Transcript PPT
Test Cross Tall (dominant phenotype) plants of the F1 generation will have one of the following genotypes: DD or Dd Test Cross By crossing to a homozygous recessive individual, the genotype of an unknown individual will be revealed in the offspring. Test Cross If the tall plant of unknown genotype is heterozygous and is crossed to a homozygous recessive plant: D d Dd d Dd d dd dd Ratio=1:1 tall:dwarf Test Cross If the tall plant of unknown genotype is homozygous and is crossed to a homozygous recessive plant: D D d Dd Dd d Dd Dd Resulting ratio= All Tall Dihybrid Cross Dihybrid Cross: A cross involving two pairs of contrasting traits. For example, cross true breeding plants with yellow, round seeds to true breeding plants with green, wrinkled seeds will yield an F1 generation of all yellow, round seeds. Dihybrid Cross: First Cross P1 P1 F1 Dihybrid Cross: First Cross What does the phenotype of the F1 generation tell you? Yellow is dominant to green Round is dominant to wrinkled What happens when the F2 is created? Dihybrid Cross: Second Cross F1 F1 9/16 yellow, round 3/16 green, round 3/16 yellow, wrinkled 1/16 green, wrinkled Dihybrid Cross This dihybrid cross is best considered as two independent monohybrid crosses. Each trait is inherited independently; the probability of inheriting a particular color is not influenced by the probability of inheriting a particular color, and vice versa. Punnett Square Genotype for color: Yellow = G Green = g Genotype for shape: Round = W Wrinkled = w Punnett Square of Dihybrid Cross: F2 GW GW Gw gW gw Gw gW gw Punnett Square of Dihybrid Cross: F2 GW Gw gW gw GW GGWW GGWw GgWW GgWw Gw gW gw Punnett Square of Dihybrid Cross: F2 GW Gw gW gw GW GGWW GGWw GgWW GgWw Gw GGWw GGww GgWw Ggww gW GgWW GgWw ggWW ggWw gw Punnett Square of Dihybrid Cross: F2 GW Gw gW gw GW GGWW GGWw GgWW GgWw Gw GGWw GGww GgWw Ggww gW GgWW GgWw ggWW ggWw gw GgWw Ggww ggWw ggww 9 yellow, round 3 yellow, wrinkled 3 green, round 1 green, wrinkled GW Gw gW gw GW GGWW GGWw GgWW GgWw Gw GGWw GGww GgWw Ggww gW GgWW GgWw ggWW ggWw gw GgWw Ggww ggWw ggww Independent Assortment Two monohybrid crosses: Yellow x green yields all yellow F1 Round x wrinkled yields all round F1 F2 of yellow F1 yields ¾ yellow, ¼ green F2 of round F1 yields ¾ round, ¼ wrinkled Dihybrid Cross: The Product Law When two independent events occur simultaneously, the combined probability of the outcomes is equal to the product of the individual probabilities of occurrence. What is the probability that an F2 plant of a dihybrid cross will be yellow and round? p= (3/4)(3/4)=9/16 Dihybrid Cross: The Product Law What is the probability that a given F2 plant would be yellow, wrinkled? p= (3/4)(1/4)=3/16 Remember, this probability calculation rests on the assumption that the traits are independent, therefore the probability of all gamete combinations is equal. Trihybrid Cross What if you added back stem height? Treat it as three monohybrid crosses and use the probability of each outcome. DDGGWW x ddggww You do the math for these outcomes: Tall, green, round Short, yellow, wrinkled