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Transcript of Genetics
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THE STUDY OF HEREDITYLH – Winter 2011
Genetics
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Gregor Mendel
The scientific study of heredity is called GENETICS!
Augustinian MonkBegan working on pea plants in his
monasteryCorrectly believed that heritable factors (genes) retain their individuality from generation to generation
i.e. – marbles
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Why Pea Plants?
Mendel chose to study garden peas, because: They reproduce quickly & have a short life cycle They have seven distinct & observable traits They produce many offspring in one cross Ease in manipulating pollination
Self-fertilization vs. Cross Pollination
Mendel produced seeds by cross pollinatingNote: Since pea plants can self-pollinate, they are called true-breeding. This means they will produce offspring
identical to themselves.
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Pea Plant Characteristics
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Mendel (con’d)
Mendel worked with his pea plants until he was sure that all were true-breeding varieties
He could finally begin his studies: What would happen if different characteristics were crossed? Purple flower x White flower?
Hybrid the offspring of two different varieties (also called a monohybrid cross)
P generation (P1) parental generation have offspring called the F1 generation
If F1 generations cross, their offspring are called the F2 generation
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P F1 F2
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Mendel’s Experiments
After studying pea plants, Mendel concluded that: Traits are passed from one generation to the next
through genes. Each trait is controlled by a different form of a gene
called an allele Some alleles are dominant to others called recessive
traits
New question: Have the recessive alleles disappeared or are they still present in the parents?
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Mendel’s Experiments
Mendel crossed the first generation and saw that the recessive trait showed up in about 1 of 4 plants.
Conclusion: Law of Segregation! Segregation of the alleles happens during the formation of gametes. Each gamete will carry one form of the allele.
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Using Probability to Predict Offspring
Punnett square – a diagram that shows the gene combinations that might result from a genetic cross of two parents
Phenotype – a description of what an individual LOOKS like (tall, red)
Genotype – a description of the genetic make-up of an individual (TT, Rr)
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Important Vocabulary
Dominant – allele that appears more frequently. It masks the recessive. Represented by a capitol letter (R=red)
Recessive – allele that appears less frequently (b/c it is repressed when paired with a dominant allele) Represented by a lower case letter (r=white)
AA – HOMOZYGOUS dominantaa – homozygous recessiveAa – HETEROZYGOUS one of each allele
Homozygous – two identical alleles for a traitHeterozygous – two different alleles for a trait
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LH BIOLOGYWINTER 2011
Solving Punnett Squares
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Punnett squaresStep 1
R = roundr = wrinkled
STEP 1 Define the alleles
If a homozygous round pea plant is crossed with a heterozygous round pea plant, what will their offspring look like?
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Step 2
Define the parents
RR x Rr
If a homozygous round pea plant is crossed with a heterozygous round pea plant, what will their offspring look like?
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Step 3Draw the Punnett square
R R
R
r
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Step 4
RR RR
Rr Rr
R R
R
r
Cross the parents find the probability of offspring
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Step 5
RR RR
Rr Rr
R R
R
r
Genotype: genetic make-up (letters)
Phenotype: physical characteristics
Find the genotype and phenotype of the offspring
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Finished Product
RR RR
Rr Rr
R R
R
r
Genotype ratio: 2 RR: 2Rr
Phenotype ratio: 100% Round
R=roundR=wrinkled
RR x Rr
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Dihybrid Crosses
Dihybrid Cross – a cross of parents differing in TWO characteristics For example: homozygous round, yellow x
homozygous wrinkled, green seeds RRYY x rryy
Law of Independent Assortment – each pair of alleles for different traits segregate independently of other pairs of alleles during gamete formation This explains genetic diversity among organisms
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Setting up a dihybrid
#1- list all 4 alleles For example: R=round, r=wrinkled, Y=yellow, y=green
#2 – Create the parental genotypes (4 letters each) Example: RRYY (Round, yellow) x rryy (wrinkled,
green) #3 – Using the “foil” method, determine the
sets of gametes (up to 4 possibilities) Example: RRYY RY
RrYy RY, Ry, rY, ry#4 – Fill in the tops and sides of punnett
square with gamete combinations#5 - Genotype and Phenotype as usual
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Dihybrid Example Problem #1
Round is dominant over wrinkledYellow is dominant over greenTwo pea plants produce offspring. One is
round and heterozygous for yellow seed color. The other is wrinkled and heterozygous for yellow seed color.
