9-2: genetic crosses help to predict the genetic makeup + appearance of offspring
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9-2: 9-2: GeneticGenetic CrossesCrossesHelp to predict the Help to predict the genetic makeup + genetic makeup +
appearance of appearance of offspringoffspring
Genotype & PhenotypeGenotype & Phenotype GenotypeGenotype – genetic makeup of an – genetic makeup of an
organismorganism– Consists of alleles inherited from Consists of alleles inherited from
parentsparents
– Ex. Ex. PPPP or or PpPp = purple flower; = purple flower; pppp = = white flowerwhite flower
PhenotypePhenotype – the appearance of an – the appearance of an organism as a result of its genotypeorganism as a result of its genotype– Ex. PP or Pp = Ex. PP or Pp = purplepurple flower; pp = flower; pp =
whitewhite flower flower
More words to know…More words to know… HomozygousHomozygous – both alleles in a pair – both alleles in a pair
are alikeare alike– Also known as a Also known as a PUREBREDPUREBRED
– Homozygous dominant = Homozygous dominant = PPPP– Homozygous recessive =Homozygous recessive = pppp
HeterozygousHeterozygous – alleles are different – alleles are different– Also known as a Also known as a HYBRIDHYBRID
– Heterozygous = Heterozygous = PPpp
ProbabilityProbability The likelihood that a specific event The likelihood that a specific event
will occurwill occur– May be expressed as a May be expressed as a decimaldecimal, , percentagepercentage, , fractionfraction, or , or ratioratio
– Equation:Equation: # times an event is expected to # times an event is expected to
happenhappen # opportunities for # opportunities for it to happenit to happen
Probability (cont)Probability (cont) Examples:Examples:
– Mendel observed that Mendel observed that yellowyellow seed color seed color in Fin F22 appeared 6,022 times and appeared 6,022 times and greengreen seed color appeared 2,001 timesseed color appeared 2,001 times Total number of seeds = 8,023Total number of seeds = 8,023
– Probability of dominant trait appearingProbability of dominant trait appearing 6,022/8,023 = .75, 75%, or ¾6,022/8,023 = .75, 75%, or ¾
– Probability of recessive trait appearingProbability of recessive trait appearing 2,001/8,023 = .25, 25%, or ¼2,001/8,023 = .25, 25%, or ¼
Probability (cont)Probability (cont) What does probability tell us about What does probability tell us about
the offspring of 2 heterozygous the offspring of 2 heterozygous individualsindividuals??– There are 3 out of 4 chances the offspring will There are 3 out of 4 chances the offspring will
have the dominant traithave the dominant trait– There is 1 out of 4 chances the offspring will There is 1 out of 4 chances the offspring will
have the recessive traithave the recessive trait
*The results of probability are more *The results of probability are more likely to occur if you have more likely to occur if you have more trials*trials*
Probability (cont)Probability (cont) Ex. Flipping a coinEx. Flipping a coin What is the What is the
probability a coin probability a coin will land on heads?will land on heads?– 50%50%
What is the What is the probability a coin probability a coin will land on tails?will land on tails?– 50%50%
Monohybrid CrossesMonohybrid Crosses
A cross between individuals involving A cross between individuals involving one pair of contrasting traits is called one pair of contrasting traits is called a a monohybrid crossmonohybrid cross– There are 4 possible outcomes (2There are 4 possible outcomes (222))
Results are created with the use of Results are created with the use of Punnett SquarePunnett Square– A diagram that helps biologist predict A diagram that helps biologist predict
the probability that certain traits will be the probability that certain traits will be inherited by offspringinherited by offspring
Example 1: Homozygous Example 1: Homozygous (Dominant) x Homozygous (Dominant) x Homozygous
(recessive)(recessive)PP – – purplepurple flower; flower; pp – white flower – white flower Cross = Cross = PPPP x x pppp
Genotypes = 4 PGenotypes = 4 Ppp Phenotypes = 4 Phenotypes = 4 purplepurple flowers flowers
PPpp PPpp
PPpp PPpp
P P
p
p
Example 2a: Homozygous Example 2a: Homozygous (Dominant) x Heterozygous(Dominant) x Heterozygous
B – B – black coat; b – coat; b – brownbrown coat coat Cross = BB x BbCross = BB x Bb
Genotypes = 2 BB, 2BbGenotypes = 2 BB, 2Bb Phenotypes = 4 Phenotypes = 4 black coat coat
BB Bb
BB Bb
Example 2b: Homozygous Example 2b: Homozygous (recessive) x Heterozygous(recessive) x Heterozygous
B – B – black coat; b – coat; b – brownbrown coat coat Cross = bb x BbCross = bb x Bb
Genotypes = 2 bb, 2 BbGenotypes = 2 bb, 2 Bb Phenotypes = 2 Phenotypes = 2 brownbrown coat, 2 coat, 2 black
coatcoat
Bb bb
Bb bbb
b
B b
Example 3: Heterozygous x Example 3: Heterozygous x HeterozygousHeterozygous
B – B – black coat; b – coat; b – brownbrown coat coat Cross = Bb x BbCross = Bb x Bb
Genotypes = 1 BB, 2Bb, 1bbGenotypes = 1 BB, 2Bb, 1bb Phenotypes = 3 Phenotypes = 3 black, 1 , 1 brownbrown
BB Bb
Bb bbb
B
B b
Gentoypic vs. Phenotypic Gentoypic vs. Phenotypic RatioRatio
Genotypic ratio for the last crossGenotypic ratio for the last cross
–1BB : 2Bb : 1bb1BB : 2Bb : 1bb Phenotypic ratio for the last crossPhenotypic ratio for the last cross
–3 3 black :1 :1 brownbrown
Example #4: TestcrossExample #4: Testcross
A A testcross is used to find an is used to find an unknown genotypeunknown genotype– Can help determine genotype of any Can help determine genotype of any
individual whose phenotype is dominantindividual whose phenotype is dominant– cross individual w/ homozygous cross individual w/ homozygous
recessiverecessive– Examples:Examples:
CompleteComplete vs. vs. Incomplete Incomplete DominanceDominance
Complete dominance– When one allele is completely dominant When one allele is completely dominant
over the otherover the other Ex. Ex. BBbb
Incomplete dominanceIncomplete dominance– No dominant or recessive allele; a No dominant or recessive allele; a
heterozygous heterozygous individual has an individual has an intermediate phenotypeintermediate phenotype Ex. Ex. BBbb
Example #5: Example #5: Incomplete Incomplete DominanceDominance
Japanese 4 o’clock flowers Japanese 4 o’clock flowers (snapdragons)(snapdragons)– R = R = redred– r = r = whitewhite
Cross: RR x rrCross: RR x rr
Genotypes: 4 RrGenotypes: 4 Rr Phenotypes: 4 Phenotypes: 4 pinkpink flowers flowers
Rr Rr
Rr Rr
Cross: Cross: RrRr x x RrRr
What is the probable genotypic ratio?What is the probable genotypic ratio?
