Trihybrid Cross Genotypic Ratio

**Science 10 Honours** **Genetics Problems** 1. Why must a recessive trait always be pure and never a hybrid when it appears in an organism? Explain. > If a dominant trait is present, then it is always expressed. 2. Brown eyes are dominant over blue eyes. A brown-eyed man (Andrew) whose father (Mike) was brown-eyed and whose mother (Jill) was blue-eyed married a blue-eyed woman (Sarah) whose father (Rob) and mother (Kate) were brown-eyed. The couple has a blue-eyed son (Brian). For which of the individuals mentioned, can you be sure of the genotypes? What are their genotypes? What genotypes are possible for the others? > Jill (bb), Andrew (Bb or BB), Sarah (bb), Rob (Bb or BB), Kate (Bb or BB), Brian (bb) 3. Cross a purebred tall (TT) plant with a purebred dwarf (tt) plant. Show both the F₁ and F₂ generations. Show the genotypic ratio and phenotypic ratio for both crosses. 4. If all the offspring in a cross are Gg, what are the genotypes of the parents? > GG and gg 5. If 50% of the offspring in a cross are round and 50% are wrinkled, what are the genotypes of the parents? > Heterozygous and Homozygous Recessive 6. Red carnations were crossed with white carnations. 25 seeds were produced which all developed red flowers. Was dominance shown? What could you do to prove this? > Yes, red cross was dominance - crossing of parents fits for standard pure-bred crossing of parents (9:3:3:1) ratio 7. A pure-breeding red flecker was mated with a pure-breeding white flecker. They produced 200 white flecker offspring. What are the genotypes of the F₁ and how many of each genotype were produced? What are the phenotypes of the F₁ and how many of each phenotype were produced? > WW (100%) > White (100%) 8. You decide to start your own flecker farm. You take two of the F₁ generation from the previous question and you successfully get them to mate. They produce 600 offspring. What genotypes are produced and how many of each type should you get? What are the phenotypes and how many of each should you get? > WW = 150 > Ww = 300 > ww = 150 > 450 white / 150 red 9. Mendel did not limit his experiments on peas to ones in which single characteristics were involved. For example, he crossed plants characterized by wrinkled green seeds. This is called a dihybrid cross. Parental generation: RRGG (round yellow) x rrgg (wrinkled green) F₁ generation: RrGg (round yellow) You want to determine what the F₂ generation would look like. To do this, you must determine the possible gametes that the F₁ produce. There are four possible types of gametes that could be produced. Show a Punnett square for this cross (over) [Image of a Punnett square with the genotypes of the F₁ generation: RG, Rg, rG, rg] **9 round yellow** **3 round green** **3 wrinkled yellow** **1 wrinkled green** 10. Complete the following dihybrid cross between a tall blue rose and a dwarf red rose. Tall is dominant to dwarf. Red is dominant to blue Parents: TTbb x ttRR Gametes: all Tb all tR F1: TtRr (All) Crossing hybrids: TtRr x TtRr Gametes: TR, Tr, tR, tr x TR, Tr, tR, tr [Punnett Square] Phenotypic ratio: 9 tall red : 3 tall blue : 3 dwarf red : 1 dwarf blue Bonus: 1. In a trihybrid cross (such as RrTtBb x RrTtBb) how many possible offspring genotypes are possible? Answer: 27 Bonus: 2. What is a flecker? Answer: Bird