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BIOLOGY 207 - Dr. Locke
Lecture#10 – Alleles, Dominance, and Morphs
Required readings and problems:
Reading: Open Genetics, Chapter 3, 4
Problems: Chapter 3, 4
Optional
Griffiths (2008) 9th Ed. Readings: pp 31-42, 223-230, 54-55
Problems: 9th Ed. Ch. 6: 1-7,10,12,13,16,18,21,22,25,28
Campbell (2008) 8th Ed. Readings: Concept 14.4
See: Supplement on "Muller's Morphs. See web site.
Concepts:
How do mutant alleles interact at a single locus?
1. Multi-cellular organism can have somatic and germline mutations.
2. DNA sequence can be altered and a mutant or variant can result.
3. The functional allele is usually dominant to the non-functional allele in individuals
with both alleles (called a heterozygote).
4. Dominance/recessiveness is not always complete.
5. Genetic tests can identify mutations that have elevated, reduced, eliminated, or
changed functions (Muller's Morphs
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Lecture#10
Fall'11
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Multi-cellular organisms (animals)
A mutation, occurs in the DNA of a single cell.
- will be passed it on to the cell's descendants directly.
Multi-cellular organism: will pass on to the cell's descendants and MAY be passed on
to the organism's progeny.
Somatic vs Germline Cells in multi-cellular organisms
Somatic cells
 - form the tissues of the organism and not used in reproduction
 - not passed on to the next generation
Germline cells
 - form the germ cells (eggs or sperm)
 - are passed on to the next generation
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Somatic vs Germline Mutations
Somatic Mutations – Occur in somatic cells (example: Fox)
- mutant cells descendants will form a clone of cells
- population of mutant cells among wild type cells
- In humans -> mutation ->cell that is cancerous -> clone of cells -> cancer
- only affect (phenotype) in the individual in which they occur
- will NOT be passed on to the next generation (progeny)
Mosaic - an individual composed of two (or more) types (lines) of cells that differ in
their genetic composition
Germline (germinal) Mutations – Occur in germline cells
- can be passed on to the next generation of organisms
- don’t affect the individual in which they occur
Animals -> segregate these cells from somatic cells
Plants -> somatic cells become germ cells;
therefore somatic mutations can become germline mutations
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Fall'11
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Genotype and Phenotype
(P= G+E+I)
Genotype
 -the genetic constitution of an organism
 -in a diploid this means the two alleles
Phenotype
 -the observed properties of an organism
 -produced by the genotype in conjunction with the environment (plus an
interaction between the two)
Mutations in haploid and diploid organisms
Geneticists use the phenotype to infer genotype or possible genotypes for an
individual.
Haploids - e.g. bacteria, haploid yeast
- usually one copy per gene
- single cells
- expression of gene affects the cell's phenotype directly
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Diploids e.g. diploid yeast, Drosophila and other animals and
plants
- two copies of each gene - interactions of alleles
- often multicellular - different tissues
- expression is NOT direct
How does one infer genotype from phenotype?
Haploids - direct genotype -> phenotype
Diploids - directly inferring is not always correct since there are two alleles to be
determined
Dominant and Recessive
Note: The concepts of alleles, dominance, recessiveness, pairs of genes,
homozygotes, heterozygotes all come to us from the work of Gregor Mendel and his
peas.
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Heterozygotes vs homozygotes
Diploid individuals have 3 possible combinations of two alleles:
RR ---> homozygote - both alleles are R
rr ---> homozygote - both alleles are r
Rr ---> heterozygote - one of each allele
True-breeding strains - a population of individuals that shows no variation for a
particular character. Example: Mendel's Round or wrinkled character of the pea
Parents:
Gametes:
Heterozygote
Progeny:
Phenotype:
Dominant vs recessive
 Heterozygote ---> phenotype is key to determining dominance/recessiveness
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The most common circumstance:
Rr has a normal (wild type) phenotype,
- the single normally functioning allele, R,
provides sufficient function to produce a normal phenotype.
Example: gene encodes an enzyme
Note 1:
Because the "R" allele determines the phenotype in the diploid heterozygote it is
considered dominant to the "r" allele.
Because the "r" allele is masked by the "R" allele, the "r" allele is considered recessive
to the "R" allele.
