Transcript Chapter 4: Modification of Mendelian Ratios

Chapter 4: Modification
of Mendelian Ratios
Honors Genetics 2012-2013
Chapter 3 Lessons
MENDEL’S POSTULATES
#1: Unit factors come in pairs.
#2: Unit factors have either a
dominant or recessive form.
#3: Unit factors segregate/
separate during gamete formation.
#4: Unit factors assort
independently from one another.
#1: Chromosomes come in pairs.
#2: GENES have either a dominant
or recessive form.
#3: Chromosomes segregate/
separate during gamete formation.
#4: Chromosomes assort
independently from one another.
Chapter 3 Lessons
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Mendel’s postulates for OTHER INHERITANCE PATTERNS do NOT
hold true in all respects
These both hold TRUE for other types of inheritance.
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#3: Unit factors segregate/ separate during gamete formation.
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#4: Multiple unit factors assort independently from one
another.
These postulates DO NOT.
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#1: Unit factors come in pairs.
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#2: Unit factors have either a dominant or recessive form.
4.1: Alleles Alter Phenotypes in Different
Ways
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Alleles are alternative forms of the same gene.
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Wild-Type Allele
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Appears most frequently in a population
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Arbitrary designation of NORMAL
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Often DOMINANT
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Used as the standard which all alterations/mutations are
compared.
Mutant Allele
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Contains modified genetic information.
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Specifies an altered gene product.
Loss-of-Function
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Mutation that results in reduced function of a protein
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Mutation that results in increased function of a protein
Null Allele
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Mutation that results in COMPLETE loss of function in proteins
Gain-of-Function
4.2: Geneticists Use a Variety of
Symbols for Alleles
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Mendel Abbreviations
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Dominant allele = capital letter of trait of interest
Recessive allele = lowercase letter of trait of interest
Work with Drosophila melanogaster (fruit fly)
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Mutant allele = lowercase letter if recessive; capital
letter if dominant.
Wild type allele = uses same letter designation
with superscript +
A slash (/) between the letters designates the
location of the allele on homologous chromosomes.
4.3: Neither Allele is Dominant in
Incomplete (Partial) Dominance
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Cross between parents with contrasting traits
may produce offspring with intermediate
phenotypes.
Occurs when the phenotype is controlled by a
single gene with two alleles, neither of which is
dominant.
Because there is no dominant trait, abbreviations
can vary:
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Red = R1 / White = R2
White = W1 / Red = W2
Red = CR / White = CW
R = Red
W = White
C = Color
Incomplete/Partial Dominance
Snapdragons:
Red + White = Pink
Red = CR / White = CW
Incomplete/Partial Dominance
Human Example:
• NORMAL have
100% activity of the
affected enzyme.
• CARRIERS have
50% activity of the
affected enzyme.
• AFFECTED have 0%
activity of the
affected enzyme.
QUESTION #1, PAGE 87
IN SHORTHORN CATTLE, COAT COLOR MAY BE RED,
WHITE, OR ROAN. ROAN IS AN INTERMEDIATE
PHENOTYPE EXPRESSED AS A MIXTURE OF RED AND
WHITE HAIRS. THE FOLLOWING DATA ARE OBTAINED
FROM VARIOUS CROSSES:
RED X RED = ALL RED
WHITE X WHITE = ALL WHITE
RED X WHITE = ALL ROAN
ROAN X ROAN = ¼ RED; ½ ROAN, ¼ WHITE
HOW IS COAT COLOR INHERITED?
WHAT ARE THE GENOTYPES OF PARENTS AND OFFSPRING
FOR EACH CROSS?
RED X RED = ALL RED
CR/CR X CR/CR = CR/CR
WHITE X WHITE = ALL WHITE
CW/CW X CW/CW = CW/CW
RED X WHITE = ALL ROAN
CR/CR X CW/CW = CR/CW
CR
CR
CW
CW CR CW C R
CW
CW CR CW C R
ROAN X ROAN = ¼ RED; ½ ROAN, ¼ WHITE
CR/CW X CR/CW = CR/CR, CR/CW, CW/CW
CR
CW
CR
CRCR
CW CR
CW
CRCW CWCW
Incomplete/Partial vs. Codominance
Incomplete:
• Phenotype expression
different than either
parent.
• MIXTURE
Codominance:
• Phenotype expression
that is equal to BOTH
parent’s phenotypes.
4.4: Codominance and MN Blood Groups
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Joint expression of BOTH alleles.
In humans, 2 forms/alleles for the glycoprotein are
present on the red blood cell surface, M and N
The gene for the glycoprotein is located on
chromosome #4.
The 2 alleles are designated LM and LN
Genotype
Phenotype
LMLM
M
LMLN
MN
LNLN
N
4.5: ABO Blood Groups: Multiple Alleles
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Identified by Landsteiner in 1901.
The alleles for ABO blood groups are located on
chromosome 9.
4.5: ABO Blood Groups
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3 alleles
I = isoagglutinogen; agglutination means to clump.
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IA = A antigens; B antibodies
IB = B antigens; A antibodies
IO = NO antigens; A and B antibodies
Genotype
Antigen
Phenotype
IAIA
A
IAIO/IAi
A
IBIB
B
IBIO/IBi
B
IAIB
A and B
AB
IOIO/ii
NO antigens
O
A
B
Bombay Phenotype
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Rare cases of incomplete formation of
carbohydrates that form the A and B antigens,
called the H substance.
Results in an O phenotype, although they do not
have O blood; they will still have A and/or B
antigens on their red cell surface.
