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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology (BIOL 107); Instructor: Elmar Schmid, Ph.D.
Chapter 9: Variations to Mendel’s patterns of inheritance

Usually the patterns of inheritance are more complex than the mono- or dihybrid
crosses which described by Mendel with the common garden pea

There are many patterns observable in nature where, e.g. one allele is not completely
dominant to the other allele, or where more than one gene is responsible for the
expression of a certain phenotype

In the 20th century, other geneticists, including Tschermak, de Vries, Correns and
Morgan, have extended Mendelian principles not only to diverse organisms, but also to
patterns of inheritance more complex than Mendel described. Looking back today, we
know that Mendel had the good fortune (or insight) to choose a system that was
relatively simple genetically. He was lucky that in the garden pea, each of the
characters (but one) he studied is controlled by a single gene.
R
Ruullee ooff IInnccoom
mpplleettee D
Doom
miinnaannccee

When Mendel crossed his true-bred and parental yellow-seed plants with green-seed
plants, he always received yellow-seed pea plants. He observed complete dominance

However, there are many examples known in nature, where crossing of two different
pure-bred varieties leads to offspring which show a blending of a the inbred phenotype;
geneticist speak then of an incomplete dominance; in this case neither of the two
inherited Alleles is dominant.

A clear example of incomplete dominance is seen in flower color of snapdragons. A
cross between a white-flowered plant and a red-flowered plant will produce all
pink F1 offspring.

However the outcome of this breeding experiment does NOT disprove Mendel’s earlier
stated principles and argue in favor of Lamarck’s blending hypothesis, since selfpollination of the F1 offspring produces 25% white, 25% red, and 50% pink offspring.
The “pure” traits reappear in F2 generational plants, clearly arguing against the blending
hypothesis.
O
Otthheerr eexxaam
mpplleess ooff iinnccoom
mpplleettee ddoom
miinnaannccee iinn nnaattuurree
1. Hair form in mice
 Crossing of straight- and curly haired mice always
produces mice with wavy hairs
2. Color and petal form of orchids
 Crossing of red orchids with straight petals with white
orchids with curly petals results in pink orchids with wavy
petals
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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology (BIOL 107); Instructor: Elmar Schmid, Ph.D.
3. Petal color of snap-dragons
RR
 crossing of homozygous parental red (R
R) and white ( )
snap dragons leads to heterozygous ppiinnkk ppllaannttss in F1generation
(see Graphic below)
 the F1-offspring have an appearance in between the phenotypes of the two parent
varieties
 both parental phenotypes (red and white blossom) reappear again in the F2
generation after crossing two heterozygous F1 offspring (Rr x Rr)
 in agreement with Mendel’s law , ¼ of the F2 offspring have red blossoms and ¼ is
white colored, while 2/4 of the F2 offspring are pink-colored
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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology (BIOL 107); Instructor: Elmar Schmid, Ph.D.
Incomplete dominance in snap dragon plants
• e.g. Heritable trait = petal color of snapdragon plants
Phenotype
X
P
RR
Genotype
rr
F1
X
Rr
Rr
Sperm
F2
R
R
RR
Rr
r
Rr
rr
r
Eggs
Punnett Square
F2
25%
Genotypes
50%
25%
Phenotypes
Graphics©E.Schmid/2002
4. Familial Hypercholesterolemia (fHC) in humans
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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology (BIOL 107); Instructor: Elmar Schmid, Ph.D.
- this human geentic disorder involves a recessive h allele which is responsible for the
onset of the health complications, which is i.e. high blood cholesterol levels, heart
problems and stroke in young age
- the HH genotype has normal cholesterol levels, while heterozygote persons (Hh)
also have elevated cholesterol levels (about twice as normal) ( see Graphic below)
- the H allele codes for a gene which is responsible for the expression of a receptor
protein (= LDL receptor), which helps to clear cholesterol-containing lipid particles
(= LDLs) from the blood stream
- heterozygotes (hH) have only half the number of LDL receptors on their (liver) cells,
while homozygous hypercholesteremic individuals do not show any LDL receptors
on their cells
Familiar Hypercholesterolemia (fHC):
An example of incomplete dominance in Homo sapiens
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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology (BIOL 107); Instructor: Elmar Schmid, Ph.D.
Clinical manifestations/symptoms:
 High LDL and cholesterol levels in blood plasma
 Early arteriosclerosis (usually before age 40)
 Xanthomas in skin and tendons
 Reduced life expectancy
1000
800
(mg/dl)
Phenotype
= Plasma Cholesterol
Concentration
600
400
200
0
FH
Homozygous
FH
Heterozygous
Normal
Genotype
-/-
-/+
+/+
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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology (BIOL 107); Instructor: Elmar Schmid, Ph.D.
E
Exxaam
mpplleess ooff iinnhheerriittaannccee ppaatttteerrnnss w
wiitthh m
moorree tthhaann 22 aalllleelleess ppeerr ggeennee ((ccoo--ddoom
miinnaannccee))

There are cases where there are more than two alleles responsible for one
characteristic (or heritable trait) and in some cases the genotype does not always
dictate the phenotype in a classical Mendelian pattern.

