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Mendelelian Mendelelian GeneticsGenetics
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Gregor Johann MendelGregor Johann MendelStudied the Studied the inheritanceinheritance of of traits in traits in pea plantspea plants
Developed the Developed the laws of inheritancelaws of inheritance
Mendel's work Mendel's work was not was not recognized until recognized until the turn of thethe turn of the 20th century20th century
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Gregor Johann MendelGregor Johann Mendel
He found that He found that the plants' the plants' offspring offspring retained retained traits of traits of the parentsthe parents
Called theCalled the “Father of “Father of Genetics"Genetics"
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Genetic TerminologyGenetic Terminology
GeneGene – a segment of DNA that is – a segment of DNA that is located in a chromosome & located in a chromosome & codes for a specific heredity traitcodes for a specific heredity traitTraitTrait - any characteristic that - any characteristic that can be passed from parent to can be passed from parent to offspring offspring HeredityHeredity - passing of traits from - passing of traits from parent to offspring parent to offspring GeneticsGenetics - study of heredity - study of heredity
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Types of Genetic Types of Genetic CrossesCrosses
Cross Cross – mating of two individuals– mating of two individualsGameteGamete – male and female sex cells (egg & – male and female sex cells (egg & sperm)sperm)FertilizationFertilization – the union of male and – the union of male and female gametes to form a zygote.female gametes to form a zygote.Monohybrid cross Monohybrid cross - - cross involving a single cross involving a single trait e.g. flower color trait e.g. flower color Dihybrid crossDihybrid cross - - cross involving two traits cross involving two traits e.g. flower color & plant heighte.g. flower color & plant height
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Punnett SquarePunnett Square
Used to help Used to help solve solve genetics genetics problemsproblems
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Designer Designer “Genes”“Genes”
AllelesAlleles - - two forms of a two forms of a gene gene
Ex. on chromosomes that code for ht. Ex. on chromosomes that code for ht. there would be 2 alleles, short & tall)there would be 2 alleles, short & tall) DominantDominant - - visible, observable trait; visible, observable trait; stronger of two genes expressed in the stronger of two genes expressed in the hybrid and masks the recessive. hybrid and masks the recessive. represented byrepresented by aa capital letter (R)capital letter (R) RecessiveRecessive – – hidden trait; gene that hidden trait; gene that shows up less often in a cross; shows up less often in a cross; represented by arepresented by a lowercase letter (r)lowercase letter (r)
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More TerminologyMore TerminologyGenotypeGenotype - - gene combination gene combination
for a trait; genetic make up of an for a trait; genetic make up of an organismorganism (e.g. RR, Rr, rr) (e.g. RR, Rr, rr)
PhenotypePhenotype - - the physical the physical feature or visible traits of an feature or visible traits of an organismorganism
(e.g. red, white) (e.g. red, white)
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GenotypesGenotypesHomozygousHomozygous genotype – both genotype – both
alleles are identical for a specific alleles are identical for a specific trait; 2 dominant or 2 recessive trait; 2 dominant or 2 recessive genes; genes;
(e.g. RR or rr);(e.g. RR or rr); also called also called pure pure HeterozygousHeterozygous genotype – having genotype – having 2 different alleles for a given 1 2 different alleles for a given 1 dominant & 1 recessive allele dominant & 1 recessive allele
((e.g. Rr);e.g. Rr); also called also called hybridhybrid
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Mendel’s Pea Plant ExperimentsMendel’s Pea Plant Experiments
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Mendel crossed 2 plants with different characters, or forms, for the same trait.
Ex. 1 tall & 1 short*The plants that grew were hybrid.
Hybrid – are the offspring of crosses between parents with different traits.
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Why peas,Why peas, Pisum Pisum sativumsativum??Can be grown in a Can be grown in a
small areasmall area Produce Produce lots of lots of offspring offspring Produce Produce purepure plants when allowed plants when allowed to to self-pollinateself-pollinate several generations several generations Can be Can be artificially artificially cross-pollinatedcross-pollinated
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How Mendel BeganHow Mendel BeganMendel Mendel produced produced purepure strains by strains by allowing allowing the plants the plants to to self-self-pollinatepollinate for for several several generatiogenerationsns copyright cmassengale
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Eight Pea Plant TraitsEight Pea Plant Traits
Seed shapeSeed shape --- Round --- Round (R)(R) or Wrinkled or Wrinkled (r)(r)
Seed ColorSeed Color ---- Yellow ---- Yellow (Y)(Y) or Green or Green ((yy))
Pod ShapePod Shape --- Smooth --- Smooth (S)(S) or wrinkled or wrinkled ((ss))Pod ColorPod Color --- Green --- Green (G)(G) or Yellow or Yellow (g)(g)Seed Coat ColorSeed Coat Color ---Gray ---Gray (G)(G) or White or White (g)(g)Flower positionFlower position---Axial ---Axial (A)(A) or Terminal or Terminal (a)(a)Plant HeightPlant Height --- Tall --- Tall (T)(T) or Short or Short (t)(t)
Flower color Flower color --- --- Purple Purple (P)(P) or white or white ((pp))
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Experiment I Concluded:*Each trait is based on two genes, one from the
mother and the other from the father *The hybrid plants looked like only 1 parent
and the character of the other parent seemed to disappear.
