This document discusses incomplete dominance, codominance, and multiple alleles in genetics. It provides examples of incomplete dominance in flowering time in plants and feather color in birds, where the heterozygote expresses an intermediate phenotype between the two homozygotes. It also describes codominance in human blood types, where both alleles are expressed simultaneously in the heterozygote (type A and B blood). Finally, it introduces the concept of multiple allele systems, where there are more than two possible alleles for a given gene, as seen in expanded human blood group systems with multiple antigens.

1. Chapter 9
Dominance relation and multiple
alleles in diploid organisms
Genetics Third edition
Monroe W.strickberg

2. Introduction
• The basic contribution of Mendel was his
discovery of the unit nature of inheritance-that it
consists of particulate factors, or genes, whose
presence can be traced from one generation to
another without change or dilution.
• F2 mendelian ratios of 3:1 in monohybrid
crosses and 9:3:3:1 in dihybrid crosses among
diploids are found as expected.
• These ratios has been found as expected ratio
derive from segregation of two different alleles
for each gene pair, one dominant and one
recessive.
• Experiments have revealed appearance of
phenotypes in novel proportions that can't be
explained on basis of simple dominance or
presence of only two kind of alleles.

3. Incomplete dominance

4. Incomplete
Dominance
• In simple or complete dominance, the heterozygote,
although genetically different has same phenotype
as one of the homozygotes (i.e.,Aa=AA)
• In Mendel's example the chosen gene differences all
showed simple dominance except for one-character,
flowering time, for which his experiments were
unfortunately incomplete.
• Rasmusson, found that influenced by different
factors among a pair of alleles called A and a.
• There was a 5-day difference in flowering time
between homozygous AA and aa.

5. Incomplete dominance
• If we designate the aa flowering time as zero(early), the flowering time
of different genotypes are:
aa 0.0 early
Aa 3.7 intermediate
AA 5.2 late
• Selfing the heterozygotes result in the following ratios:
Aa×Aa
1AA: 2Aa : 1aa
late: intermediate: early

6. Incomplete
Dominance
• The genotypic ratios are same as we expected
in Mendelian segregation, it is the phenotype
that has changed from their usual 3:1 dominant
to recessive ratio
• The absence of complete dominance by one
allele thus make each genotype separately
distinguishable
• The usual allelic symbols implying dominance
and recessiveness (e.g., A and a) can be
replaced by symbols in which the alleles
affecting the observed character are merely
differently numbered. e.g., A1 and A2

7. Incomplete dominance
• Many examples of departure
from complete dominance
relationship have been found
for various traits in both plants
and animals.
• For example, Correns found
that red-flowered four-o’clock
crossed to white-flowered
plants give pink heterozygotes.
• If the pink F1 is self fertilized
the F2 ratio is:
1red: 2pink: 1white.

9. Incomplete dominance
• Certain feather color genes in birds act similarly, the blue
Andalusian fowl arises from the combination of alleles for
white and black feathers.
• A mating between two blue fowl gives offspring in the
ratio 1white:2blue:1black.

11. Incomplete
dominance
• The pink and blue colors in these two types of
heterozygotes may not be exactly intermediate
to red, white and black and white.
• A pink and blue heterozygote, measured on a
calorimetric scale, may inclined more towards
one parental colors than other.
• If the heterozygous phenotype, i.e,A1A2
Concides with the phenotype of either one of
the homozygotes i.e. ,A1A1 or A2A2 is complete
for one of these alleles. Any lesser phenotypic
effect of the heterozygote i.e., towards A1A1 or
towards A2A2 can then be termed incomplete or
partial dominance.

12. Incomplete dominance
• There are some instances where dominance appears to be complete, it is
not usual to find some functional effects of the recessive gene in
heterozygote. This can be seen in one of the Mendel’s own crosses, that
between smooth and wrinkled-seeded plants.
• Investigation by Darbishire have shown that this physical appearance of the
starch grain is closely connected with the shape and appearance of starch
grains in the seed.
• Smooth seeds have many large, round starch grains they can retain more
water and consequently appear fuller and rounder.
• Wrinkled seeds on other hand are more sugary than starchy and lose water
upon ripening. Hybrids although smooth in appearance, have starchy grains
that are intermediate in type and amount.

