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CHAPTER 13
Beyond Mendel
• In the 20th century, geneticists have
extended Mendelian principles not only to
diverse organisms, but also to patterns of
inheritance more complex than Mendel
described.
• In fact, Mendel had the good fortune to
choose a system that was relatively simple
genetically.
– Each gene has only two alleles, one of which
is completely dominant to the other.
– Each character (but one) is controlled by a
single gene.
– In Mendel’s pea plants each gene has only
two alleles, one of which is completely
dominant to the other. However, in early
twenty century, geneticists trying to determine
of Mendel’s principles apply to all traits in all
organisms were faced with the following
dilemma.
– In Anthirrinum (Snapdragons), when two pure
lines, red and white, are crossed, it would be
expected that the F1 shows one of the two
parental phenotypes (the dominant one).
(Following Mendel’s principles).
– However, a cross between a
white-flowered plant
and a red-flowered
plant will produce all
pink F1 offspring.
– Self-pollination of the
F1 offspring produces
25% white , 25% red,
and 50% pink
offspring.
– This is a textbook example of
incomplete dominance .
• Incomplete dominance is an allelic
relationship where dominance is only
partial. In a heterozygote, the recessive
allele is not expressed. The one dominant
allele is unable to produce the full
phenotype seen in a homozygous
dominant individual. The result is a new,
intermediate phenotype.
A similar system is found
in carnations.
Another example is plumage color in chickens:
a. Crossing a true-breeding black chicken (CB CB) with a
true-breeding white one (CWCW) produces an
Andalusian blue F1 (CBCW).
b. When the F1 interbreed, the
F2 include black (CBCB),
Andalusian blue (CBCW), and
white (CWCW) birds, in a ratio
of 1;2;1.
c. At the molecular level, two
copies of CB produce black,
while one copy is sufficient to
produce only the grey
“Andalusian blue” phenotype.
b. When the F1 interbreed,
the F2 include black
(CBCB), Andalusian blue
(CBCW), and white
(CWCW) birds, in a ratio
of 1;2;1.
c. At the molecular level,
two copies of CB produce
black, while one copy is
sufficient to produce only
the grey “Andalusian blue”
phenotype.
Another example of incomplete dominance
is found in cats. In cats, the gene S codifies
for white spots. The gene s is for solid
color or spot-less. The gene is incomplete
dominant.
A cat that is SS cat has white spots
in more than 2/3 of their body
independently of their base color.
For example a black cat with
white spots or a white cat with
white spots. If the spot is in the
eye region, it may cause that the
eyes are also blue.
Booger aka Boo 1992-2006
Baby kitty 1994-2005
• A Ss cat has also white spots but since S
is incomplete dominant a Ss cat has 1/3 to
½ spots.
KinnerD, 1995
A ss cat does not have white spots expressing a solid color
phenotype.
Kooguakoogua
1995-1997
• Another textbook example of
incomplete dominance in found in
horses.
• When two true breeding horses with
the C gene for Chesnut is mated
with a homozygous Ccr for cremello,
they produce an F1 of intermediate
color known as palomino.
• Write the genotypes for these three
phenotypes.
Multiple Alleles
• Not all genes have only
two alleles; many have
multiple alleles.
• No matter how many
alleles for the gene exist in
the multiple allelic series, a
diploid individual will have
only two alleles, one on
each homologous
chromosome
A well known multiple
allele system is the one
expressing the fur
color of rabbits. The
gene C (color) has
four alleles. C+, Cch,
Ch and Ca.
In this case + is the wild type or normally
gray, ch is chinchilla phenotype, h is
himalayan phenotype and a, albino
phenotype. These genes are completely
dominant. Write all the genotypes that you
could have in this allele system.
Multiple alleles in cats.
• There are two chemically different kinds of melanin:
eumelanin and phaeomelanin. Eumelanin granules are
thought to be spherical in shape and absorb almost all
light, giving black pigmentation. Phaeomelanin
granules are thought to be elongated "footballs" in
shape, and reflect light in the red-orange-yellow range.
• The multiple allele system is comprised of three
genes in order of dominance: B, b and b’. The
alleles are completely dominant. A cat B– is
black a b is brown (chocolate) and a b’ is light
brown or cinnamon.
B
b
b’b’
Codominance
• Most genes have more than two alleles in a
population (multiple alleles).
• The ABO blood groups in humans are
determined by three alleles, IA, IB, and I.
– Both the IA and IB alleles are dominant to the i allele
– 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.
Both IA and IB are dominant to i, while IA and IB are
codominant to each other. The resulting phenotypes are:
a.
b.
type A.
c.
type B.
d.
People with genotype i/i are blood type O.
People with genotype IA/IA or IA/i are blood
People with genotype IB/IB or IB/i are blood
People with genotype IA/IB are blood type AB.
•Karl Landsteiner discovered human ABO blood groups
in the early 1900s, and received the 1930 Nobel Prize in
Physiology or Medicine for this work.
•Landsteiner's Rule: If an individual has the antigen,
he/she will not have the antibody. This is a universal law
with few exceptions.
•ABO antigens are glycolipids (on the surface of the RBC)
or glycoproteins (in secretions). ABO antigens are found
on RBC's , lymphs, platelets, tissue cells, bone marrow,
and organs. These antigens can be secreted by tissue cells
if the appropriate genes are present. These antigens must
be matched for ransfusions and wrong combinations can
lead to pregnancy/newborn problems
Biochemistry of the RBC antigens.
• The A & B antigens are derived from a common precursor
known as the H antigen.
• Activity of the IA gene product, a-N-acetylgalactosamyl
transferase, converts the H antigen to the A antigen.
• Activity of the IB gene product, a-D-galactosyltransferase,
converts the H antigen to the B antigen.
• Both enzymes are present in an IAIB individual, and some
H antigens will be modified to the A antigen while others are
modified to the B antigen.
• Neither enzyme is present in an ii individual, and so the
H antigen remains unmodified.
• Production of the H antigen is controlled
by a different genetic locus from the ABO
enzymes. Rarely, an individual lacks the
dominant allele H needed for H antigen
production.
• This hh genotype results in the Bombay
blood type, which is similar to type O
except that Bombay blood type individuals
produce anti-O antibodies that are not
seen in true type O individuals.
Figure 14.10x ABO blood types
ƒ ABO antigens are developed in the uterus at 5-6
weeks of gestation. And full expression of ABO
antigens occurs between 2-4 years of age.
ƒ
O 45%
Type Frequency
A 40%
B 11%
AB 4%
ƒ Another group of cell antigens are the M-N red
blood cell antigens that are less important in
transfusions but also show codominance. There
are three types:
Type M, with genotype LM/LM.
Type MN, with genotype LM/LN.
Type N, with genotype LN/LN.
In Summary:
In codominance, since both alleles are dominant,
the heterozygote’s phenotype includes the
phenotypes of both homozygotes. Examples
include:
The ABO blood series, in which a heterozygous
IA/IB individual will express both antigens,
resulting in blood type AB. And the human M-N
blood group where individuals with genotype
LM/LN will express both antigens.