the probability of a chance or event is independent of any other chance or event. the probability of...
TRANSCRIPT
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• The probability of a chance or event
is independent of any other chance
or event.
• The probability of two or more
events occurring together is equal to
the product of their individual
probabilities.
Laws of Probability
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• Expressivity – the extent to which a phenotype shows in an individual in a population. E.g. how brown is hair color?
• Penetrance – the extent to which a phenotype is carried by members of a population
Expressivity & Penetrance
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• Amniocentesis
• CVS - chorionic villus sampling
• Linkage maps/cytological maps
Analysis and Diagnosis
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Mendel and
His Peas
A Look Into A Look Into GeneticsGenetics
http://www.accessexcellence.org/RC/AB/BC/Gregor_Mendel.php
http://en.wikipedia.org/wiki/Image:NCI_peas_in_pod.jpg
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• Gregor Mendel - 1800’s
• Austrian (modern Czech) Monk
• Observed “traits” or “factors” in pea
plants in his monastery garden
• Kept good records
• Never published...
Gregor Mendel:Classical Genetics
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• Traits - logical, probability terms used
by Mendel to describe his work.
• Allele - one of two (or more)
expressions of a trait in a trait pair.
• Gene - section of DNA responsible for
the translation of a protein (enzyme)
Traits, factors, alleles, genes...
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• Round or wrinkled peas
• Sweet or starchy peas
• Green or yellow peas
• Inflated or wrinkled pods
• Green or gray pods
• Tall or short plants
• Terminal or axial flowers
Traits of Peas….Mmm
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Peas – True Breeding
Round pea seed
wrinkled pea seed
Round pea seeds produce round plantswrinkled pea seeds produce wrinkled plants
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Crossing Peas
x • Mendel crossed a
purebred round with
a purebred wrinkled
pea, thinking that
the offspring would
be a blend of the
two traits...
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Crossing Peas
x • Instead he found that
the first offspring
generation was all
round…
• This led him to assert
that round was
dominant to wrinkled.
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Crossing Peas
x • When he allowed the
first offspring
generation to self-
pollinate..
• …the second
offspring generation
showed both traits.
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Naming the generations
x P1 Parental generation
F1 First filial
generation
F2 Second filial generation
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The Genes that Cause the Traits...
xRR
Rr
rr
RR Rr rrRr
R
R
r rRr Rr
RrRr
R
r
R rRR Rr
rrRr
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• Genotype - letters or symbols for
the genes involved: RR, Rr, rr; TT,
Tt, tt
• Phenotype - adjectives that describe
the trait: round, wrinkled, tall, short
Genotype and Phenotype
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Genotype and Phenotype
• The genotype causes the
phenotype…
• The phenotype is the expression of
the genotype...
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• Homozygous (purebred) - both
alleles in the allele pair are identical
• Heterozygous (“hybrid”) - the two
alleles in the allele pair are different
Purebreds and Hybrids
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Another set of alleles...
SHORTPLANT
TALLPLANT
Cross a pure bred tall plantwith a pure bred short plant...
T = tall genet = short gene
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• Just as in the P1 cross of
purebred dominant with
purebred wrinkled…
• All the F1 individuals will
show the dominant trait
(and heterozygous
genotype)
P1 Cross: Homozygous Tall with Homozygous Short...
TT ttx
T
T
t t
Tt Tt
TtTt
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Tt Ttx
T
t
T t
TT Tt
ttTt
• Now the F1 can cross
• And produce the
characteristic 3:1 ratio
of phenotypes…
• (and 1:2:1 ratio of
genotypes…)
F1 Cross: Self-pollination of hybrid
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• Because the F1 (self-pollinations)
crosses are ones between hybrids
for one allele, they are often called
monohybrid crosses.
• It is an excellent example of
classical genetics.
Monohybrid Cross
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Monohybrid Cross
• Monohybrid Crosses show the
segregation of two alleles in the
same allele pair.
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• Monohybrid cross shows the
segregation of alleles of the
same allele pair into different
gametes...
• Dihybrid cross ?
More Classical Genetics
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Green - Yellow Pea
GREENx
GREEN
YELLOW x YELLOWx
ALL YELLOWOFFSPRING
ALL GREENOFFSPRING
x
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Green/yellow crossesYELLOW x GREEN
ALL YELLOWOFFSPRING
(SELF-POLLINATION)
x
3 TO 1 RATIO:YELLOW TO GREEN
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G/y Phenotypes/GenotypesYELLOW x GREEN
ALL YELLOWOFFSPRING
(SELF-POLLINATION)
x
3 TO 1 RATIO:YELLOW TO GREEN
yyYY
Yy
YY,2Yy,yy
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G/y Punnett SquaresYY yyx
Y
Y
y yYy Yy
YyYy
Yy Yyx
Y
y
Y yYY Yy
yyYy
PHENOTYPE RATIOS3 yellow: 1greenGENOTYPE RATIOS1YY:2Yy:1yy
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Dihybrid CrossCross a homozygous round yellow pea plant with a homozygous wrinkled green plant.
Then cross the F1 (self-pollination).
What are the genotypes?
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P1 CrossRRYY x rryyP CROSS:
GAMETES: RY ry
RY
RY
ry ryRrYy RrYy
RrYy RrYy
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F1 Cross
RrYy x RrYyF1 CROSS:
GAMETES: RY, Ry, rY, ry
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RrYy Dihybrid CrossRY Ry
RY
Ry
rY
ry
ryrY
RRYY RRYy
RRyyRRYy
RrYY RrYy
RryyRrYy
RrYy
Rryy
rrYy
rryy
RrYY
RrYy
rrYY
rrYy
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RY Ry
RY
Ry
RRYY RRYy
RRyyRRYy
rY
ry
RrYY RrYy
RryyRrYy
ry
RrYy
Rryy
rrYy
rryy
rY
RrYY
RrYy
rrYY
rrYy
RrYy Dihybrid Cross
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Phenotype RatiosRY Ry
RY
Ry
rY
ry
ryrY
Round Yellow
Round Green
Wrinkled Yellow
Wrinkled Green
9
3
3
1
9:3:3:1
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Genetic Recombination
R tTr•This is a diploid cell
right after mitosis.
•Note that there are 4 paired chromosomes
•Each chromosome has only one chromatid
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RR ttTTrr
S-phase chromosomes
•After S-phase (i.e. DNA synthesis)…
•There are still only 4 chromosomes…
•Each chromosome now has two chromatids.
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Metaphase-1
ORRR
ttTT
rr rr
TT tt
RR
•Because the chromosomes synapse at random on the metaphase plate..•Two different combinations are possible
with two pairs of chromosomes.
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Anaphase-1
POSSIBLEANAPHASE 1
ARRANGEMENTS
ORRR
ttTT
rr RR
ttTT
rr
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POSSIBLEMETAPH 2ARRANGE-
MENTS
R
T
r
t
R
r R R
r r
t
T T T
tt•…and this means more varied recom-
binations are possible.
Metaphase-2
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Anaphase-2
ANAPHASE 2AND
TELOPHASE 2
R
T
r
t
R
r R R
r r
t
T T T
tt
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Gametes
ALLPOSSIBLE GAMETES
2 CHROMOSOMESEACH
RT RT rT rT
Rtrtrt Rt