Download Modifications of dominance relationships – Incomplete dominance

Modifications of dominance relationships – Incomplete
dominance
Modifications of dominance relationships – Incomplete domina
Complete dominance in terms of gene
product
1. Threshold effect – the
phenomenon that “50% of the protein is
enough” is fairly common among many
genes
Example: Tay- Sachs disease
At the molecular level, the mutation that
causes TSD is in a gene that encodes for
the enzyme hexaminidase A (hex A).
Enzyme is responsible for the breakdown
of gangliosides.
Modifications of dominance relationships
Co-dominance
MN blood group
Genotype
LMLM
Phenotype
M
LMLN
MN
LNLN
N
Which phenotypes may be produced in a mating
between two heterozygotes?
Multiple alleles – allelic forms of a gene
Table 4.1
Biochemical basis of
ABO blood groups
The Bombay phenotype
Lethal alleles
Other variations:
Essential genes and lethal alleles
Dihybrid cross
Two modes of
inheritance
Modification of
9:3:3:1 ratio
Phenotypes are often affected by more than one ge
Gene Interaction – Novel phenotypes
rrP_
R_pp
R_P_
rrpp
Genetic Interaction
Epistasis
Recessive epistasis
Genetic Interaction – Recessive epistasis
Black lab = B_E_
Yellow lab = __ e/e
Chocolate lab = bbE_
Genetic Interaction – Dominant epistasis
Dominant epistasis – hypothetical explanation
Genetic Interaction – Duplicate recessive epistas
Duplicate recessive epistasis – hypothetical explana
In some plants a red pigment, cyanidin, is synthesized from
colorless precursor. The addition of a hydroxyl group (OH)
to cyanidin molecules causes it to become purple. In a cros
between two randomly selected purple plants the following
results were obtained:
94 purple
31 red
43 white
1. Construct a pathway to represent the synthesis of the pu
pigment. How many enzymes? How many genes?
2. Are the results of the cross in agreement with your pathw
Explain.
3. What are the genotypes of the purple parental plants?
Complementation test
Two different genes
affecting the same trait
Complementation test
Phenotypic Expression
Pleiotropy
Penetrance and Expressivity
Sex-limited and sex-influenced
traits
Genetic background –
suppression and
position effects
Conditional mutations –
temperature
Other variations: penetrance and expressivity
Variable expressivity - neurofibromatosis
Variable expressivity
Sex-limited traits
Sex influenced traits
Other variations:
Sex influenced
traits
Temperature sensitive
genes
Position effect
Imprinting:
The Igf2 gene
in mice
X-linked inheritance
Pedigree analysis – sex linked traits
what to look for:
Recessive trait:
1. More males that females affected
2. Affected sons are usually born to unaffected
mothers, zig-zag pattern – from grandfather to
grandson through an unaffected female.
3. Approximately 50% of the progeny of a carrier
female are affected
4. It is never passed from father to son
5. All daughters of affected fathers are carriers
Pedigree analysis – sex linked traits
what to look for:
Dominant trait:
1. Both males and females are affected; often
more females than males
2. Does not skip a generation. Affected sons
must have an affected mother; affected daughters
must have either an affected mother or an
affected father.
3. Affected fathers will pass the trait on to all their
daughters.
4. Affected mothers ( if heterozygous) will pass
on the trait to approximately 50% of their
daughters and 50% of their sons.