Issues arise at the time of transfusion; if they test
RBC’s only in the patient, they could receive
incompatible blood and will suffer a transfusion
reaction.
See pedigree, Page 64
Rh Factor
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Rh inheritance (+/-) follows traditional
Mendelian Dominant/Recessive inheritance
patterns.
Rh + is Dominant
Rh – is Recessive
Rh pos can be homozygous dominant or
heterozygous (+/+ or +/-)
Rh neg must be homozygous recessive (-/-)
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Referenced as the D antigen
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Rh + = D
Rh - = d
MN/ABO Practice
A man is suing his wife for divorce on the grounds of
infidelity. Their first child and second child, whom
they both claim, are blood groups O and AB,
respectively. The third child, whom the man disclaims,
is blood type B.
(a)Can this information be used to support the man's
case?
(a)Can this information be used to
support the man's case?
Child #1: IOIO
Child #2: IAIB
Child #3: IBIB or IBIO
MOTHER’S GENOTYPE: IAIO
FATHER’S GENOTYPE:
IB IO
MN/ABO Practice
A man is suing his wife for divorce on the grounds of
infidelity. Their first child and second child, whom
they both claim, are blood groups O and AB,
respectively. The third child, whom the man disclaims,
is blood type B.
(b) Another test was made using the M-N blood
group system. The third child was group M, the man
was group N. Can this information be used to
support the man's case?
(b) Another test was made using the M-N blood
group system. The third child was group M, the
man was group N. Can this information be used to
support the man's case?
Child 3: LMLM
Father: LNLN
Impossible for the man to be the child’s father due to
the difference in M and N antigen on the red cell
surface.
4.6: Lethal Alleles Represent Essential Genes
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Mutations resulting in the production of a gene
product that is nonfunctional can often be
tolerated in the heterozygous form.
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Behaves as a recessive allele
If present in the homozygous condition it is
considered LETHAL and the organism will not
survive.
The time of death is dependent on when the
product is needed.
Example: Huntington’s Disease
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Gradual nervous and motor degeneration
Late-onset
Phenotypes are Often Affected by More
Than One Gene
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Gene interaction is when several genes influence a
particular characteristic.
It doesn’t necessarily mean that genes directly
interact with each other but that the cell products
of gene expression influence a common
phenotype.
Often described as a CASCADE event.
Epigenesis is the increased complexity in the
development of an organ/system and is under the
control of one or more genes.
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Example: Insect eyes and mammalian ears
A mistake in the CASCADE of events can lead to
mutations and poor development, leading to blindness
or deafness.
Epistasis
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Expression of one gene/gene pair masks or
modifies the expression of another gene/gene
pair.
Can be antagonistic or complementary
#1: RECESSIVE EPISTASIS: Epistatic/Hypostatic
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Homozygous presence of a recessive allele
prevents/masks the expression of other alleles at a
second location.
The alleles at the first locus are epistatic to the alleles at
the second locus.
The alleles at the second locus are hypostatic to the
alleles at the first locus.
EXAMPLE: Bombay phenotype
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The FUT1 gene overrides the expression of the IA and IB alleles.
Epistasis
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#2: DOMINANT EPISTASIS:
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A single Dominant allele at locus 1 influences the
expression of the alleles at locus 2.
EXAMPLE: Squash color; the presence of the
dominant A allele for white squash is expressed
regardless of what is coded for at the second
location. Absence of the A allele will produce either
yellow (BB, Bb) or green (bb).
#3: COMPLEMENTARY GENE INTERACTION:
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Two gene pairs COMPLEMENT one another so that at
least one dominant allele must be present from each
pair of genes to express a particular phenotype.
EXAMPLE: Pea flower color; 2 proposed
locations/genes for flower color; the presence of at
least one dominant allele at either location allows
for expression of allele.
EPIGENETICS
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The study of
how the
genome is
influenced by
outside factors.
EPIGENESIS
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Cascade of
events that
leads to
development of
complex
structure.
Gene
interaction is
essential.
EPISTASIS
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Expression of
one gene or
gene pair
masks,
modifies, or
complements
the expression
of another gene
or gene pair.
4-10: Expression of a Single Gene May Have
Multiple Effects
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PLEIOTROPY: expression of a SINGLE gene has
multiple phenotype effects.
Marfan Syndrome:
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Mutation of fibrillin protein, a necessary connective
tissue protein that provides structural integrity for many
body tissues.
Single m]gene mutation = multiple effects.
Porphyria variegata:
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Mutation in enzyme that breaks down porphyrin, a
component of hemoglobin.
Porphyrins build up in body tissues, causing a wide
range of symptoms.
4.11: X-linkage
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Covered with Chapter 3 pedigree Practice.
Refers to genes located ONLY on the X
chromosome.
Female carry 2 copies
Males have only 1 copy
Remember the rules
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In X-linked RECESSIVE pedigrees, affected mothers
will have 100% affected sons.
In X-linked DOMINANT pedigrees, affected fathers
will have 100 % affected daughters.
For Quiz:
1) Correct and Review Chapter 4 Quiz
2) Review Powerpoints, focus on Vocabulary
#1: Chapter 4
#2: Epigenetics
3) Practice Punnett Squares and Pedigrees for
Sex-linked (X-linked) traits.
4) MN/ABO/Rh Blood Group Practice
Still to finish…
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4.12 Sex-influenced inheritance
4.13 Environmental Affect of phenotype
expression
4.14 Extranuclear inheritance