Parental genotypes = RRYy x rrYy
Possible gametes RY, Ry rY, ry
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Dihybrid Example Problem #1
Set up the dihybrid cross
RY Ry rY
ry
RrYY RrYy
RrYy Rryy
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Dihybrid Example Problem #1
Determine the genotype and phenotype!
RY Ry
Genotype: Phenotype: 1 RrYY: 2 RrYy : 1 Rryy 3 Round, yellow
1 Round, green
rY
ry
RrYY RrYy
RrYy Rryy
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Dihybrid Example Problem #2
Key: Black fur is dominant (B) to white fur (b)Long hair is dominant (L) to short hair
(l) Two guinea pigs mate. The dad is
homozygous for black fur and long hair. The mom is also homozygous, but for white fur and short hair. 1) Determine the possible gametes of each 2) What is the only gamete possibility for their
offspring?
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Dihybrid Example Problem #2
Key: Black fur is dominant (B) to white fur (b)Long hair is dominant (L) to short hair (l)
Two guinea pigs mate. The dad is homozygous for black fur and long hair. The mom is also homozygous, but for white fur and short hair.
1) Determine the possible gametes of eachDad 4 BL Mom 4 bl
2) What is the only gamete possibility for their offspring?
100% BbLl (Black, long-haired)
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Incomplete Dominance
Neither allele is dominant over the otherHeterozygous condition shows a blending of
genesAssign capital and lowercase letters for alleles
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Example problem #1
If a red four o’clock flower is crossed with a pink four o’clock flower what will their offspring look like?
RR = redrr = whiteRr = pink
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Perform cross
RR x Rr
Genotype ratio:
Phenotype ratio:
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Codominance
Both alleles are equalBoth alleles are expressed in the organismUse capital letters for both alleles
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Codominance Example
Black feathers and white feathers in chickens is codominant. In the heterozygous condition the feathers are called “erminette” and appear blue. BB = black WW or B’B’ = White BW or BB’ = blue
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Codominance Problem #1
Cross a Black chicken with a blue roosterParents = BB x BW (BBx BB’)
Genotype ratio:
Phenotype ratio:
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Sex-linked Problems
Some traits are only carried on the X chromosome
Traits can be dominant or recessiveSex-linked traits include:
Color blindness Hemophilia Duchene muscular dystrophy Fragile X syndrome
Males sex chromosome (XY)Females sex chromosome (XX)Sex-linked traits are more common in males
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It is good to be female!
Males are more likely to have these disorders since they only have 1 X chromosome (XY)
Females have two chances (XX), but they can be carriers and pass it on to their offspring
Ex: H = normal h = hemophilia
XHXH = normal female XHY = normal maleXHXh = carrier female XhY = hemophiliac male
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Example Problem
A carrier female marries a normal male. What are the chances that they will have a hemophiliac child?
Key:ParentsGenotype ratio:Phenotype ratio:
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Multiple Alleles
Blood Type ProblemsThere are 4 blood types: A, B, AB, OA and B are codominantO is recessive
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Blood Type Key
AA = homozygous Type AAO= heterozygous Type ABB= homozygous Type BBO= heterozygous Type BAB= codominant ABOO = Type O
SIX total combination possibilities
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Example Problem
A type AB woman marries a type O man. What are the possible phenotypes of their offspring?
AB x OO
B A
O
O
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Dihyrbid Example Problem
Round is dominant over wrinkledYellow is dominant over greenTwo homozygous pea plants produce
offspring. One is round and yellow. The other is wrinkled and green. What will their F2 generation look like? RY rY Ry ryRRYY RrYY RRYy RrYy
RrYY rrYY RrYy rrYy
RRYy RrYy Rryy Rryy
RrYy rrYy Rryy rryy
RYrYRyry
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RRYY RrYY RRYy RrYy
RrYY rrYY RrYy rrYy
RRYy RrYy Rryy Rryy
RrYy rrYy Rryy rryy