genetics 2
DESCRIPTION
2nd yearTRANSCRIPT
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GeneticsGenetics
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Heredity and GeneticsHeredity and Genetics• Heredity is the passing of
traits from parents to offspring
• Traits are controlled by genes, so GENETICS is the study of how traits are inherited through the action of alleles
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• Austrian monk born in 1822 who is responsible
for the laws governing the inheritance of traits
• Between 1856 and 1863, Mendel cultivated and tested over 28,000 pea plants
• Mendel performed cross-pollination by transferring pollen from one plant to selected ova of other plants, thereby controlling which plants mixed
Gregor Mendel – “Father of Genetics”
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Mendel’s ExperimentsMendel’s Experiments• Mendel produced pure strains by allowing
plants to self-pollinate, I.e., pollen (male gamete) from one plant fertilizes ova (female gamete) of same plant
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Mendel’s Peas….Mendel’s Peas….
S
G
iI
yY
s
g
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Mendel’s Peas….Mendel’s Peas….
T t
a
p
A
P
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Mendel’s Results….Mendel’s Results….(F2 from crossing to heterozygous F1s)
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Genetics TerminologyGenetics Terminology• Traits – any characteristic that can be passed from
parents to offspring• Heredity – the passing of traits from parents to
offspring• Alleles – one form (dominant or recessive) of a gene
• Sex cells have ONE form of a gene on their chromosomes
• Body cells have TWO forms or alleles for a single gene (you get one from mom and the other from dad)
• Dominant – is always expressed; masks a recessive trait
• Recessive – can only be expressed if there are no dominant alleles present
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Genetics TerminologyGenetics Terminology• Dominant alleles are represented by a capital letter• Recessive alleles are represented by a lower case letter
Example: B = Brown eye color (dominant) b = Blue eye color (recessive)
• “Purebred” species have two of the same alleles; this is also called HOMOZYGOUS, e.g., BB (homozygous dominant) or bb (homozygous recessive)
• Species with two different alleles are called HETEROZYGOUS, e.g., Bb
• Genotype: the alleles present in the organism, i.e., BB, Bb, or bb
• Phenotype: the expression of the genes; what is observed
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Genotype and Genotype and Phenotype in FlowerPhenotype in Flower
• All genes occur in pairs, so TWO alleles affect a trait.
• Possible combinations if:R = Red flowerr = Yellow flower
• Genotypes RR Rr rr• Phenotypes Red Red Yellow
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Genetic CrossesGenetic Crosses• Monohybrid Cross: cross involving ONE
trait, e.g., eye color • Dihybrid Cross: cross involving TWO traits,
e.g., eye color and hair color• Offspring’s genotype and phenotype is
determined using a Punnett square
B
b
B
b
BB
bbBb
Bb
BrainPop - Heredity
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Punnett Square, contPunnett Square, cont.
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Generations in CrossesGenerations in Crosses• Parental Generation (P1) = the parental
generation in a breeding experiment• First Filial Generation (F1) = the first
generation of offspring in a breeding experiment
• Second Filial Generation (F2) = the second generation of offspring in a breeding experiment
TT Tttt TtTTTtTt tt
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P1 Monohybrid CrossP1 Monohybrid Cross• Trait: Seed Color• Alleles: Y – Yellow y – Green• Cross: Yellow seeds X Green seeds
YY X yy Crossing two true-breeding (pure)
plants
Yy Yy
Yy Yy
y
y
Y YGenotype: Yy
Phenotype: Yellow
Genotypic Ratio: 100% Yy
Phenotypic Ratio: 100% Yellow
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F1 Monohybrid CrossF1 Monohybrid Cross• Trait: Seed Color• Alleles: Y – Yellow y – Green• Cross: Yellow seeds X Green seeds
Yy X YyCrossing to heterozygotes (hybrids)
YY Yy
Yy yy
Y
y
Y yGenotype: YY, Yy, yy
Phenotype: Yellow and Green
Genotypic Ratio: 25% YY, 50% Yy, 25% yy (1:2:1)
Phenotypic Ratio: 75% Yellow, 25% Green (3:1)
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Dihybrid CrossesDihybrid Crosses• A breeding experiment that tracks the
inheritance of two traits• Mendel’s “Law of Independent Assortment”
• Each pair of alleles segregates independently during gamete formation
• Formula: 2n (n = # of heterozygotes)Example:
1. RrYy: 2n = 22 = 4 possible gametes RY Ry rY ry
2. AaBbCCDd: 2n = 23 = 8 gametes ABCD ABCd AbCD AbCd
aBCD aBCd abCD abCD
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Dihybrid CrossesDihybrid CrossesTraits: Seed shape & Seed colorTraits: Seed shape & Seed colorAlleles:Alleles: R round r wrinkled Y yellow y green
RrYy x RrYy
RY Ry rY ryRY Ry rY ry RY Ry rY ryRY Ry rY ry
All possible gamete combinationsAll possible gamete combinations
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Dihybrid CrossDihybrid Cross
RYRY RyRy rYrY ryry
RYRY
RyRy
rYrY
ryry
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Dihybrid CrossDihybrid Cross
RRYY
RRYy
RrYY
RrYy
RRYy
RRyy
RrYy
Rryy
RrYY
RrYy
rrYY
rrYy
RrYy
Rryy
rrYy
rryy
Round/Yellow: 9
Round/green: 3
wrinkled/Yellow: 3
wrinkled/green: 1
9:3:3:1 phenotypic ratio
RYRY RyRy rYrY ryry
RYRY
RyRy
rYrY
ryry
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Dihybrid CrossDihybrid Cross
Round/Yellow: 9Round/green: 3wrinkled/Yellow: 3wrinkled/green: 1
9:3:3:1
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Test CrossTest Cross
Yy Yy
Yy Yy
y
y
Y YYy yy
Yy yy
•Test crosses involve breeding the individual in question with another individual that expresses a recessive version of the same trait. If all offspring display the dominant phenotype, the individual in question is homozygous dominant; if the offspring display both dominant and recessive phenotypes, then the individual is heterozygous yY
y
y
Offspring all yellow!