13. Incomplete dominance
• In Drosophila, Ziegler-Gunder and Hadorn showed that effects of
normal eye color genes appear dominant on superficial examination,
some recessive mutations affect the amount of eye pigments in
heterozygotes.
• Thus, the sepia eye-colored mutant for example acts as recessive
gene to normal red eye color, but still reduces the quantity of some of
the fluorescent pteridine pigments in the heterozygotes.
• Wild type alleles normally found in diploids are dominant because of
their advantageous affects on the organism, mutant alleles are often
nonfunctional or only partly functional.

14. Overdominance

15. Overdominance
On occasion for some traits heterozygote may exceed
the phenotypic measurement for both homozygous
parents. Such heterozygotes are described as
overdominance.
Example: in drosophila the white-eyed gene (w) in
heterozygous condition (w+/w) causes a marked
increase in the amount of certain fluorescent
pigments( sepiapteridine and himmelblaus) over
both the white and wild-wild type homozygotes.
Many gene differences are usually involved in such
crosses, it is difficult to determine the exact
dominance relations of particular individual genes.

16. Codominance and
blood types

17. Codominance
and Blood
types
• Codominance occurs when both substances appear
together in heterozygote. For example, if unique
phenotypic substances X and Y are associated with
the homozygotes A1A1 and A2A2 respectively, the
codominant heterozygote A1A2 would produce both
substances, X and Y at the same time.
• There are examples in which visible effects are
produced by each allele of a gene pair, but the
detection of such qualitative differences is usually
difficult.
• The blue Andalusian fowl, which exhibit incomplete
dominance between black and white feather color
alleles, is really a fine mosaic of black and white
areas that appear to be blue.

18. Codominance and Blood types
When a foreign material, an antigen when enters a bloodstream of an organism it will
elicit the production of substance in the host called antibodies which react with the
material and thereby reduce harmful effects.
Thus, the bloodstream(specifically the blood serum) of an individual may contain many
different kind of antibodies for many different kind of antigens.
Human RBCs(erythrocytes) can be used to elicit the production of antibodies. If such
cells are washed and then injected into a rabbit, the rabbits blood serum(antiserum)
soon contain antibodies that can be extracted by special methods.

20. Codominance
and Blood
types
• The type of reaction that usually occurs is a
clumping or agglutination of cells into groups.
• More formal terminology describes the antigen
as an agglutinogen and the antibody as
agglutinin
• Landsteiner and Levine tested the red blood
cells of various people, they found at first three
general types, called M,N and MN, respectively.
• The M type elicited antibodies (anti M-serum)
specific for M which could not agglutinate N,
while the N red blood cells caused the
production of antibodies specific for N (anti N-
serum). Both types of antibodies however, could
agglutinated the MN red blood cells.

22. Codominance
and Blood types
• By analyzing the relationship between
people carrying the various blood types, it
was found that the genes for M and N
appeared to be alleles to each other.
• in honor of Landsteiner the gene was
named L and the alleles were named LM
and LN respectively.
• LMLM Individuals had the M phenotype and
produce only LM gametes, LNLN individuals
had the N phenotype and produced only
LN gametes, LMLN individuals had the MN
phenotype and produced both LM andLN
gametes and these alleles are also
distinguished simply M or N.

23. Codominance
and Blood
types
Table 9.1, there are three different
genotypes , there are six possible
matings, each of which, as shown.
Among different mating
combinations some exclude the
appearance of particular classes of
offsprings; i.e. N×N matings can't
give rise to M and MN offspring.

24. Parents Offspring
ratios
M
MN N
LMLM ×LMLM or MM×MM
all ------- -----------
LMLM ×LMLN or MM×MN 1 1 ------------
LMLM ×LNLN or MM×NN ------------ all -------------
LMLN ×LMLN or MN×MN 1 2 1
LMLN ×LNLN or MM×NN ------------- 1 1
LNLN ×LNLN or NN×NN ------------- ---------- all

25. Multiple alleles

26. Multiple Alleles
• Alleles can be defined as genes that are members of the same gene
pair, each kind of allele affecting a particular character somewhat
differently than the others.
• In Mendel's experiments there were two possible kinds of alleles in a
gene pair, i.e, smooth or wrinkled (S,s) yellow or green (Y, y)
• There are more than only two possible kinds of alleles in a gene;
hundreds or perhaps thousands of possibilities exist.
• The grouping of all different alleles that may be present in a gene pair
is defined as a system of multiple alleles.

