Practice Genetics Questions on Mendelian Genetics

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Below are 5 analytical questions on Mendelian genetics with step-by-step solutions, followed by 5 unsolved questions for further exploration.

Solved Questions:

1. Question: What is the phenotypic ratio of the offspring resulting from a cross between two heterozygous tall plants with genotype Tt?

  • Step 1: List possible gametes for each parent: T, t
  • Step 2: Use the Punnett Square to find all possible combinations of these gametes.
  • Step 3: Count the number of each type of phenotype.

Answer: Phenotypic ratio is 3 tall (TT, Tt, Tt) to 1 short (tt).

2. Question: If you cross a homozygous dominant yellow pea plant (YY) with a homozygous recessive green pea plant (yy), what will the F1 generation look like?

  • Step 1: List possible gametes for each parent: Y for the dominant, y for the recessive.
  • Step 2: Use the Punnett Square to combine these gametes.
  • Step 3: Identify the genotype and phenotype of the offspring.

Answer: All the F1 offspring will have the genotype Yy and will be yellow in color.

3. Question: What is the expected ratio of genotypes in the offspring of a dihybrid cross between two heterozygous plants with genotypes RrYy?

  • Step 1: List the possible gametes for each parent: RY, Ry, rY, ry.
  • Step 2: Use a 4×4 Punnett Square for the dihybrid cross.
  • Step 3: Count the genotypes.

Answer: The genotype ratio is 1:2:1:2:4:2:1:2:1 (RRYY:RrYY:rrYY:RRYy:RrYy:rrYy:RRyy:Rryy:rryy).

4. Question: If a heterozygous round seed (Rr) is crossed with a wrinkled seed (rr), what is the phenotypic ratio of the offspring?

  • Step 1: List possible gametes for each parent: R, r for the first parent and r for the second parent.
  • Step 2: Use a Punnett Square to find all possible combinations of these gametes.
  • Step 3: Identify the phenotypes of the offspring.

Answer: The phenotypic ratio will be 1 round seed (Rr) to 1 wrinkled seed (rr).

5. Question: What will be the genotype ratio of the offspring when two dihybrid plants AaBb are crossed?

  • Step 1: List the possible gametes: AB, Ab, aB, ab.
  • Step 2: Use a 4×4 Punnett Square for the dihybrid cross.
  • Step 3: Count the genotypes.

Answer: The genotype ratio will be 1 AABB : 2 AABb : 1 AAbb : 2 AaBB : 4 AaBb : 2 Aabb : 1 aaBB : 2 aaBb : 1 aabb.

Unsolved Questions:

  1. What would be the genotype ratio if a heterozygous plant with genotype AaBb is crossed with another heterozygous plant with genotype AaBb?
  2. In a trihybrid cross between AaBbCc and AaBbCc, what is the phenotypic ratio of the offspring?
  3. If a homozygous dominant tall plant (TT) is crossed with a heterozygous tall plant (Tt), what would be the phenotypic ratio of the offspring?
  4. What is the probability of having a homozygous recessive offspring from a cross between two heterozygous parents with genotype Aa?
  5. In a dihybrid cross between AABB and aabb, what will the F1 and F2 generations look like?

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