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Lecture 5

HMB265H1 Lecture Notes - Lecture 5: Consanguinity, Gamete, Penetrance


Department
Human Biology
Course Code
HMB265H1
Professor
Maria Papaconstantinou
Lecture
5

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Lecture 5 Mendelian Genetics I: Dominance, Co-Dominance, Penetrance, Expressivity
Friday, September 22nd, 2017 1pm-2pm
Hartwell et al. pp. 36-45, 56-59
LECTURE 5 OUTLINE:
“Wrinkles” in Mendel’s Laws
o Incomplete dominance and codominance
o Multiple alleles
o Pleiotropy
o Variable expressivity
o Incomplete penetrance
o Environmental influence
Accommodating the Mendelian Model of Inheritance
Consider:
Different alleles at the same locus will likely
have different levels of “effect” on the trait
Problem:
Variable levels in allele “activity” will somehow
be manifest at the phenotypic level
How does one accommodate this into the Mendelian
model?
Dominance is Not Always Complete
Crosses between true-breeding strains can produce hybrids
with phenotypes different from both parents
Incomplete dominance:
o F1 hybrids that differ from both parents, expressing an
intermediate phenotype
o Neither allele is dominant nor recessive to the other
o Phenotypic ratios are the same as genotypic ratios
Codominance:
o F1 hybrids express the phenotype of both parents
equally
o Phenotypic ratios are the same as genotypic ratios
Summary of dominance relationships
There are many different types of mutations, all with different effects on phenotype
Variable levels of allele activity will show up at the phenotypic level
When Mendel proposed his laws and sorted out his ratios, he relied on complete
dominance and recessiveness, but this isn’t always the case
In a cross between two-breeding parents that differ at only one trait,
the phenotype of the F1 heterozygote defines the dominance
relationship between two alleles of a single gene (for complete
dominance)
These crosses can also produce hybrids with phenotypes that differ
from the parents (incomplete dominance)
o F1 hybrids express an intermediate novel phenotype
o Neither allele is dominant or recessive to the other
o Phenotypic and genotypic ratios are the same
With codominance, these crosses produce hybrids with phenotypes
of both parents in an equal manner
o F1 hybrids express a novel phenotype that includes both
parents’ phenotypes
o Both parental alleles are equally expressed
We’re looking at the phenotype of the F1 hybrid which defines the
dominance pattern of a single gene.
The phenotype of the heterozygote defines the dominance relationship
between two alleles of the same gene (A1 and A2).
Dominance is complete when the hybrid resembles one of the two
pure-breeding parents
Dominance is incomplete when the hybrid resembles neither parent
o Novel phenotype is usually intermediate, but typically
more similar to one parent than the other
Codominance occurs when the hybrid shows the traits from both
pure-breeding parents
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Incomplete Dominance in Antirrhinum
The 1:2:1 ratio signifies that the alleles of a single
gene determine these three colours
Incomplete Dominance in Whippets
Snap dragon flower colour
Cross pure-breeding parents and get an intermediate phenotype
Crossing two F1 individuals, F2 generation shows 1:2:1 genotypic ratio AND a
1:2:1 phenotypic ratio (not 3:1 in complete dominance monohybrid cross)
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Incomplete Dominance in Humans
E.g., familial hypercholesteraemia
Heterozygous phenotype is distinct form either homozygous phenotype (i.e., intermediate phenotype)
Excessive cholesterol leads to deposits of fatty substances under the skin
Individuals that are pure breeding or heterozygous, the heterozygous
phenotype is different between the two homozygous phenotypes
o Intermediate phenotype
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