29. Multiple
Alleles
• Dominance in this series appears to be
incomplete, although it would be difficult to
make such a decision without this type of
refined analysis.
• Alleles of this type which act within the same
phenotypic range of each other have been
termed isoalleles.
• Some of these alleles has also been discovered
in abnormal character, mutant isoalleles and
some within the phenotypic range of wild type,
normal isoalleles.
• A multiple allele system may therefore be quite
complex, including within it various subsidiary
isoallelic systems.

30. Multiple allelic blood group systems

31. Multiple-
Allelic Blood-
Group
Systems.
• In animals, tissues that are removed from
one individual and grafted onto another are
frequently sloughed off or rejected because
of incompatibility between the introduced
material and that of host.
• In 1947, Walsh and Montgomery found that
a certain portion of human blood serum
could be used to distinguish new varieties of
MN blood system.
• There appear to be four codominant alleles
LMS, LMs, LNS, LNs, (or MS, Ms, Ns, NS) which
would give nine different phenotypic
combinations.

33. Multiple-
Allelic
Blood-
Group
Systems
The first case of multiple alleles demonstrated in
man was really that of another blood group system
which has been discovered by Landsteiner and his
students in early 1900s.
This system called ABO was shown by Bernstein in
1925 to consist of three alleles of a single gene, IA,
IB and IO forming four different phenotypic groups:
A(IAIA or IAIO), B(IBIB or IBIO), AB(IAIB) and O(IoIo).
In this case, the blood serum of man himself
manufactured the antibodies that reacted with the
blood-cell antigens of other individuals.

35. Multiple-Allelic Blood-Group
Systems
• According to Kabat, Watkins and others it is the terminal
sugars of these compounds which differ between the A
and B antigens.
• The A substance bears an N-acetyl group at the number
2position of the galactose sugar; the B substance carries
a hydroxyl group at this position and the O substance
lacks the terminal galactose sugar entirely.
• The distinction between the A and B substances thus
arise from the distinctive difference in the kinds of
terminal galactose sugars transferred to a precursor
substance by the action of the a and B alleles; each allele
produce transferase enzyme but one function as an N-
acetyl galactosaminyl transferase(A) and other as
galactosyl transferase(B).
• In case of O, no terminal transferase enzyme appears to
be produced and it can therefore be called a null allele.

38. Multiple-
Allelic
Blood-
Group
Systems
• In respect to A and B, we can see the antigenic
differences, although small, are nevertheless
significant, so that antibodies can discriminate
between one antigen and the other.
• In fact, additional multiple alleles at the ABO
locus have recently been found(A2, A3, Ax and
Am) which probably differ in even more minor
respects.
• According to Stormont, the number of
different alleles for a particular blood type
gene called B reached more than 300.

39. RH and ABO
incompatibility

40. RH and ABO
incompatibility
The agglutination reaction that occurs when red blood
cells are clumped by serum antibodies may also occur in
the circulation of the mammalian embryo having a
blood type antigenically different from its mother.
The first instance of such compatibility was noted in
checking the blood types of children born with serious
anemia(erythroblastosis fetalis or hemolytic disease of
the newborn) caused by the breakdown (hemolysis) of
their normal red blood cells.
Before 1940, this disease was present in about one out
of 200 births.

41. RH and ABO
incompatibility
As determined by Levine and others, these children had a blood
type, Rh positive, inherited from their fathers, but antigenically
different from their Rh-negative mothers.
This Rh factor, first detected in the red blood cells of Rhesus
monkeys, was initially thought to be caused by a gene with only
two alleles, R and r.
The events leading to erythroblastosis thus arose from the Rh-
negative genotype of the mother (rr) producing antiserum
against the antigens of the Rh-positive offspring(Rr)
Since the R allele acted as a dominant to r, Rh positive males
married to Rh negative females could have either all or half their
offspring phenotypically Rh positive depending on parent's
genetic constitution was respectively homozygous (RR) or
heterozygous (Rr).