Note 2:
The terms dominant and recessive are always used in relation to another alleles - in a
diploid individual
Dominance and recessive are not an innate property of an allele but a relative one.
Note 3:
Notation - Frequently capital letters are used to denote dominant alleles while the
recessive alleles are given the lower case equivalent.
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Incomplete dominance (semi-dominance)
Alleles of genes do not always exhibit simple Dominance/Recessive relationship
Sometimes there is an intermediate (or blended) phenotype in the heterozygote
Example in "Four-o'clock" plants
Strains have flowers with either:
CRed / CRed - homozygotes
White petals CWhite / CWhite - homozygotes
Heterozygote CRed / CWhite = has a pink colour (intermediate between the two
 Red petals


homozygotes)
Co-dominance
In certain circumstances both alleles can be seen in the phenotype.
Example: Human Blood types HbS and HbA
Fig: http://www.mun.ca/biology/scarr/Hemoglobin_Electrophoresis.html
Can see both alleles and thus distinguish the homozygotes from the heterozygote.
Note: sickle-cell anemia disease is recessive phenotype of HbS/HbS individuals
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Lecture#10
Fall'11
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Allele - a changed form of a gene; an allelic form; an allele.
Allelic forms:
Wild type allele, or "normal" allele
- functional - What is wild type?
Variant allele
- the DNA sequence is different from wildtype
- produce a product that is active and functional (yet have bp changes in the gene)
-
Mutant Allele (Fig vg alleles)
- the DNA sequence is different from wild type; a change in gene structure ->
substitution, deletion or duplication
- makes the gene (Muller's morphs)
1) non-functional – null - loss of function
2) less functional - leaky
3) more functional – gain of function (same or different)
4) new function
5) dominant negative function
Allelic series – many different mutations (alleles) at the same locus
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General classification of germline mutants
The mutation's effect on the organism.
1) Morphological mutations
- change in the form of the organism
- size, shape, color, number, etc.
2) Lethal mutations
- mutation that leads to or causes death of an organism
- most commonly recessive lethal
- homozygotes die - heterozygotes are viable
3) Conditional mutations - relies on the concept of:
Phenotype = Genotype + Environment + Interaction
- expresses mutant phenotype only under specific environmental conditions
Under restrictive conditions they express the mutant phenotypic; while
under permissive conditions they show a wild type phenotype
- e.g.: temperature sensitive -> Siamese cat
4) Biochemical mutations
- produce auxotrophic mutants from prototrophic parents
- a specific type of the conditional mutation class. e.g. - your lab exercises
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"Muller's morphs" classification - (1932)
H.J. Muller (1946) was awarded Nobel prize for his contributions to radiation genetics.
He designated five classes of mutant as "amorph, hypomorph, hypermorph, neomorph and
antimorph - "morph" meaning "form"
Amorph - A- absence
- mutation with complete absence of wild type function
- also called "null" mutation
- no function
- no expression of wild type gene at any level
- eg. gene deleted - deletion
- loss-of-function - loss of enzymatic function
Hypomorph - hypo -> less than
-> less than wild type level of expression
of the wild type gene - same quality
- range from slightly less than wild type
to almost complete loss (amorph)
- reduction-of-function
- also referred to as "leaky"
- reduced activity of enzyme
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Lecture#10
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Hypermorph - hyper -> more than
-> more than wild type level of gene expression - same quality more quantity
- gene duplication - 2 genes -> more product- more enzyme activity
- gain-of-function - more of the same
Neomorph - neo -> new
- mutations which create a new and different from the wild type function
- gain-of-function - novel function
e.g. Drosophila antennapedia, aristapedia
Antimorph - anti -> against
- mutant gene product results in a reduction or loss in the function and interferes with
normal gene product from any wild type gene that may be present
Class demonstration. See: Supplement on "Muller's Morphs" on web site
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Summary
The several schemes for classifying mutations are not mutually exclusive.
They describe mutations at different levels
- DNA, chemical level - e.g. frameshift,
- gene product level - e.g. functional/non-functional
- phenotypic level, mutant vs wild type
"Muller's Morphs" classify all types of mutations
- amorph,
- hypomorph,
- hypermorph,
- neomorph
- antimorph
Allelic interactions give us:
 Dominant vs. recessive
 Co-dominant
 Incomplete (semi-) dominant
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Lecture#10
Fall'11
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