*Each trait is controlled by 1 gene.
Alleles- controls the different forms of a gene.Genes- chemical factors that determine traits.Phenotype is based on Genotype
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Law of DominanceLaw of Dominance *States that some alleles are dominant & others are recessive.
*Whenever a living thing inherits a dominant allele, that trait is visible.
*All the offspring will be heterozygous and express only the dominant trait.
RR x rr yields all Rr (round seeds)
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Law of DominanceLaw of Dominance
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Law of SegregationLaw of Segregation
During the During the formation of formation of gametesgametes (eggs or sperm), the (eggs or sperm), the two allelestwo alleles responsible for a responsible for a trait trait separateseparate from each other. from each other.
Alleles for a trait are then Alleles for a trait are then "recombined" at fertilization"recombined" at fertilization, , producing the genotype for producing the genotype for the traits of the offspringthe traits of the offspring.
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Experiment II: Law of Segregation
Mendel crossed a tall plant (dominant) with a short plant (recessive), the F1 plant inherited an allele for tallness from the tall parent & an allele for shortness from the short parent. Pg. 265
Mendel allowed his hybrid plants to self-pollinate.
Some showed recessive traits, the recessive traits did not disappear.
Earlier, the dominant masked the recessive, so it was not visible.
Alleles for the same trait can be separated.copyright cmassengale 19
Law of Segregation
Segregation – When sex cells, or gametes, are formed. Each gamete carries only 1 copy of each gene.
Therefore, each F1 plant produces 2 types of gametes (some with an allele for tallness & some with an allele for shortness). Ex. T, t, T, t = TT, Tt, Tt, tt
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Applying the Law of Applying the Law of SegregationSegregation
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Probability – the likelihood that a particular event will occur. Ex. Flipping a coin. The
probability that it will land on tails is ½.
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Law of Independent Law of Independent AssortmentAssortment
Alleles for Alleles for differentdifferent traits are traits are distributed to sex cells (& distributed to sex cells (& offspring) independently of one offspring) independently of one another.another.
This helps account for genetic This helps account for genetic variations.variations.
This law can be illustrated using This law can be illustrated using dihybrid crossesdihybrid crosses. Ex Pea shape & . Ex Pea shape & pea colorpea color
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Generation “Gap”Generation “Gap”Parental PParental P11 Generation Generation = the parental = the parental
generation in a breeding experimentgeneration in a breeding experiment..
FF11 generation generation = the first-generation = the first-generation offspring in a breeding experiment. offspring in a breeding experiment. (1st (1st filial generation)filial generation)
From breeding individuals from the PFrom breeding individuals from the P11 generationgeneration
FF22 generation generation = the second-generation = the second-generation offspring in a breeding experiment. offspring in a breeding experiment. (2nd filial generation)(2nd filial generation)
From breeding individuals from the FFrom breeding individuals from the F11 generationgeneration
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Following the Following the GenerationsGenerations
Cross 2 Cross 2 Pure Pure
PlantsPlantsTT x ttTT x tt
Results Results in all in all
HybridsHybridsTtTt
Cross 2 Cross 2 HybridsHybrids
getget3 Tall & 1 3 Tall & 1
ShortShortTT, Tt, ttTT, Tt, tt
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A Punnett Square shows:
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*All the possible results of a genetic cross.
*The genotypes of the offspring.
*The alleles in the gametes of each parent.