43. RH and ABO incompatibility
• The Rh blood group is only one of the systems which may cause mother-offspring
incompatibilities.
• Other blood group antigens may also travel across the placenta and produce
maternal antisera.
• Rh incompatibility is estimated to occur in about 10percent of all pregnancies;
nevertheless, only 1/20 to 1/50 of these incompatible offspring turn out to be
affected by hemolytic anemia.
• This diffusion not seem to occur very frequently and even when it occurs the
amount of diffused antigen may be low enough so that the amount of maternal
antibody production is not very high.

44. RH and ABO
incompatibility
• Although Rh incompatibility may exist
between mother and offspring
• ABO incompatibility may prevent anti-Rh
serum from developing.
• Incidence of Rh hemolytic disease occur when
mother and offspring are compatible for the
ABO blood groups.
• When they are incompatible, and the diffusing
fetal cells can be destroyed by maternal ABO
antibodies, the frequency of Rh hemolytic
disease decreases.

46. Histocompatibility
genes and antibody
formation

47. Histocompatibility
genes and
antibody
formation
Blood group incompatibility is only one possible
interaction between different vertebrate
individuals.
Another type of interaction may occur when
transplants of skin or most other organs are
attempted between individuals.
This rejection and second transplant is usually
rejected because of production of antibodies.
Unless individuals are identical twins or come
from an inbred stock with a high degree of
genetic similarity.

48. Histocompatibility
genes and
antibody
formation
Antibody producing cells in vertebrates are
known to arise in the bone marrow and are
then processed by two main types of
lymphatic organs to B and T lymphocytes.
It involves many steps in the recognition of
antigens and the stimulation and release of
either serum antibodies (B cells) or cell
surface antibodies (T-cells and macrophages)
The fact that an organism may be exposed to
thousands of different kinds of antigens
places emphasis on the ability to produce a
wide variety of possible kind of antibodies.

50. Histocompatibility
genes and
antibody formation
Genes involved in the production of cell surface antigens
that are recognized by the rejection or tolerance of
tissue transplants are collectively called
histocompatibility genes or loci
The action of antigen-producing alleles at many of these
gene's loci seems to be codominant, and individuals will
reject the tissue of donors that carry alleles which they
themselves do not carry.
In humans' histocompatibility is believed to be primarily
determined by a counterpart of the mouse H-2 system,
called the HLA system (human lymphocyte antigens),
although antigens produced by the ABO blood group
and other systems are also involved.

51. Histocompatibility genes and
antibody formation
• Four separate genes linked together on the same
chromosome (number 6) have been proposed for the
HLA system, each gene producing 8-40 multiple
alleles, each allele specifying a particular antigen.
• Number 6 chromosome may have one of the 20
possible alleles at genes HLA-A, one of 40 possible
alleles at gene HLA-B, etc.
• There are over 75,000 theoretically possible
combinations of HLA alleles on a single number 6
chromosome (20×40×8×12) each combination called a
haplotype (haploid genotype).

52. Histocompatibility
genes and
antibody
formation
• The fact that an individual usually possesses two
different haplotypes, one from each parent,
probably provides millions of different possible HLA
diploid genotypes. (theoretically, if N is the number
of haplotypes, there can be N(N+1)/2 different
diploid genotypes).
• Tests to obtain as close an HLA match as possible
between transplant recipient and donor are
therefore a necessity.
• HLA test can also be used to help decide questions
of genetic relatedness (such as paternity problems)
and to provide information on anthropological
matters (such as the almost complete absence of
the HLA-A1 allele in oriental populations).

54. Histocompatibility genes and antibody
formation
• One of the most interesting aspects of the HLA complex is the correlation between HLA
antigens and the incidence or severity of particular disease.
• The most dramatic of such associations is that between ankylosing spondylitis and
antigen B27.
• Individuals carrying this allele in Caucasian populations suffer from the disease about 87
times more frequently than individuals carrying other B alleles.
• A long list of such autoimmune diseases can be compiled that ranges from the rejection
of specific organs because of antibodies formed against thyroid glands and adrenal
glands to the widespread breakdown of numerous tissues.