gene action:
•affected individuals have at
least one affected parent
•the phenotype generally
appears every generation
•two unaffected parents only
have unaffected offspring
And for those traits exhibiting
recessive gene action:
•unaffected parents can
have affected offspring
•affected progeny are both
male and female
Figure 14.16 Large families provide excellent case studies of human genetics
Figure 14.17 Testing a fetus for genetic disorders