genetics
DESCRIPTION
2nd yearTRANSCRIPT
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GENETICSGENETICS
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GeneticsGenetics
• The study of heredityheredity.
•• Gregor Mendel (1860Gregor Mendel (1860’’s)s) discovered the fundamental principles principles of geneticsgenetics by breedingbreedinggarden peasgarden peas.
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GeneticsGenetics•• AllelesAlleles
1. Alternative forms of genes.genes.
2. Units that determine heritable traits.
3. Dominant alleles Dominant alleles (TTTT - tall pea plantstall pea plants)a. homozygous dominanta. homozygous dominant
4. Recessive alleles Recessive alleles (tt tt - dwarf pea plantsdwarf pea plants)a. homozygous recessivea. homozygous recessive
5. HeterozygousHeterozygous (TtTt - tall pea plantstall pea plants)
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PhenotypePhenotype
•• Outward appearanceOutward appearance•• Physical characteristicsPhysical characteristics
•• Examples:Examples:1.1. tall pea planttall pea plant2.2. dwarf pea plantdwarf pea plant
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GenotypeGenotype•• Arrangement of genes that produces the Arrangement of genes that produces the
phenotypephenotype•• Example:Example:
1.1. tall pea planttall pea plantTT = tall (homozygous dominant)(homozygous dominant)
2.2. dwarf pea plantdwarf pea planttt = dwarf (homozygous recessive)(homozygous recessive)
3.3. tall pea planttall pea plantTt = tall (heterozygous)(heterozygous)
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Punnett squarePunnett square
• A Punnett square Punnett square is used to show the possible combinationscombinations of gametesgametes.
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Breed the P generationP generation
•• tall (TT) vs. dwarf (tt) pea plantstall (TT) vs. dwarf (tt) pea plants
t
t
T T
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tall (TT) vs. dwarf (tt) pea plantstall (TT) vs. dwarf (tt) pea plants
t
t
T T
Tt
Tt
Tt
Tt All Tt = tall(heterozygous tall)
produces theFF11 generationgeneration
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Breed the FF11 generationgeneration
•• tall (Tt) vs. tall (Tt) pea plantstall (Tt) vs. tall (Tt) pea plants
T
t
T t
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tall (Tt) vs. tall (Tt) pea plantstall (Tt) vs. tall (Tt) pea plants
TT
Tt
Tt
tt
T
t
T t
produces theFF22 generationgeneration
1/4 (25%) = TT1/2 (50%) = Tt1/4 (25%) = tt
1:2:1 genotype1:2:1 genotype3:1 phenotype3:1 phenotype
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Monohybrid CrossMonohybrid Cross
• A breeding experiment that tracks the inheritance of a single trait.single trait.
•• MendelMendel’’s s ““principle of segregationprinciple of segregation””a. pairs of genes separate during gamete gamete
formation (meiosis).(meiosis).
b. the fusion of gametesgametes at fertilization pairs genes once again.
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Homologous ChromosomesHomologous Chromosomes
eye color locusb = blue eyes
eye color locusB = brown eyes
Paternal Maternal
This person would have brown eyes (Bb)
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Meiosis Meiosis -- eye coloreye color
Bb
diploid (2n)
B
b
meiosis I
B
B
b
b
sperm
haploid (n)
meiosis II
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Monohybrid CrossMonohybrid Cross
•• ExampleExample: Cross between two heterozygotesheterozygotesfor brown eyes (Bb)
BB = brown eyesBb = brown eyesbb = blue eyes
B
b
B b
Bb x Bb
malegametes
female gametes
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Monohybrid CrossMonohybrid Cross
BB
Bb
Bb
bb
B
b
B b
Bb x Bb
1/4 = BB - brown eyed1/2 = Bb - brown eyed1/4 = bb - blue eyed
1:2:1 genotype3:1 phenotype
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Dihybrid CrossDihybrid Cross
• A breeding experiment that tracks the inheritance of two traits.two traits.
•• MendelMendel’’s s ““principle of independent assortmentprinciple of independent assortment””
a. each pair of alleles segregates independently during gamete formation (metaphase I)(metaphase I)
b. formula: 22nn (n = # of heterozygotes)(n = # of heterozygotes)
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Independent AssortmentIndependent Assortment•• Question: Question: How many gametes will be produced
for the following allele arrangements?
• Remember: 22nn (n = # of heterozygotes)(n = # of heterozygotes)
1.1. RrYyRrYy
2.2. AaBbCCDdAaBbCCDd
3.3. MmNnOoPPQQRrssTtQqMmNnOoPPQQRrssTtQq
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Answer:Answer:1. RrYy: 21. RrYy: 2nn = 2= 222 = = 4 gametes4 gametes
RY Ry rY ryRY Ry rY ry
2. AaBbCCDd: 22. AaBbCCDd: 2nn = 2= 233 = = 8 gametes8 gametesABCD ABCd AbCD AbCdABCD ABCd AbCD AbCdaBCD aBCd abCD abCD aBCD aBCd abCD abCD
3. MmNnOoPPQQRrssTtQq: 23. MmNnOoPPQQRrssTtQq: 2nn = 2= 266 = = 64 gametes64 gametes
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Dihybrid CrossDihybrid Cross
•• Example:Example: cross between roundround and yellowyellowheterozygous pea seeds.
