Transcript Chapter 4: Modification of Mendelian Ratios

Chapter 4: Modification
of Mendelian Ratios
Honors Genetics 2012-2013
Chapter Focus
1
While alleles are transmitted from parent to offspring according
to Mendelian principles, they sometimes fail to display the clearcut dominant-recessive relationship observed by Mendel.
2
In many cases, contrast to Mendelian genetics, two or more genes
are known to influence the phenotype of a single characteristic.
3
Another exception to Mendelian inheritance is the presence of
genes on sex chromosomes, where males only receive a single
copy of genes on that chromosome.
Chapter Focus
4
Phenotypes are the combined result of both genetics and the
environment within which genes are expressed.
5
The result of the various exceptions to Mendelian principles is the
occurrence of phenotypes that differ from those resulting from
mono-, di-, and tri-hybrid crosses.
6
Extranuclear inheritance, resulting from the expression of DNA
found in mitochondria (and chloroplasts) can modify Mendelian
inheritance patterns. These genes are transmitted through the
female gamete.
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
•
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.
•
#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.
Example
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Recessive body color
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Ebony = e
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Gray = e+
Normal Wild Type body color
Possible genotypes for diploid fly:
e+/e+ = gray homozygous (wild type)
e+/e = gray heterozygous (wild type)
e/e = ebony homozygous (mutant)
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
Humans:
Carriers have 50%
activity of 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
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Identified by Landsteiner in 1901.
A and B antigens 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
A
IBIB
B
IBIO
B
IAIB
A and B
AB
IOIO
NO antigens
O
A
B
PAGE 87
QUESTIONS #3 AND #11
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.
4.5: ABO/Bombay Phenotype
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Bombay phenotype results from the incomplete
production of an enzyme needed to express the A
or B antigen.
Although they may genetically be IA or IB, the
antigen is not expressed and the phenotype is
type O blood.
See pedigree, page 64.