Transcript Complementation - Arkansas State University

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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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The sex chromosomes
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• The rest of the chromosomes are autosomes.
– Sex chromosomes determine sex of individual.
• X and Y differ in size, X much bigger in humans
– But still synapse during meiosis, so still a pair
• In humans, fruit flies, XX = female; XY = male.
• In several other organisms, other combinations
of sex chromosomes determine sex.
• Because there are genes on sex chromosomes,
inheritance of certain traits can be sex-linked.
Sex inheritance
• In humans, fruit flies, XX = female; XY = male.
• Inheritance of sex is just like any other trait,
except it involve inheritance of an entire
chromosome.
Because there are
genes on sex
chromosomes,
inheritance of certain
traits can be sex-linked.
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Sex Linkage and Determination
• Early 1900s, Thomas Hunt Morgan was doing
classical genetics on fruit flies, looking for
mutants and checking out the patterns of
inheritance.
• He studied the white eye phenotype and
discovered something odd…
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What Morgan saw
Reciprocal cross
produced a
different result:
Inheritance of
eye color
depended on
sex of the fly.
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Morgan and the F2 generation
When the all red eyed F1
heterozygotes were crossed,
close to a 3:1 ratio was
observed, but the traits were not
evenly divided between the
sexes.
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Explanation
• The traits of sex and eye color did not assort
independently (as the traits in peas did).
• The traits are linked.
• The gene is NOT PRESENT on the Y
chromosome.
R = red eye
R = white eye
XX = female
XY = male
significance
• With regard to X linked traits, males have only
one allele, not two. They are said to be
hemizygous.
• Morgan’s work led to the understanding that
genes are located on chromosome’s because
inheritance of certain traits corresponded to
inheritance of a visibly different chromosome.
• Inheritance of X-linked traits results in typical
crisscross inheritance: mother to son.
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Crisscross inheritance
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Carrier mother passes allele to son who expresses it,
passes allele to daughter who carries it, etc.
Hemophilia & color blindness: examples in humans.
http://www.udl.es/usuaris/e4650869/docencia/segoncicle/genclin98/temes_teoria/imatge
s_temes_teoria/image3.gif
Sex linked, limited, & influenced
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• Sex linked inheritance is when the allele is
present on a sex chromosome (usually X).
• Sex limited: when other genetic factors restrict
expression to one sex
http://rwqp.rutgers.edu/i
mages/dairy%20cow.jpg
– Bulls don’t give milk.
• Sex influenced: other genetic factors modulate
expression. Example: pattern baldness
– Females must be homozygous recessive for trait
– Even then, hormonal factors
restrict expression.
imsc.usc.edu/.../contem
porary_sb_3.html
Sex determination
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• Different organisms have different
chromosomal mechanisms for determining sex.
– XX/XO: typically, the male has one copy.
• Nematodes, e.g. C. elegans
– XX/XY: as in humans, fruit flies, XX = female; XY =
male.
• Heterogametic sex is the one that produces a
mixture of gametes. Usually the male but:
– Female can be heterogametic in some species
– Designation is ZZ/ZW where female is ZW
More on sex determination
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• Temperature affects sex determination in many
reptile species
– Females result from low, high, or extremes of
temperature.
– Hypothesis:
History
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• Sex determination studies began in late 1800’s
• Work in humans started around 1912, but didn’t
get it right until 1956.
• Keys to understanding sex determination in
humans:
– Improved karyotype methods
– Study of aneuploidy of sex chromosomes
– Aneuploidy is the wrong number of a particular
chromosome.
– Aneuploidy results from non-disjunction
Abnormalities in chromosome number result
from non-disjunction
Homologues fail to separate during Meiosis I.
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Abnormalities in chromosome number result
from non-disjunction-2
Sister chromatids fail to separate during Meiosis II.
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Evidence for XX/XY
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• 47, XXY Klinefelter syndrome
– male in appearance, but some feminization; sterile.
– slow to learn, but not retarded.
– XXXY etc. similar, but more severe symptoms
• 45, XO Turner syndrome
– Monosomy, the only one occurring in humans
– female, sterile, short webbed neck, broad chest,
short.
– majority aren’t born
• If XXY is still male and XO is female
– Y must be determinant of maleness
About the Y
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• Y chromosome has been shrinking.
– Now missing many of genes that X has.
• Two regions: PAR and MSY
• PAR= pseudoautosomal region
– Regions near p telomere and q telomere are
homologous to X chromosome. Crossing over can
occur there during meiosis. Because of this, genes
in this location do not behave as sex-linked traits,
thus said to be pseudoautosomal because they
behave like genes on autosomes rather than sex
chromosomes.
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Structure of Y
universe-review.ca/R11-14-Ychromosome.htm
http://www.asiaandro.com/
1008-682X/4/259fig.jpg
Human Genome
project has
revealed much
about the Y
chromosome.
Male specific region Y (MSY)
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• X-transposed region
– 99% identical to X chromosome region, but only 2
genes; the rest are not expressed.
• X-degenerative region
– Contains DNA related to X chromosome regions
– Several functional genes and pseudogenes
– Contains SRY that codes for testis-determining
factor, necessary for maleness during development.
• Ampliconic region
– Highly similar or repeated genes, some related to
male development and fertility.
Evidence for SRY
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• SRY contains gene for testis determining factor
• Crossing over in meiosis
– Males with two X chromosomes; SRY found on one
– Females with X and Y; SRY is missing from Y
• Transgenic mice
– Remove SRY from Y chromosome
– Mice are XY but are female
– Reciprocal experiment also done
Article about Y chromosome
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• http://images.google.com/imgres?imgurl=http://www.tx
twriter.com/onscience/OSpictures/Y%2520chromosom
e%2520repair.jpg&imgrefurl=http://www.txtwriter.com/
onscience/Articles/ychromosome.html&h=927&w=504
&sz=160&hl=en&start=14&tbnid=hTINd2RIkH59cM:&t
bnh=147&tbnw=80&prev=/images%3Fq%3DY%2Bchr
omosome%26svnum%3D10%26hl%3Den%26rls%3D
GGLG,GGLG:2005-29,GGLG:en
• Copy and paste