A classical example for this is the inheritance of blood types in humans. The ABO blood
groups in humans are determined by not only two but three alleles.

The IA and IB alleles are codominant to each other. Because each individual carries two
alleles, there are six possible genotypes and four possible blood types.
1. the ABO blood group system in humans

There are three alleles (IA, IB and i) which lead to six different genotypes (IAIA, IAi, IBIB,
IBi, IAIB and ii)

The IA, IB alleles are dominant to the i allele, which means that both genotypes IAIA
and IAi express blood type A; the recessive homozygotes ii have blood type O

IA and IB exhibit co-dominance, which means that both alleles are expressed in
heterozygous individuals
TThhee tthhrreeee A
Alllleelleess ooff tthhee hhuum
maann bblloooodd ttyyppee
Allele
IA
For simplicity we call
these:
A
IB
B
i
O

These ssiixx ggeennoottyyppeess lead to ffoouurr pphheennoottyyppeess or so-called ABO blood groups (A, B,
AB and O)

Each of us has two ABO blood type alleles, because we inherit one blood type allele
from our biological mother, and one Allele from our biological father.

In the case of the genotype AB, the information of both Alleles is expressed in the
phenotype. A and B are co-dominant.

the letters for the phenotype refer to two carbohydrate molecules designated A and B
which are exposed on the surface of red blood cells (= erythrocytes)
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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology (BIOL 107); Instructor: Elmar Schmid, Ph.D.
TThhee A
A,, B
B,, aanndd O
OA
Alllleelleess aanndd tthhee hhuum
maann A
AB
BO
O bblloooodd ttyyppeess
Parent
(haploid gametes)
Offspring
(diploid)
Allele
((M
Mootthheerr))
Allele
((FFaatthheerr))
G
Geennoottyyppee
P
Phheennoottyyppee = B
Blloooodd
ttyyppee
A
A
AA
A
A
B
AB
AB
A
O
AO
A
B
A
AB
AB
B
B
BB
B
B
O
BO
B
O
O
OO
O

A person with blood group A or B has erythrocytes coated with either carbohydrate A or
B; a person with blood type AB exposes both carbohydrates ( A and B) on the surface
of its red blood cells (see Graphic below)
 a person with O-type blood doesn’t expose any of these two sugars

The ABO blood type plays an important role in the successful transfusion of donated
blood into a patient. Donating blood can be life saving for people who suffer from
massive blood loss due to accidents or caused by severe blood diseases.

If the donor’s blood has an A or B oligosaccharide that is foreign to the recipient,
antibodies in the recipient’s blood will bind to the foreign molecules, cause the donated
blood cells to clump together, and can kill the recipient.

The donor blood has to be compatible (= matching) with the patients blood group,
otherwise the donor blood cells will be recognized as foreign by the patient’s immune
system.
 the red blood cells will be attacked and destroyed by antibodies and white blood cells
 the patients antibodies recognize the foreign carbohydrate on the surface of the
donor blood cell and clump (= agglutinate) it, which can lead to a lethal clotting
(= agglutination) of the blood in the recipients body

A person with blood type A will generate antibodies against transfused B-type blood
cells and vice versa. A person with blood type AB doesn’t agglutinate blood cells from
either a A-type or B-type donor and can therefore receive blood from both blood types

A person’s ABO blood group can be experimentally determined by performing a socalled agglutination test. This test is sometimes also used in so-called paternity law
suits to confirm the identity of the biological father of a child.
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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology (BIOL 107); Instructor: Elmar Schmid, Ph.D.
The ABO phenotype: The Antigens on the surface of red blood cells (RBCs)
alleles
IA
i
IB
gene expression
Chromosome
#9
(9q34.1)
no enzyme
“A”
“B” enzyme
Sugar residues
Pr
“H” enzyme  all humans have it!
“A” enzyme
“B” enzyme
Pr
“A”
antigen
A
Pr
“B”
antigen
“A” + “B”
antigen
B
AB
Blood type
“O”
antigen
O
Graphics©E.Schmid/2002
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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology (BIOL 107); Instructor: Elmar Schmid, Ph.D.
P
Plleeiioottrrooppyy

Pleiotropy means, that one gene influences many characteristics; one genotype 
many phenotyes

Pleiotropy is observed in the human disease sickle cell anemia

In sickle cell anemia, the defect of one single allele (which leads to an abnormal
hemoglobin molecule in homozygous individuals) affects the body in multiple ways, i.e.
clumping of red blood cells, clogging of small blood vessels, heart failure, pain & fever,
etc. Only homozygous individual usually suffer from the disease.

But since the sickle cell and non-sickle cell alleles are co-dominant, heterozygous
carriers can suffer from complications under certain conditions (e.g. low oxygen
pressure, high elevations)
 1 out of 10 African Americans are carrier of the sickle cell allele!
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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology (BIOL 107); Instructor: Elmar Schmid, Ph.D.
P
Poollyyggeenniicc iinnhheerriittaannccee ppaatttteerrnnss

Some heritable traits do not fit the either-or basis that Mendel studied.

