Patterns of Inheritance
Chapters 14 and 15
A. P. Biology
Liberty Senior High School
Mr. Knowles
How do you make a
giraffe?
X
G. camelopardalis
Early Ideas of Genetics
• Saw patterns of inheritance in people
and domesticated plants and animals.
• Bizarre chimeras explained variation- not
true – heredity occurs within species.
• Thought traits were “blended” from
parents.
• Traits are transmitted directlyexplained by a seed “gonons”
(Hippocrates) or “humuculus”
(Leewenhoek)
Gregor Mendel (1866)
Wrinkled
Smooth
Pea Color
Why Peas (Pisum sativum)?
• Many varieties or strains of plant.
• These strains are true-breeding or
pure – produce the same trait
generation after generation.
• The strains can be hybridized or
strains crossed (T. A. Knight, 1790s).
• Can be self-fertilized or crossfertilized.
Table 14.1
• First, alternative versions of genes
– Account for variations in inherited characters, which
Allele for purple flowers
are now called alleles
Locus for flower-color gene
Figure 14.4
Allele for white flowers
Homologous
pair of
chromosome
s
Homologous
Chromosomes
A
C
G
Brown
T
A Allele
C
Locus
Gene
A
C
G
Blue G
Allele C
T
Some Terms
• Locus (i)- position on a chromosome
where a gene is located.
• Alleles- alternative forms of a gene.
Different genetic information for a
protein.
• Phenotype- “form that is shown”physical appearance of a trait.
• Genotype- the sum of an organism’s
alleles.
Phenotype versus Genotype
Phenotype
Purple
3
Purple
Genotype
PP
(homozygous)
1
Pp
(heterozygous)
2
Pp
(heterozygous)
Purple
1
Figure 14.6
White
pp
(homozygous)
Ratio 3:1
Ratio 1:2:1
1
Some Terms
• Dominant Allele- an allele whose
expression is readily seen; affects the
phenotype more.
• Recessive Allele-an allele whose
expression is less seen; affects the
phenotype less.
• Homozygous- organism with two
identical alleles at the same locus.
• Heterozygous- organism with two
different alleles at one locus.
Summary of Mendel’s Crosses
• A cross between homozygous
dominant X homozygous
recessive, F1 progeny are all
heterozygous, and resemble the
homozygous dominant parent in
phenotype.
• Two alternative alleles of a gene
segregate randomly.
A Testcross
APPLICATION An organism that exhibits a dominant trait,
such as purple flowers in pea plants, can be either homozygous for
the dominant allele or heterozygous. To determine the organism’s
genotype, geneticists can perform a testcross.

Dominant phenotype,
unknown genotype:
PP or Pp?
Recessive phenotype,
known genotype:
pp
If PP,
then all offspring
purple:
If Pp,
then 2 offspring purple
and 1⁄2 offspring white:
TECHNIQUE In a testcross, the individual with the
unknown genotype is crossed with a homozygous individual
expressing the recessive trait (white flowers in this example).
By observing the phenotypes of the offspring resulting from this
cross, we can deduce the genotype of the purple-flowered
parent.
p
1⁄
p
p
p
RESULTS
P
P
Pp
Figure 14.7
Pp
P
Pp
Pp
pp
pp
p
Pp
Pp
Testcross (Backcross)
• How can you tell if an organism
with a dominant phenotype is a Het.
or Homo.?
• To determine whether an individual
is a Het or Homo., cross the
individual with a known
homozygous recessive- Testcross.
Summary of Mendel’s Crosses
• If cross or self-fertilize the F2
generation, the result is a 3:1 ratio.
• Crosses with individuals that are
heterozygous at one locus-Monohybrid
Cross.
• The two alternative alleles segregate
independently from one another and are
distinct- Law of Segregation.
Law of Segregation
• Alternative forms of a gene (alleles)
are discrete and do not blend in
Hets.
• Alleles independently assort from
each other into gametes.
• Each gamete has an equal
probability of receiving either
allele.
Do different genes also
segregate independently?
• Examine crosses which involve two
genes. (Ex. seed shape and seed color).
Fig. 13.16, p. 282.)
• Crosses with individuals heterozygous at
two different loci- Dihybrid Crosses.
• Genes assort independently in the F2 with
a 9:3:3:1 ratio.