RR = round= roundrr = wrinkled= wrinkledYY = yellow= yellowyy = green= green
RY Ry rY ry RY Ry rY ry x RY Ry rY ryRY Ry rY rypossible gametes produced
RrYyRrYy x RrYyRrYy
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Dihybrid CrossDihybrid CrossRYRY 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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Test CrossTest Cross• A mating between an individual of unknown genotypeunknown genotype
and a homozygous recessivehomozygous recessive individual.•• Example:Example: bbC__ bbC__ x bbccbbcc
BB = brown eyesBb = brown eyesbb = blue eyes
CC = curly hairCc = curly haircc = 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 Dominance•• F1 hybrids F1 hybrids have an appearance somewhat in in
betweenbetween the phenotypes phenotypes of the two parental varieties.
•• Example:Example: snapdragons (flower)snapdragons (flower)• red (RR) x white (rr)
RR = red flowerRR = red flowerrr = white flower
r
r
R R
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Incomplete DominanceIncomplete Dominance
Rr
Rr
Rr
Rr
r
r
R R
All Rr = pink(heterozygous pink)
produces theFF11 generationgeneration
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CodominanceCodominance
•• Two allelesTwo alleles are expressed (multiple allelesmultiple alleles) in heterozygous individualsheterozygous individuals.
•• Example:Example: bloodblood
1. type A = IAIA or IAi2. type B = IBIB or IBi3. type AB = IAIB4. type O = ii
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CodominanceCodominance
•• Example:Example: homozygous male B (IBIB)x
heterozygous female A (IAi)
IAIB IAIB
IBi IBi
1/2 = IAIB1/2 = IBi
IA
IB IB
i
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CodominanceCodominance
•• Example:Example: male O (ii) x female AB (IAIB)
IAi IBi
IAi IBi
1/2 = IAi1/2 = IBi
i
IA IB
i
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CodominanceCodominance
•• QuestionQuestion: 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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SexSex--linked Traitslinked Traits
• Traits (genes) located on the sex sex chromosomeschromosomes
•• Example:Example: fruit fliesfruit flies
(redred-eyed male) X (whitewhite-eyed female)
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SexSex--linked Traitslinked TraitsSex ChromosomesSex Chromosomes
XX chromosome - female Xy chromosome - male
fruit flyeye color
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SexSex--linked Traitslinked Traits•• Example:Example: fruit fliesfruit flies
(red-eyed male) X (white-eyed female)•• Remember:Remember: the Y chromosomeY chromosome in males
does not carry traits.
RR = red eyedRr = red eyedrr = white eyed
Xy = maleXX = female
Xr
XR y
Xr
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SexSex--linked Traitslinked Traits
XR Xr
XR Xr
Xr y
Xr y
1/2 red eyed and female1/2 white eyed and male
Xr
XR y
Xr
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Population GeneticsPopulation Genetics• The study of genetic changesgenetic changes in populationspopulations.
• The science of microevolutionary changesmicroevolutionary changes in populationspopulations.
•• HardyHardy--Weinberg equilibrium:Weinberg equilibrium:the principle that shuffling of genes that occurs during sexual reproduction, by itself, cannot change the overall genetic makeup of a population.
•• HardyHardy--Wienberg equation:Wienberg equation: 1 = p1 = p22 + 2pq + q+ 2pq + q22
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Question:Question:
•• How do we get this equation?How do we get this equation?
Answer:Answer: ““SquareSquare”” 1 = p + q1 = p + q
↓↓1122 = (p + q)= (p + q)22
↓↓1 = p1 = p22 + 2pq + q+ 2pq + q22
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HardyHardy--Wienberg equationWienberg equation
•• Five conditions Five conditions are required for Hardy-Wienberg equilibrium.1.1. large populationlarge population2.2. isolated populationisolated population3.3. no net mutationsno net mutations4.4. random matingrandom mating5.5. no natural selectionno natural selection
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•• Need to remember the following:Need to remember the following:
pp2 2 = homozygous dominant= homozygous dominant2pq = heterozygous2pq = heterozygousqq22 = homozygous recessive= homozygous recessive
ImportantImportant
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• Iguanas with webbed feet (recessive trait) (recessive trait) make up 4% of the population. What in the population is heterozygousheterozygous and homozygoushomozygous dominantdominant.
Question:Question:
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Answer:Answer:
1. q1. q2 2 = 4% or .04= 4% or .04 qq22 = .04= .04 q = .2q = .2
2. then use 1 = p + q2. then use 1 = p + q1 = p + .21 = p + .2 1 1 -- .2 = p.2 = p .8 = p.8 = p
3. for heterozygous use 2pq3. for heterozygous use 2pq2(.8)(.2) = .32 or 32%2(.8)(.2) = .32 or 32%
4. For homozygous dominant use p4. For homozygous dominant use p22
.8.822 = .64 or 64%= .64 or 64%
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HardyHardy--Wienberg equationWienberg equation
1 = p1 = p22 + 2pq + q+ 2pq + q22
•• 64% = p64% = p2 2 = homozygous dominant= homozygous dominant•• 32% = 2pq32% = 2pq = heterozygous= heterozygous•• 04% 04% = q= q22 = homozygous recessive= homozygous recessive•• 100%100%