Quantitative characters vary in a population along a continuum. These are usually due
to polygenic inheritance, which means the additive effects of two or more genes on a
single phenotypic character.

Therefore, if many genes influence and contribute to the expression of one single
characteristic, geneticists speak of polygenic inheritance.

Classical examples of polygenic inheritance patterns in humans are certain body
features, such as skin color, hair color or body height all of which are controlled by at
least three different genes.

All of these body features rather follow a continuum and are the result of the additive
contribution of two or more genes on one distinct phenotype

e.g. in a hypothetical model lets say a ‘dark skin’ allele is responsible for a strong
pigmentation and consists of 3 genes called A,B,C
 each gene contributes ‘one unit of darkness’ and is
incompletely dominant to the three corresponding genes
aa,,bb,,cc of the ‘‘lliigghhtt sskkiinn’’ aalllleellee

The genotype of the homozygous ‘dark skin model’ phenotype would be AABBCC,
while the genotype of the homozygous ‘light skin model’ would be aaaabbbbcccc

AaaB
Crossing of both would lead to a heterozygous genotype (A
BbbC
Ccc) in the F1 generation
which would have light-brown skin color

After crossing of both heterozygous F1 ‘light brown model’ offspring we would expect 64
different genotypes and 7 different ‘skin color’ phenotypes in the F2 generation
( see Punnett square!). Since there are probably more than only three genes
responsible for our skin type, we would indeed expect a continuum expression of our
skin color within the human population.
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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology (BIOL 107); Instructor: Elmar Schmid, Ph.D.
•
To make things in the world of genetics even more challenging (and interesting at
the same time), heredity not only depends on genes but on the impact the
environment may have on these genes as well.
•
For example, a single tree has leaves that vary in size, shape, and greenness,
depending on exposure to wind and sun. For humans, nutrition influences height,
exercise alters build, sun-tanning darkens the skin, and experience improves
performance on intelligence tests.
•
Even identical twins, genetic equals, accumulate phenotypic differences as a result
of their unique experiences.
•
The relative importance of genes (“Nature”) and the environment (“Nurture”) in
influencing human characteristics is a very old and still a hotly contested debate.
•
This debate gained more momentum in recent years with the discovery of “microRNA” and other regulatory RNA in diverse life forms which have powerful controlling
effects on critical genes.
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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology (BIOL 107); Instructor: Elmar Schmid, Ph.D.
•
The product of a genotype is generally not a rigidly defined phenotype, but a range
of phenotypic possibilities, the norm of reaction, that are determined by the
environment.
•
In some cases the norm of reaction has no breadth (for example, blood type). Norms
of reactions are broadest for polygenic characters. For these multifactorial
characters, such as petal color in many flowering plants, environment
contributes to their broad quantitative nature.
LLiinnkkeedd ggeenneess

Geneticists observed in crossing experiments with plants and certain flies, that genes
on the same chromosome tend to be inherited together (= linked genes)

Linked genes are genes whose loci are on the same chromosome and usually close
together

They are therefore often inherited together and don’t exactly follow the Mendelian law of
independent assortment of genes

The first evidence of linked genes was reported in 1908 by W. Bateson & R. Punnett
(the inventor of the Punnett square) when they did crossing experiments with doubly
heterozygous sweet pea plants. They studied two characteristics:
Allele
1. flower color:
dominant Purple
recessive Red
P
p
2. polen shape:
dominant Long
recessive short
L
l
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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology (BIOL 107); Instructor: Elmar Schmid, Ph.D.
After crossing two heterozygous F1 offspring (hybrid plants), they did not receive the
predicted 9 : 3 : 3 : 1 ratio for the dihybrid cross in the F2 generation.
genotype
PpLl
genotype
x
PpLl

The discrepancy in the phenotypic ratio could be explained by the fact that the gene
coding for the color (P= purple; r= red) and the gene coding for the polen shape (L =
long; l = round) were located on the same chromosome;

Both genes were so-called linked genes. They were not independently assorted during
meiosis.
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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology (BIOL 107); Instructor: Elmar Schmid, Ph.D.
Genetics: Linked genes/alleles
• The Bateson/Punnett experiment with sweet pea plants (1908)
Two F1 hybrid
Pea plants
Phenotype
X
PpLl
PpLl
Phenotypes
Genotype
Predicted
Offspring
(9:3:3:1)
Purple-Long
Purple-Round
Red-Long
Red-Round
Observed
Offspring
215
71
71
24
284
21
21
55
recombinants
Interpretation: Genes/alleles are on the same chromosome –
Geneticists say alleles are linked
P L
F1
P generation
p l
Sperm
p
l
P L
Meiosis
p l
P L
P L
Oocytes
p
l
Expected Phenotypes
without crossing over
3 Purple-Long
1 Red-Round
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