½ Offspring yellow; ½ Offspring green!
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Test CrossTest Cross• A mating between an individual of unknown
genotype and a homozygous recessive individual.• Example: bbC__ x bbcc
• BB = brown eyes• Bb = brown eyes• bb = blue eyes
• CC = curly hair• Cc = curly hair• cc = straight hair
bCbC b___b___
bcbc
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Test CrossTest Cross
Possible results:Possible results:bCbC b___b___
bcbc bbCc bbCc
C bCbC b___b___
bcbc bbCc bbccor
c
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Incomplete DominanceIncomplete Dominanceandand
CodominanceCodominance
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Incomplete DominanceIncomplete Dominance• F1 hybrids have an appearance somewhat in
between the phenotypes of the two parental varieties.
• Example: snapdragons (flower)red (RR)red (RR) x x white (rrwhite (rr)
RR = red flowerRR = red flowerrr = white flower
R
R
r r
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Incomplete DominanceIncomplete Dominance
RrRr
RrRr
RrRr
RrRr
RR
RR
rr
AllAll Rr Rr = = pink pink(heterozygous pink)(heterozygous pink)
produces theproduces theFF11 generation generation
r
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Incomplete DominanceIncomplete Dominance
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CodominanceCodominance• Two alleles are expressed (multiple Two alleles are expressed (multiple
alleles) in heterozygous individuals.alleles) in heterozygous individuals.• Example: blood typeExample: blood type
• 1.1. type Atype A = I= IAAIIAA or I or IAAii• 2.2. type Btype B = I= IBBIIBB or I or IBBii• 3.3. type ABtype AB= I= IAAIIBB
• 4.4. type Otype O = ii= ii
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Codominance ProblemCodominance Problem• Example:homozygous male Type B (IBIB)
xheterozygous female Type A (IAi)
IAIB IBi
IAIB IBi
1/2 = IAIB
1/2 = IBi
IB
IA i
IB
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Another Codominance ProblemAnother Codominance Problem
• Example:Example: male Type O (ii) x female type AB (IAIB)
IAi IBi
IAi IBi
1/2 = IAi1/2 = IBi
i
IA IB
i
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CodominanceCodominance
•Question:If a boy has a blood type O and his sister has blood type AB, what are the genotypes and phenotypes of their parents?
boy-type O (ii) X girl-type AB (IAIB)
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CodominanceCodominance
• Answer:Answer:
IAIB
ii
Parents:Parents:genotypesgenotypes = IAi and IBiphenotypesphenotypes = A and B
IB
IA i
i
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Sex-linked TraitsSex-linked Traits• Traits (genes) located on the sex
chromosomes• Sex chromosomes are X and Y• XX genotype for females• XY genotype for males• Many sex-linked traits carried
on X chromosome
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Sex-linked TraitsSex-linked Traits
Sex ChromosomesSex Chromosomes
XX chromosome - female Xy chromosome - male
fruit flyeye color
Example: Example: Eye color in fruit Eye color in fruit fliesflies
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Sex-linked Trait ProblemSex-linked Trait Problem
• Example: Eye color in fruit flies• (red-eyed male) x (white-eyed female)
XRY x XrXr
• Remember: the Y chromosome in males does not carry traits.
• RR = red eyed• Rr = red eyed• rr = white eyed• XY = male• XX = female
XR
Xr Xr
Y
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Sex-linked Trait Solution:Sex-linked Trait Solution:
XR Xr
Xr Y
XR Xr
Xr Y
50% red eyed female
50% white eyed male
XR
Xr Xr
Y
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Female CarriersFemale Carriers
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