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Trait: Seed ShapeTrait: Seed Shape
Alleles: Alleles: RR – Round – Round rr – Wrinkled – Wrinkled
Cross: Cross: RoundRound seedsseeds xx Wrinkled Wrinkled seedsseeds
RRRR xx rr rr
PP11 Monohybrid Cross Monohybrid Cross
R
R
rr
Rr
RrRr
Rr
Genotype:Genotype: RrRr
PhenotypePhenotype: RoundRound
GenotypicGenotypicRatio:Ratio: All alikeAll alike
PhenotypicPhenotypicRatio:Ratio: All alike All alikecopyright cmassengale
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PP11 Monohybrid Cross Monohybrid Cross ReviewReviewHomozygous dominant x Homozygous dominant x
Homozygous recessiveHomozygous recessiveOffspringOffspring allall HeterozygousHeterozygous (hybrids)(hybrids)Offspring calledOffspring called FF11 generation generation
Genotypic & Phenotypic ratio Genotypic & Phenotypic ratio isis ALL ALIKEALL ALIKE
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Trait: Seed ShapeTrait: Seed Shape
Alleles: Alleles: RR – Round – Round rr – Wrinkled – Wrinkled
Cross: Cross: RoundRound seeds seeds xx Round Round seedsseeds
RrRr xx Rr Rr
FF11 Monohybrid Cross Monohybrid Cross
R
r
rR
RR
rrRr
Rr
Genotype:Genotype: RR, Rr, RR, Rr, rrrr
PhenotypePhenotype: Round Round && wrinkled wrinkled
G.Ratio:G.Ratio: 1:2:11:2:1
P.Ratio:P.Ratio: 3:1 3:1copyright cmassengale
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FF11 Monohybrid Cross Monohybrid Cross ReviewReviewHeterozygous x heterozygousHeterozygous x heterozygousOffspring:Offspring:
25% Homozygous dominant25% Homozygous dominant RRRR50% Heterozygous50% Heterozygous RrRr25% Homozygous Recessive25% Homozygous Recessive rrrrOffspring calledOffspring called FF22 generation generationGenotypic ratio isGenotypic ratio is 1:2:11:2:1Phenotypic RatioPhenotypic Ratio is 3:1 is 3:1
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……And Now the Test And Now the Test CrossCross
Mendel then crossed a Mendel then crossed a purepure & a & a hybridhybrid from his from his FF2 2 generationgeneration
This is known as an This is known as an FF22 or test cross or test cross
There are There are twotwo possible testcrosses: possible testcrosses:Homozygous dominant x HybridHomozygous dominant x HybridHomozygous recessive x HybridHomozygous recessive x Hybrid
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Trait: Seed ShapeTrait: Seed Shape
Alleles: Alleles: RR – Round – Round rr – Wrinkled – Wrinkled
Cross: Cross: RoundRound seedsseeds xx Round Round seedsseeds
RRRR xx Rr Rr
FF22 Monohybrid Cross (1 Monohybrid Cross (1stst))
R
R
rR
RR
RrRR
Rr
Genotype:Genotype: RR, RR, RrRr
PhenotypePhenotype: RoundRound
GenotypicGenotypicRatio:Ratio: 1:11:1
PhenotypicPhenotypicRatio:Ratio: All alike All alike
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Trait: Seed ShapeTrait: Seed Shape
Alleles: Alleles: RR – Round – Round rr – Wrinkled – Wrinkled
Cross: Cross: WrinkledWrinkled seedsseeds xx Round Round seedsseeds
rrrr xx Rr Rr
FF22 Monohybrid Cross (2nd) Monohybrid Cross (2nd)
r
r
rR
Rr
rrRr
rr
Genotype:Genotype: Rr, rrRr, rr
PhenotypePhenotype: Round & Round & WrinkledWrinkled
G. Ratio:G. Ratio: 1:11:1
P.Ratio:P.Ratio: 1:1 1:1copyright cmassengale
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FF22 Monohybrid Cross Monohybrid Cross ReviewReviewHomozygous x Homozygous x
heterozygous(hybrid)heterozygous(hybrid)Offspring:Offspring:50% Homozygous 50% Homozygous RR or rrRR or rr50% Heterozygous50% Heterozygous RrRrPhenotypic RatioPhenotypic Ratio is 1:1 is 1:1Called Called Test CrossTest Cross because the because the offspring have offspring have SAMESAME genotype genotype as parentsas parents
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Dihybrid CrossDihybrid CrossA breeding experiment that tracks A breeding experiment that tracks
the the inheritance of two traitsinheritance of two traits..
Mendel’s Mendel’s “Law of Independent “Law of Independent Assortment”Assortment”
a. Each pair of alleles segregates a. Each pair of alleles segregates independentlyindependently during gamete during gamete formationformation
b. Formula: 2b. Formula: 2nn (n = # of heterozygotes) (n = # of heterozygotes)
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Question:Question:How many gametes will be How many gametes will be
produced for the following allele produced for the following allele arrangements?arrangements?