A dihybrid cross:
– Illustrates the inheritance of two characters
• Produces four phenotypes in the F2 generation
P Generation
EXPERIMENT Two true-breeding pea plants—
one with yellow-round seeds and the other with greenwrinkled seeds—were crossed, producing dihybrid F1 plants.
Self-pollination of the F1 dihybrids, which are heterozygous
for both characters, produced the F2 generation. The two
hypotheses predict different phenotypic ratios. Note that
yellow color (Y) and round shape (R) are dominant.
YYRR
yyrr
Gametes
YR
F1 Generation

yr
YyRr
Hypothesis of
independent
assortment
Hypothesis of
dependent
assortment
Sperm
Sperm
1⁄
2
RESULTS
F2 Generation
(predicted
offspring)
YR
1⁄
2
yr
Eggs
1⁄
2 YR
CONCLUSION The results support the hypothesis of
independent assortment. The alleles for seed color and seed
shape sort into gametes independently of each other.
YYRR YyRr
1⁄
2
1⁄
4
YR
1⁄
4
1⁄
4
Yr
yR
1⁄
4
yr
Eggs
1⁄
4
YR
1⁄
4
Yr
1⁄
4
yR
YYRR
YYRr
YyRR YyRr
YYrr
YYrr
YyRr
Yyrr
YyRR
YyRr
yyRR
yyRr
YyRr
Yyrr
yyRr
yyrr
yr
YyRr
3⁄
4
yyrr
1⁄
4
Phenotypic ratio 3:1
1⁄
4
yr
9⁄
16
3⁄
16
3⁄
16
1⁄
16
Phenotypic ratio 9:3:3:1
Figure 14.8 315
108
101
32
Phenotypic ratio approximately 9:3:3:1
Law of Independent
Assortment
• Genes located on different
chromosomes assort
independently of one
another- Independent
Assortment.
How would separate genes
located close to one another
on a chromosome be
inherited?
Linked Genes-do not assort
independently.
Was Mendel lucky?
Non-Mendelian
Inheritance
Complex Patterns of
Inheritance: How Genes
Interact
Incomplete Dominance
• Red (CRCR) X White (CWCW)
Snapdragons
• F1 generation are all pink (CRCW)
• F2 generation is 1 red:2 pink:1
white
• Not blending, parental phenotype
is recovered in the F2.
Incomplete Dominance
Red (CRCR)
Roan (CRCW)
X White (CWCW)
Codominance
• MN Blood Type: a single gene
locus (B) at which two alleles
(M and N) are possible.
Genotype
Phenotype
B M BM
M blood group
B N BN
N blood group
B M BN
MN blood group
Codominance
• The MN phenotype is not
intermediate between M and the
N phenotypes.
Codominance
• A,B,O Blood Type: a specific
locus (I) at which there are three
common alleles (A, B, and O).
They are modifying enzymes.
They modify cell surface
glycolipids.
Codominance in Blood Types
Enzyme
A
B
O
Function
adds a galactosamine
adds a galactose
does not add
anything
A, B, O, AB Blood Types
Genotype
IAIA or IAIO
IBIB or IBIO
IAIB
I OI O
Phenotype
+ galactosamine,
Blood Type A
+ galactose
Blood Type B
+ both Blood Type AB
neither added, Type O
Distribution of O Allele
Distribution of A allele
Distribution of B allele
A “Typical” Antibody
Compatible Blood Groups
• Donors and recipients must have
matching cell surface molecules.
• If not “self,” the recipient will produce
proteins called antibodies to
agglutinate (clump together) the
donated blood cells.
• The foreign cell surface molecule is an
antigen.
Agglutination Reactions
A Blood
B Blood
Agglutination Reactions
AB Blood
O Blood
Agglutination for the Rh or D
Antigen
Rh Positive Blood
Rh Negative Blood
Blood Group Compatibility
Blood Type
A
B
AB
O
Antibodies Produced
anti-B
anti-A
neither antibody
(universal recipient)
anti-A and anti-B
(universal donor)
Rh Factor in Humans
• Rh Blood Group: another cell surface
marker on RBC’s controlled by > 7
closely linked genes.
Genotype
Phenotype
R R or R r
cell surface marker
(about are Rh+ 85%)
rr
lack molecule, Rh-
Rh Factor in Humans
-
• What happens when an Rh female X Rh+
male?
• Offspring is possibly Rh+.
• If fetal Rh+ RBCs cross the placenta and
treated as a foreign antigen.
