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