Genetic CrossesGenetic Crosses

Section 9.2Section 9.2

GenotypeGenotype The genetic makeup of an organismThe genetic makeup of an organism

Consists of the alleles that the organism Consists of the alleles that the organism inherits from its parentsinherits from its parents

Example: white flowering pea plantsExample: white flowering pea plants (recessive trait (recessive trait pppp) )

PhenotypePhenotype The appearance of an organism as a The appearance of an organism as a

result of its genotyperesult of its genotype

Example: pea plants that are PP or PpExample: pea plants that are PP or Pp will have purple flowerswill have purple flowers

HomozygousHomozygous Both alleles of a pair are alikeBoth alleles of a pair are alike

May be homozygous dominant (PP) or May be homozygous dominant (PP) or homozygous recessive (pp)homozygous recessive (pp)

HeterozygousHeterozygous The two alleles in the pair are differentThe two alleles in the pair are different

Example: a pea plant that is Example: a pea plant that is heterozygous for flower color heterozygous for flower color

would be Pp (purple flowers)would be Pp (purple flowers)

ProbabilityProbability The likelihood that a specific event will The likelihood that a specific event will

occuroccur

May be expressed as a decimal, a May be expressed as a decimal, a percentage, or a fractionpercentage, or a fraction

Probability = Probability = number of times an event is number of times an event is expected to happen_____________expected to happen_____________ number of opportunities for an eventnumber of opportunities for an event to happento happen

Example: yellow pea seeds appeared 6022 times and Example: yellow pea seeds appeared 6022 times and green pea seeds appeared 2001 times in the Fgreen pea seeds appeared 2001 times in the F22

Total (6022 + 2001 = 8023) Total (6022 + 2001 = 8023)

What is the probability that the dominant trait will appear in What is the probability that the dominant trait will appear in a similar cross?a similar cross?

P = P = 60226022 = .75 (or 75% or ¾ ) = .75 (or 75% or ¾ ) 80238023

Monohybrid CrossMonohybrid Cross A cross between individuals that involves A cross between individuals that involves

one pair of contrasting traitsone pair of contrasting traits

Example: crossing a pea plant that is Example: crossing a pea plant that is pure for white flowers (pp) with one that pure for white flowers (pp) with one that is pure for purple flowers (PP)is pure for purple flowers (PP)

Punnett SquarePunnett Square Diagram used to predict the outcome of Diagram used to predict the outcome of

certain crossescertain crosses

Predicts the probability of inherited traitsPredicts the probability of inherited traits

Homozygous X HomozygousHomozygous X Homozygous

white flowerwhite flower (pp)(pp) p pp ppurple purple flower (PP) P Pp Ppflower (PP) P Pp Pp

P Pp PpP Pp Pp

Homozygous X HeterozygousHomozygous X Heterozygous

black coatblack coat (Bb)(Bb) B bB bblack coatblack coat (BB) B BB Bb(BB) B BB Bb

B BB BbB BB Bb

Heterozygous X HeterozygousHeterozygous X Heterozygous

black coatblack coat (Bb)(Bb) B bB bblack coatblack coat (Bb) B BB Bb(Bb) B BB Bb

b Bb bbb Bb bb

TestcrossTestcross An individual of an unknown genotype is An individual of an unknown genotype is

crossed with a homozygous recessive crossed with a homozygous recessive individualindividual

Can be used to determine the genotype Can be used to determine the genotype of any individual whose phenotype is of any individual whose phenotype is dominant (example: BB or Bb)dominant (example: BB or Bb)

Incomplete DominanceIncomplete Dominance Occurs when two or more alleles Occurs when two or more alleles

influence the phenotypeinfluence the phenotype

Example: four o’clock flowersExample: four o’clock flowers RR = redRR = red Rr = pinkRr = pink rr = whiterr = white

Incomplete Dominance in Incomplete Dominance in Four O’clock FlowersFour O’clock Flowers

Pink flowerPink flower (Rr)(Rr) R r R r Pink flowerPink flower (Rr) R RR Rr (Rr) R RR Rr

r Rr rrr Rr rr

Phenotypic ratio: 1 red: 2 pink:1 white Phenotypic ratio: 1 red: 2 pink:1 white

CodominanceCodominance Occurs when both alleles for a gene are Occurs when both alleles for a gene are

expressed in a heterozygous offspringexpressed in a heterozygous offspring

Neither allele is dominant or recessiveNeither allele is dominant or recessive

Example: roan horses R = red coatExample: roan horses R = red coat (RR`) R` = white(RR`) R` = white

Dihybrid CrossDihybrid Cross A cross between individuals that involves A cross between individuals that involves

two pairs of contrasting traitstwo pairs of contrasting traits

Must consider how the four alleles from Must consider how the four alleles from each parent can combineeach parent can combine

Homozygous X HomozygousHomozygous X Homozygous

wrinkled & greenwrinkled & green (rryy)(rryy) ry ry ry ryry ry ry ry

round & round & RY RrYyRY RrYy RrYy RrYy RrYy RrYy RrYy RrYy

yellow yellow RY RrYy RrYy RrYy RrYy RY RrYy RrYy RrYy RrYy

(RRYY) (RRYY) RY RrYy RrYy RrYy RrYyRY RrYy RrYy RrYy RrYy

Ry RrYy RrYy RrYy RrYyRy RrYy RrYy RrYy RrYy

Heterozygous X HeterozygousHeterozygous X Heterozygous

round & yellowround & yellow (RrYy)(RrYy) RY Ry rY ryRY Ry rY ry

round & round & RY RRYY RRYy RrYY RrYyRY RRYY RRYy RrYY RrYy

yellow yellow Ry RRYy RRyy RrYy Rryy Ry RRYy RRyy RrYy Rryy

(RrYy) (RrYy) rY RrYY RrYy rrYY rrYyrY RrYY RrYy rrYY rrYy ry RrYy Rryy rrYy rryyry RrYy Rryy rrYy rryy

Four PhenotypesFour Phenotypes 9/16 round & yellow9/16 round & yellow (genotypes RRYY, RRYy, RrYY, and RrYy)(genotypes RRYY, RRYy, RrYY, and RrYy)

3/16 round & green3/16 round & green (genotypes RRyy and Rryy)(genotypes RRyy and Rryy)

3/16 wrinkled & yellow3/16 wrinkled & yellow (genotypes rrYY and rrYy)(genotypes rrYY and rrYy)

1/16 wrinkled & green1/16 wrinkled & green (genotype rryy)(genotype rryy)

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