• Anitbodies (IgG) cross the placenta and
agglutinate fetal RBC’s- erythroblastosis
fetalis
• Treat with Rhogam: anit-Rh antibodies and
prevent maternal immune response.
Erythroblastosis fetalis
Genetic Diseases can be
Mendelian Dominant or
Recessive
Autosomal Dominant Diseases
• Homozygotes and
Heterozygotes can be
phenotypically the same- both
show disease phenotype.
• Lethal dominant diseases are less
common. Why?
Autosomal Dominant
Diseases
• Familial Hypercholesterolemiamost common; 1:500; 19p13.2-p13.1
• Huntington’s Disease- production of
an inhibitor of brain cell metabolism;
degeneration of nervous system at
middle age; lethal dominant;
1:10,000; 4p16.3
Familial Hypercholesterolemia
QuickTime™ and a
GIF decompressor
are needed to see this picture.
The Solution-Balloon
Angioplasty
A Stent
Marfan Syndrome- Dominant
Mutation
• Marfan’s
Syndromemutation in the
fibrillin gene
(glycoprotein in
connective
tissue).
Marfan’s Sufferer?
Mitral Valve Prolapse
Baby with Osteogenesis
Imperfecta
Osteogenesis Imperfectaautosomal dominant
Gene for Neurofibromatosis Type
2
NeurofibromatosisAutosomal Dominant
Joseph Merrick-N. F. or
Proteus Syndrome?
Baby with Achondroplasia
Achondroplasia- autosomal
dominant
• Affects in
1:10,000.
• Heterozygotes
have dwarf
phenotype.
• Homozygosity is
lethal.
Polydachtyly -dominant mutation at
13q21-q32, occurs only 1/400)
Autosomal Recessive
Diseases
• Heterozygotes are phenotypically
normal, called carriers.
• Only the homozygous recessive
alleles are diseased.
• Lethal Recessive Diseases are more
common. Why?
Cystic fibrosis-Autosomal Recessive
• Most common Caucasian genetic disease1: 2500 affected; 1:25 are carriers.
• Mutation in a chloride channel protein
(CFTR).
• Leads to high [Cl-]in extracellular fluid.
• Causes mucus to become thicker than
normal-favors bacterial infections.
• Untreated condition- death by fifth year.
Molecular Mechanisms of CF
C F Lung
Tay-Sachs Disease
• Recessive lethal allele-dysfunctional
hexosaminidase A; unable to metabolize
gangliosides (lipids of the CNS).
• Lipids accumulate--> lead to neuron
death and eventual death.
• Affects 1: 3600 European Jews.
• Only the homozygotes are affected and
die.
Tay-Sachs Diseased Tissue
Tay-Sachs DiseaseAutosomal Recessive
• Why are only the homozygous people
affected? In other words, why is this
disease recessive?
• Answer: the Heterozygote produces
about 1/2 the normal amount of
enzyme--> they are phenotypically
normal.
Genetic Diseases are Codominant at
the Molecular Level
• Sickle-cell Disease: a single amino acid
change at #6 (Glu-->Val) in the 146 a.a.
chain of hemoglobin.
• Mutant form of hemoglobin deforms the
RBCs at low [O2].
• Multiple Symptoms: anemia, clumping and
clogging of RBCs (heart failure and CV
disease), spleen and kidney damage.
Normal RBCs
Sickle-Celled RBCs
Sickle-Cell Clumping
Removing Damaged RBC’s by
Spleen
Frontal Bossing-Replacing the
RBCs
The Genetics of Sickle-Cell
Genotype
Phenotype
A+ A+
normal (9/10)
A+ As
Het. Carriers, usually
normal, the two alleles are
codominant; 1/10; resistance to
malaria.
As As
severe disease 1/400
African-Americans
Anopheles Mosquito
Malaria in a RBC
Dominance/Recessiveness
• Range from Complete--->Incomplete------>Codominance.
• Reflect the functions of the enzymes
encoded by the alleles and not one allele
subduing or overpowering another.
• Dominance does not determine the
relative frequency of alleles in a
population.
And That’s Dominance
and Recessiveness!
Other Patterns of
Inheritance
Complex Gene Interactions
Multiple Alleles Possible for a
Gene
• Incomplete Dominance or
Codominance• Ex. Coat color in cattle; Red X
White ---> Roan
A
B
• Ex. ABO blood type; the I and I
are equally expressed--> AB blood
type.
