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Biology 30 · Worksheets

Genetics · Set A

Ten questions of mixed difficulty, covering Genetics. Print it, or work through it on screen — the answer key starts on its own page.

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Genetics · Set A

Biology 30 · maddyhelps.com

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Circle the best answer for each question. Show your work in the space provided.

  1. Tall (T) is dominant over short (t). In the cross Tt × tt, what fraction of the offspring are expected to be tall?

    1. a) all of them
    2. b) 1/4
    3. c) 1/2
    4. d) 3/4

  2. What phenotype ratio is expected from the dihybrid cross TtYy × TtYy?

    1. a) 9 : 3 : 3 : 1
    2. b) 3 : 1
    3. c) 1 : 2 : 1
    4. d) 1 : 1 : 1 : 1

  3. Tall (T) is dominant over short (t). What phenotype ratio is expected from the cross Tt × Tt?

    1. a) all tall
    2. b) 1 tall : 2 medium : 1 short
    3. c) 1 tall : 1 short
    4. d) 3 tall : 1 short

  4. The daughter cells produced by mitosis are:

    1. a) genetically identical to the parent cell
    2. b) missing one chromosome from each pair
    3. c) always gametes
    4. d) genetically unique, with half the chromosomes

  5. Colour blindness is X-linked recessive. A carrier mother (XᶜXᶜ′) and a father with normal vision (XᶜY) have children. What fraction of their SONS are expected to be colour blind?

    1. a) 1/2
    2. b) 1/4
    3. c) 0
    4. d) all of them

  6. Non-disjunction during meiosis I produces gametes that:

    1. a) have an abnormal number of chromosomes
    2. b) are always identical to the parent cell
    3. c) contain no genetic material at all
    4. d) have the correct number of chromosomes but the wrong alleles

  7. What does DNA stand for?

    1. a) dinucleic acid
    2. b) deoxyribose nucleotide analogue
    3. c) deoxyribonucleic acid
    4. d) diribonucleic acid

  8. A father with blood type A (IAi) and a mother with blood type B (IBi) have a child. Which blood types are possible?

    1. a) O only
    2. b) A and B only
    3. c) A, B, AB and O
    4. d) AB only

  9. In a DNA molecule, which base pairs with adenine?

    1. a) cytosine
    2. b) guanine
    3. c) uracil
    4. d) thymine

  10. What phenotype ratio is expected from the testcross TtYy × ttyy?

    1. a) 1 : 2 : 1
    2. b) 1 : 1 : 1 : 1
    3. c) 9 : 3 : 3 : 1
    4. d) 3 : 1

Answer key · Genetics · Set A

Biology 30 · maddyhelps.com

  1. c) 1/2 — The Punnett square gives Tt, Tt, tt, tt — half carry a T and are tall. This 1:1 result is the classic testcross pattern.
  2. a) 9 : 3 : 3 : 1 — Two independent traits, each 3:1, multiply out to 9:3:3:1 across the 16 boxes. This is the signature result of a heterozygous dihybrid cross.
  3. d) 3 tall : 1 short — The square gives TT, Tt, Tt, tt. Three of the four have at least one T, so the phenotype ratio is 3:1 even though the genotype ratio is 1:2:1.
  4. a) genetically identical to the parent cell — Mitosis copies a cell exactly, which is what growth and repair need. Halving the chromosome number and creating variety is meiosis' job.
  5. a) 1/2 — Sons get their only X from their mother, and half her X chromosomes carry the allele. With no second X to mask it, half the sons are affected.
  6. a) have an abnormal number of chromosomes — Non-disjunction means a homologous pair fails to separate, so one gamete gains a chromosome and another loses one. Trisomy 21 arises this way.
  7. c) deoxyribonucleic acid — Deoxyribonucleic acid — named for its sugar, deoxyribose, which is ribose minus one oxygen. RNA keeps that oxygen.
  8. c) A, B, AB and O — Each parent can pass IA, IB or i, giving IAIB (AB), IAi (A), IBi (B) and ii (O) — all four types, each with probability 1/4.
  9. d) thymine — In DNA, A pairs with T and C pairs with G. Uracil replaces thymine only in RNA.
  10. b) 1 : 1 : 1 : 1 — The fully recessive parent contributes only ty, so the four offspring types mirror the four gamete types of TtYy in equal numbers. That is why a testcross reveals a genotype directly.