DNA, Inheritance & Gene Expression — Chapter quiz · Life Science: Biology, Grade 10

Use DNA and inheritance models while testing their limits with expression and variation evidence.

Problem 1

In DNA, adenine (A) always pairs with
  1. thymine (T)
  2. cytosine (C)
  3. guanine (G)
  4. another adenine (A)
Answer and explanation

thymine (T)

The complementary base pairs are A-T and C-G, so adenine always bonds with thymine.

Problem 2

One strand of DNA has 25 guanine (G) bases. On the complementary strand that pairs with it, how many cytosine (C) bases are there?
Answer and explanation

25 cytosine bases

Guanine pairs only with cytosine, so 25 G bases require 25 C partners.

Problem 3

Translation — building a protein from mRNA — takes place at the
  1. mitochondrion
  2. cell membrane
  3. ribosome
  4. nucleus
Answer and explanation

ribosome

Ribosomes read mRNA codons and link amino acids together into a protein.

Problem 4

Explain how transcription and translation together turn a gene into a protein.
Answer and explanation

In transcription, a gene's DNA is copied into mRNA inside the nucleus. The mRNA travels to a ribosome, where translation reads its three-base codons and links the matching amino acids into a protein.

Problem 5

In a cross of two Bb pea plants (B dominant), what percent of the offspring are expected to show the recessive phenotype?
Answer and explanation

25 %

Bb x Bb gives BB, Bb, Bb, bb — 1 of 4 boxes (25%) is bb, the recessive phenotype.

Problem 6

Using an example, explain the difference between an organism's genotype and its phenotype.
Answer and explanation

Genotype is the exact pair of alleles an organism carries, such as Bb; phenotype is the physical trait that results, such as brown fur. A BB and a Bb organism have different genotypes but the same brown phenotype, because B is dominant.

Problem 7

Which observation most directly supported the double-helix model's base pairing?
  1. DNA contains phosphorus
  2. The amount of A equals T and G equals C in a given species
  3. DNA is found in the nucleus
  4. DNA can be stained
Answer and explanation

The amount of A equals T and G equals C in a given species

Chargaff's finding that A equals T and G equals C pointed to specific pairing between those bases, which the double-helix model explained.

Problem 8

A student says DNA replication produces "two copies, one old and one new." Correct the statement precisely.
Answer and explanation

Replication produces two double helices, each containing one original strand and one newly made strand. There is no wholly old molecule and no wholly new one; the original pair is separated and distributed between the two products. That is what semiconservative means, and it was established by labelling experiments that followed the original strands into the products.

Response criteria

  • State a biologically accurate conclusion.
  • Show the data, mechanism, or calculation that supports it.
  • Include units, conditions, or limits where relevant.

Problem 9

Independent chapter check: Write the complementary strand opposite 5′-AACGTT-3′, then explain strand origin after one semiconservative round.
Answer and explanation

The directly paired complement is 3′-TTGCAA-5′. Each resulting duplex has one old template strand and one newly synthesized partner in the ideal model.

The directly paired complement is 3′-TTGCAA-5′. Each resulting duplex has one old template strand and one newly synthesized partner in the ideal model.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the biological mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Problem 10

The order in which genetic information flows during protein synthesis is
  1. protein → RNA → DNA
  2. DNA → RNA → protein
  3. RNA → DNA → protein
  4. DNA → protein → RNA
Answer and explanation

DNA → RNA → protein

A gene's DNA is transcribed to RNA, which is translated into protein. The sequence of steps is what makes a mutation in DNA able to change a protein.

Problem 11

Explain how a change of one DNA base can alter an organism's observable characteristics, naming every step in the chain.
Answer and explanation

A changed DNA base changes the corresponding messenger RNA codon during transcription. If the new codon specifies a different amino acid, the polypeptide's sequence changes at that position. A different amino acid interacts differently with its neighbours, so the chain may fold into a different shape, and if that region forms a binding or active site the protein's function changes. Because proteins carry out and regulate the cell's work, an altered or missing protein can change a measurable characteristic of the organism. Each step is conditional, which is why many base changes produce no visible effect.

Response criteria

  • State a biologically accurate conclusion.
  • Show the data, mechanism, or calculation that supports it.
  • Include units, conditions, or limits where relevant.