Pleiotropy
• When one gene or allele has multiple
phenotypes (pleion= many).
• Ex. Sickle-cell allele has many
symptoms:
Breakdown of RBCs--> Anemia,
Heart Failure, Physical Weakness
Clumping of RBCs--> Brain
Damage, Kidney and Spleen Damage.
Pleiotropy
• Often a gene functions in some
other unknown way.
• Ex. Lucien Cuenot- tried to
develop a true-breeding yellowfurred mouse. Y= dominant for
yellow fur color.
• Unable to get a YY strain. Why?
Pleiotropic Effects of Y
Yy (Yellow Fur Color,
Dominant)
Y allele
YY (Lethal Development,
Recessive)
Epistasis
• When a gene at one locus
alters the phenotypic
expression of another gene at
a second locus.
Epistasis
• Ex. Coat Color in Mammals:
One gene, the B locus:
B = black or b = brown
BB or Bb = both black, bb = brown
Another gene, C, deposits pigment into
hair
CC or Cc = dominant for color
cc = no pigment deposited, albino
Genetics of Coat Color in
Mammals
• What do the offspring of a
BbCc X BbCc (dihybrid) cross
look like?
• 9 Black : 3 Brown : 4 Albino
• What would Mendel predict?
• 9 : 3: 3 :1
Albinism in Humans
Another Example of Epistasis
• R. A. Emerson, 1918, Zea mays
• Crossed two pure-breeding strains that
never expressed purple pigment
(anthocyanin) in seed coat. All of the
F1 plants were purple!
• Crossed these F1 plants--> 56% of F2
purple, 44% were not. How?
Epistasis in Zea mays
Starting Molecule (Colorless)
Enzyme 1
Intermediate (Colorless)
Enzyme 2
Anthocyanin (Purple)
Epistasis in Zea mays
• Dominant alleles encode functional
enzymes and produce purple
pigment.
• Recessive alleles encode
nonfunctional enzymes.
• Requires BOTH dominant alleles
for the purple phenotype.
Another Example of EpistasisPTC Tasting
• Can two non-tasters produce a taster
child?
• Answer: Yes! tt X tt --> Taster
Offspring.
• I lied! This trait isn’t a simple
dominance/recessive trait.
• Research suggests the phenotype is
controlled by two genes.
Polygenic Traits
• These are not “either/or” characteristics,
but a continuum or gradation.
• Quantitative Characters-quantitative
variation indicates polygenic
inheritance- an additive effect of two or
more genes on a single phenotypic
character.
• Converse of Pleiotropy.
Polygenic Traits
• Ex. Skin Color in Humans controlled
by at least three separately inherited
genes.
Three Genes: A, B, C, dark-skin alleles,
each contribute one “unit” of darkness
and are incompletely dominant to the a,
b, c alleles.
AABBCC = very dark
aabbcc = very light
Human Skin Color
AaBbCc = intermediate skin color
Alleles have cumulative effect;
AaBbCc and AABbcc both make
same three unit contribution to
darkness.
Cross AaBbCc X AaBbCc
AaBbCc X AaBbCc Skin Color
aabbcc
Aabbcc
AaBbcc
AaBbCc
AABbCc
AABBCc
AABBCC
1/64
6/64
15/64
20/64
15/64
6/64
1/64
Very Light
Intermediate
Very Dark
Polygenic Traits
• Quantitative Characteristics- give
a bell-shaped curve, a normal
distribution.
• Environmental Factors (sun
exposure help smooth the curve also.
• Ex. Height and Weight
Multifactorial Inheritance
• Environmental factors interact with
genes.
• Genotype may be a phenotypic
range or possibilities- norm of
reaction for the genotype.
• The variation is due to
environmental factors.
Multifactorial Inheritance
• The norm of reaction may be
small- Example: ABO Blood type.
• Or it may be very broad- Example:
the Number of RBCs--> physical
activity, altitude, health, the genes
that control cell division.
• Hydrangea Flowers - of the same
genotype range in color from purple
(alkaline soils) to pink (acidic soils) due
to anthocyanin.
• Cardiovascular Disease- ApoE gene
(apolipoprotein E) and the angiotensin
genes affect cholesterol levels and blood
pressure levels--> genetic predisposition
+ lifestyle factors such as diet, smoking,
physical activity.
Some Defects are Multifactorial
The End !