Download Complementation

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Alterations to Mendel
Incomplete or partial dominance
Codominance
Multiple alleles and Lethal alleles
Gene interactions & multiple genes
– Epistasis and complementation
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Effect of environment
Extranuclear inheritance
Sex-linked, sex-limited, & sex-influenced
Sex determination and Gene dosage
Polygenics
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Gene dosage
• It matters how many copies of genes there are.
– Snapdragons: heterozygous flowers are pink.
– Multiple histone genes.
– Too many of some genes is deleterious.
• 3 copies of chromosome 21 = Down Syndrome
• What about sex chromosomes? XX vs. XY
– Y chromosomes are missing most of genes X has.
– So, if 1 set of genes on the X is good for males, is
two sets (2 X chromosomes) bad for females?
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Dosage compensation: Barr, Ohno, and Lyon
• Barr noticed that in the nucleus of females, but not
males, a darkly staining body is visible.
• Ohno hypothesized that this was an inactivated X
chromosome in females so that there would only be 1
functional copy of genes, as in males.
• Inactivated X is called a Barr body.
• Individuals with incorrect numbers of sex
chromosomes have appropriate number of Barr
bodies.
– E.g. XXX females have 2 Barr bodies
Lyon Hypothesis
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• X chromosome inactivation takes place early in
development.
• In placental mammals, it can be either X chromosome.
– All the descendents of that cell have the same X
chromosome inactivated.
– Results in a mosaic, patches of tissue with different
lineages. Seen with X-linked traits.
• Human females: anhidrotic epidermal dysplasia,
no sweat glands; female has patches of skin w/o
sweat glands, cells descended from a cell in
which the X chromosome with the normal gene
was inactivated.
• G6PD alleles; Patches of color blindness
Descent of cells:
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How mosaics are made.
Events during
development.
Two homologous
chromosomes, blue & red.
Black indicates
inactivation = Barr
body
Formation of Barr bodies-2
Classic example: the calico cat.
One X chromosome codes for
orange fur, the other for black. Cat
shows characteristic mosaic patterns
caused by one or the other X
chromosome being inactivated.
White fur results from the effect of
another gene.
http://www.petstreetmall.com/merchant/Embroidery/Cat/CalicoCatBody.gif.jpe
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Molecular basis of Barr body formation
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• Xic is a region on the X near the centromere.
• Xic region includes a region called Xist (X inactivation
specific transcript)
– This area is transcribed, but RNA isn’t used to make
a protein; it binds to the DNA of the rest of the X
chromosome.
– This promotes molecular changes that inactivate
the chromosome including extensive methylation
(except for XIC) and condensation of DNA (into
smaller space).
• In the OTHER X chromosome, Xic region is
methylated so it will NOT be active.
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Occurs in a
“window” of
time during
development
http://bioweb.wku.edu/courses/biol566/Images/PlathF2.jpg
Active and inactive regions
Red: active genes.
Black: inactive
Xic is responsible for this
process; if moved to an
autosome, that chromosome
will be inactivated.
Besides XIC, a few other genes on the chromosome
remain active. Logically, they are genes also found in
the pseudoautosomal region of the Y chromosome.
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Polygenic Traits
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• Polygenic traits: different from multiple genes
– Seems like it should be the same, but no
– Also called Quantitative traits
– Polygenic traits are different in AMOUNT not TYPE
• Range of heights vs. purple/white
• Traits studied by Mendel: “discrete”
– Polygenic traits usually show continuous variation
• Height, weight, eye color, etc.
– Number of phenotypic classes depends on how much
you subdivide.
Polygenic Traits-2
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• Some polygenic traits are”meristic”
– Must be integers; meristic traits must be counted
• Number of kernels of corn can’t be continuous
• Offspring of crosses appear blended
– Still fit into Mendel’s notion of unit factors
– Multiple genes, and their alleles, are additive or not
• The total number of additive alleles determines
the phenotype.
• Usually studied using statistics
– Distribution of traits follows bell curve
– Mean, standard deviation, and variance
Quantitative traits are Mendelian
• Example: red and white wheat.
– Red results from an additive
allele, “white” is the absence of
of additive alleles.
– When the F1 plants are
crossed, an apparently
continuous range of
phenotypes is produced.
Including a “white” which is 1/16 of total.
Closer view: 1:4:6:4:1
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AB
Ab
aB
ab
AB
AABB
AABb
AaBB
AaBb
Ab
AABb
Aabb
AaBb
aaBb
aB
AaBB
AaBb
aaBB
aaBb
ab
AaBb
Aabb
aaBb
aabb
Five phenotypic classes:
4+ alleles, 3+ alleles, 2+ alleles, 1+ allele, none
Summary of polygenic idea
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Continuous variation
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• Traits usually quantifiable (weighing, etc.)
• Two or more genes contribute to phenotype in an
additive way.
– Individual allele either adds to phenotype or doesn’t
• Effect of each allele is small (but adds up)
– Lots of incremental effects create wide range of
phenotypic variation,
• Study requires large numbers of individuals
Continuous variation-2
Variation appears continuous because these traits
often affected by the environment.
note bell curve.
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