–1 RR : 2 Rr : 1 rr1 RR : 2 Rr : 1 rr What is the probable phenotypic ratio?What is the probable phenotypic ratio?
–1 1 RedRed : 2 : 2 PinkPink : 1 : 1 whitewhite
Cross 2 Cross 2 heterozygousheterozygous flowersflowers
RR Rr
Rr rr
CODOMINANCECODOMINANCE When both alleles are dominant and When both alleles are dominant and
influence the phenotype of the influence the phenotype of the individualindividual– Ex. Blood typeEx. Blood type
AB AB
AB AB
A
A
B B
Dihybrid CrossesDihybrid Crosses A A dihybriddihybrid cross involves crossing 2 cross involves crossing 2
contrasting traitscontrasting traits– There are 16 possible outcomes (2There are 16 possible outcomes (244))– Punnett square:Punnett square:
Example #1: Homozygous Dominant x Example #1: Homozygous Dominant x Homozygous RecessiveHomozygous Recessive
R = R = roundround seeds seeds r = r = wrinkledwrinkled seeds seeds Y = Y = yellowyellow seed color seed color y = y = greengreen seed color seed color
Cross = RRYY x rryyCross = RRYY x rryy How do we determine the alleles How do we determine the alleles
given off by each parent?given off by each parent?– The “The “FOILFOIL”” method!!! method!!!
FOIL (First-Outer-Inner-FOIL (First-Outer-Inner-Last)Last)
Parent 1Parent 1 RRYYRRYY
Alleles : RY,RY,RY,RYAlleles : RY,RY,RY,RY
FOIL (First-Outer-Inner-FOIL (First-Outer-Inner-Last)Last)
Parent 2Parent 2 rryyrryy
Alleles: ry,ry,ry,ryAlleles: ry,ry,ry,ry
Genotypes: 16 RrYyGenotypes: 16 RrYy Phenotypes: 16 Phenotypes: 16 roundround, , yellowyellow
RrYy RrYy RrYy RrYy
RrYy RrYy RrYy RrYy
RrYy RrYy RrYy RrYy
RrYy RrYy RrYy RrYy
ry ry ryry
RY
RY
RY
RY
HeterozygousHeterozygous x x HeterozygousHeterozygous
Cross: Cross: RrYy x RrYyRrYy x RrYy Alleles from each parent:Alleles from each parent:
– Parent 1 = Parent 1 = RY, Ry, rY, ryRY, Ry, rY, ry– Parent 2 = Parent 2 = RY, Ry, rY, ryRY, Ry, rY, ry
RrYyRrYy x x RrYyRrYy
RRYY RRYy RrYY RrYy
RRYy RRyy RrYy Rryy
RrYY RrYy rrYY rrYy
RrYy Rryy rrYy rryy
RY Ry rY ry
RY
Ry
rY
ry
Results of the crossResults of the cross
Genotypes Genotypes ––– 1 RRYY, 2RRYy, 1 RRyy, 2 RrYY, 4 1 RRYY, 2RRYy, 1 RRyy, 2 RrYY, 4
RrYy, 2 Rryy, 1 rrYY, 2 rrYy, 1 rryyRrYy, 2 Rryy, 1 rrYY, 2 rrYy, 1 rryy Phenoytpes Phenoytpes ––
– 9 9 roundround, , yellowyellow; 3 ; 3 roundround, , greengreen; ; 3 3 wrinkledwrinkled, , yellowyellow; 1 ; 1 wrinkledwrinkled, , greengreen
Multiple Alleles + Polygenic Multiple Alleles + Polygenic TraitsTraits
Multiple Alleles – Multiple Alleles – 3 or 3 or more allelesmore alleles– Blood typesBlood types
A, B, OA, B, O
Polygenic – Polygenic – 2 or more 2 or more genes genes – HeightHeight– WeightWeight– Eye colorEye color– IntelligenceIntelligence– Skin colorSkin color
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