Problem 12

Independent chapter check: Using GCA=Ala, GCU=Ala and GUA=Val, compare a coding-region change that gives GCU versus one giving GUA in mRNA. Explain why neither sequence result alone determines a whole-organism trait.
Answer and explanation

GCU retains alanine; GUA specifies valine. Regulatory context, protein folding/activity and other biological conditions determine downstream effects, so a measured trait needs additional evidence.

GCU retains alanine; GUA specifies valine. Regulatory context, protein folding/activity and other biological conditions determine downstream effects, so a measured trait needs additional evidence.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the biological mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Problem 13

The expected genotype ratio from a Tt × Tt cross is
  1. 1 : 1
  2. 3 : 1
  3. 1 : 2 : 1
  4. 9 : 3 : 3 : 1
Answer and explanation

1 : 2 : 1

The genotype ratio is 1 TT : 2 Tt : 1 tt, which produces a 3:1 phenotype ratio when T is fully dominant.

Problem 14

A cross expected to give a 3:1 ratio produces 140 dominant and 60 recessive offspring out of 200. Evaluate whether the result is consistent with the expectation.
Answer and explanation

The expectation for 200 offspring is 150 dominant and 50 recessive. The observed 140 and 60 differ by 10 in each category, which is a 6.7 percent shortfall in the dominant class. A departure of this size in a sample of 200 is the kind of variation chance produces, so the data are broadly consistent with a 3:1 ratio rather than evidence against it. Deciding properly requires a statistical test of how often chance alone would produce a deviation at least this large; the honest statement is that the result does not contradict the expectation.

Response criteria

  • State a biologically accurate conclusion.
  • Show the data, mechanism, or calculation that supports it.
  • Include units, conditions, or limits where relevant.

Problem 15

Independent chapter check: Under complete dominance, Aa×aa produces independent offspring with equal genotype probabilities. Find the expected aa count among 60 and the probability that the first three are all Aa.
Answer and explanation

Expected aa count is 60×1/2=30. The probability of three Aa outcomes is (1/2)³=1/8. Neither expectation forces a specific finite family outcome.

Expected aa count is 60×1/2=30. The probability of three Aa outcomes is (1/2)³=1/8. Neither expectation forces a specific finite family outcome.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the biological mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Problem 16

In RW × WW, what percent are expected to be WW?
Answer and explanation

50 %

Half receive W from both parents.

Problem 17

Why might ten offspring fail to match an expected 1:2:1 ratio exactly?
Answer and explanation

The ratio is a probability expectation; small samples fluctuate, and the biological model or sampling may also need checking.

Problem 18

Independent chapter check: A heterozygote has an intermediate phenotype under an incomplete-dominance model. Predict genotype and phenotype probabilities from Rr×Rr and explain what happens to the alleles.
Answer and explanation

Genotypes are 1/4 RR, 1/2 Rr, 1/4 rr, giving three phenotype categories in the same ratio under this model. The alleles remain distinct and segregate; the intermediate appearance does not erase them.

Genotypes are 1/4 RR, 1/2 Rr, 1/4 rr, giving three phenotype categories in the same ratio under this model. The alleles remain distinct and segregate; the intermediate appearance does not erase them.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the biological mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Problem 19

Which conclusion is justified by a changed DNA sequence alone?
  1. A sequence change occurred
  2. The organism is definitely fitter
  3. Every protein is damaged
  4. Every offspring must inherit it
Answer and explanation

A sequence change occurred

Functional and inheritance consequences need further evidence.

Problem 20

Name evidence needed to support a claim that an edit increases drought tolerance.
Answer and explanation

Confirm the edit, compare suitable controls under matched drought and normal conditions, use replicates, measure performance, and examine tradeoffs.

Problem 21

Independent chapter check: A regulatory-region variant is associated with lower RNA but unchanged encoded amino-acid sequence. Explain a possible mechanism and two further tests needed for a causal trait claim.
Answer and explanation

It may reduce transcription or another regulatory process without changing the polypeptide sequence. Use matched genetic backgrounds/controls, protein amount/activity measurements and an appropriate trait test to examine causality and alternatives.

It may reduce transcription or another regulatory process without changing the polypeptide sequence. Use matched genetic backgrounds/controls, protein amount/activity measurements and an appropriate trait test to examine causality and alternatives.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the biological mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

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