Remember: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 gametes = 4 gametes
RY Ry rY ryRY Ry rY ry
2. AaBbCCDd: 22. AaBbCCDd: 2nn = 2 = 233 = 8 gametes = 8 gametes
ABCD ABCd AbCD AbCdABCD ABCd AbCD AbCd
aBCD aBCd abCD abCDaBCD aBCd abCD abCD
3. MmNnOoPPQQRrssTtQq: 23. MmNnOoPPQQRrssTtQq: 2nn = 2 = 266 = = 64 gametes64 gametes
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Experiment III
*Mendel wanted to see if genes that determine 1 trait have anything to do with genes that determine another.
*He followed 2 different genes as they passed from one generation to the next.
*Mendel crossed true-breeding plants - round yellow peas (RRYY) with wrinkled green peas (rryy).
*The F1 offspring were all round & yellow showing that both were dominant alleles. The genotype is RrYy. Pg. 270
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Dihybrid CrossDihybrid CrossTraits: 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 combinationscopyright cmassengale
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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 CrossA mating between an individual of A mating between an individual of unknown unknown
genotype genotype and a and a homozygous recessivehomozygous recessive individual.individual.
Example:Example: bbC__ bbC__ x x bbccbbcc
BB = brown eyesBB = brown eyesBb = brown eyesBb = brown eyesbb = blue eyesbb = blue eyes
CC = curly hairCC = curly hairCc = curly hairCc = curly haircc = straight 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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Summary of Mendel’s Summary of Mendel’s lawslaws
LAWLAW PARENT PARENT CROSSCROSS OFFSPRINGOFFSPRING
DOMINANCEDOMINANCE TT x ttTT x tt tall x shorttall x short
100% Tt 100% Tt talltall
SEGREGATIONSEGREGATION Tt x TtTt x Tt tall x talltall x tall
75% tall 75% tall 25% short25% short
INDEPENDENT INDEPENDENT ASSORTMENTASSORTMENT
RrGg x RrGgRrGg x RrGg round & round & green x green x round & round & greengreen
9/16 round seeds & green 9/16 round seeds & green pods pods
3/16 round seeds & yellow 3/16 round seeds & yellow pods pods
3/16 wrinkled seeds & 3/16 wrinkled seeds & green pods green pods
1/16 wrinkled seeds & 1/16 wrinkled seeds & yellow podsyellow pods
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Pedigrees*Pedigrees – are graphic representations
of a family tree, which allows for patterns of inheritance to be seen.
*Each horizontal row of circles and squares represents a generation.
*People that do not show the trait but are heterozygous (Tt) for the trait are called carriers.
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Pedigree on mouse color
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Different Patterns of Dominant & Recessive
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Incomplete Dominance – 1 allele is not completely dominant over another. Ex. Red flower
(RR) & white flower (WW), F1 is a pink flower (RW). Pg. 272
Codominance – both alleles contribute to the phenotype.
Ex. A cross of a black chicken (BB) with a white chicken (WW) will produce all speckled offspring
(BBWW); colors appear separately.
Different Patterns of Dominant & Recessive
Multiple Alleles – have more than 2 alleles. Ex. Coat color in rabbits.
Polygenic Traits – traits controlled by 2 or more genes; “having many genes”. Ex. Variation in human skin color.
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Incomplete DominanceIncomplete Dominance
F1 hybrids F1 hybrids have an appearance somewhat in betweenin between 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
RrRr
RrRr
RrRr
RrRr
RR
RR
rr
All Rr =All Rr = pink pink(heterozygous pink)(heterozygous pink)
produces theproduces theFF11 generation generation
r
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Incomplete DominanceIncomplete Dominance
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CodominanceCodominanceTwo allelesTwo alleles are expressed ( are expressed (multiple multiple
allelesalleles) in ) in heterozygous individualsheterozygous individuals..Example:Example: blood type blood type
1.1. type Atype A = I= IAAIIAA or I or IAAii2.2. type Btype B = I= IBBIIBB or I or IBBii3.3. type ABtype AB= I= IAAIIBB
4.4. type Otype O = ii= ii
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Codominance ProblemCodominance ProblemExample:homozygous male Type B (IBIB)
x heterozygous female Type A (IAi)
IAIB IBi
IAIB IBi
1/2 = IAIB
1/2 = IBi
IB
IA i
IB
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Another Codominance Another Codominance ProblemProblem
• 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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Sex-linked TraitsSex-linked Traits
Traits (genes) located on the Traits (genes) located on the sex chromosomessex chromosomes
Sex chromosomes Sex chromosomes are are X and YX and YXXXX genotype for females genotype for femalesXYXY genotype for males genotype for malesMany sex-linked traits carried Many sex-linked traits carried
on X chromosomeon 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 Sex-linked Trait ProblemProblem
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 eyedRr = red eyedrr = white eyedXY = maleXX = 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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