Genetic Crosses Section 9 2 Genotype The genetic

Genetic Crosses Section 9. 2

Genotype § The genetic makeup of an organism § Consists of the alleles that the organism inherits from its parents § Example: white flowering pea plants (recessive trait pp)

Phenotype § The appearance of an organism as a result of its genotype § Example: pea plants that are PP or Pp will have purple flowers

Homozygous § Both alleles of a pair are alike § May be homozygous dominant (PP) or homozygous recessive (pp)

Heterozygous § The two alleles in the pair are different § Example: a pea plant that is heterozygous for flower color would be Pp (purple flowers)

Probability § The likelihood that a specific event will occur § May be expressed as a decimal, a percentage, or a fraction

§ Probability = number of times an event is expected to happen_______ number of opportunities for an event to happen

Example: yellow pea seeds appeared 6022 times and green pea seeds appeared 2001 times in the F 2 Total (6022 + 2001 = 8023) What is the probability that the dominant trait will appear in a similar cross? P = 6022 =. 75 8023 (or 75% or ¾ )

Monohybrid Cross § A cross between individuals that involves one pair of contrasting traits § Example: crossing a pea plant that is pure for white flowers (pp) with one that is pure for purple flowers (PP)

Punnett Square § Diagram used to predict the outcome of certain crosses § Predicts the probability of inherited traits

Homozygous X Homozygous white flower (pp) p p purple flower (PP) P Pp Pp

Homozygous X Heterozygous black coat (Bb) B b black coat (BB) B BB Bb

Heterozygous X Heterozygous black coat (Bb) B b black coat (Bb) B BB Bb bb

Testcross § An individual of an unknown genotype is crossed with a homozygous recessive individual § Can be used to determine the genotype of any individual whose phenotype is dominant (example: BB or Bb)

Incomplete Dominance § Occurs when two or more alleles influence the phenotype § Example: four o’clock flowers RR = red Rr = pink rr = white

Incomplete Dominance in Four O’clock Flowers Pink flower (Rr) R r Pink flower (Rr) R RR Rr rr Phenotypic ratio: 1 red: 2 pink: 1 white

Codominance § Occurs when both alleles for a gene are expressed in a heterozygous offspring § Neither allele is dominant or recessive § Example: roan horses (RR`) R = red coat R` = white

Dihybrid Cross § A cross between individuals that involves two pairs of contrasting traits § Must consider how the four alleles from each parent can combine

Homozygous X Homozygous wrinkled & green (rryy) round & yellow (RRYY) ry ry RY Rr. Yy Rr. Yy RY Rr. Yy Ry Rr. Yy

Heterozygous X Heterozygous round & yellow (Rr. Yy) RY Ry r. Y ry RY RRYy Rr. YY Rr. Yy Ry RRYy RRyy Rr. Yy Rryy r. Y ry Rr. YY Rr. Yy Rryy rr. YY rr. Yy rryy

Four Phenotypes 9/16 round & yellow (genotypes RRYY, RRYy, Rr. YY, and Rr. Yy) 3/16 round & green (genotypes RRyy and Rryy) 3/16 wrinkled & yellow (genotypes rr. YY and rr. Yy) 1/16 wrinkled & green (genotype rryy)

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