DNA, Inheritance & Gene Expression — Chapter Practice, Grade 10

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

Problem 1

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

thymine (T)

A-T and C-G are the complementary base pairs; adenine always bonds with thymine.

Problem 2

Which best describes the overall shape of a DNA molecule?
  1. A double helix — a twisted ladder
  2. A single straight line
  3. A flat triangle
  4. A solid sphere
Answer and explanation

A double helix — a twisted ladder

Two strands wind around each other to form the double-helix, or twisted-ladder, shape.

Problem 3

One strand of DNA contains 40 adenine (A) bases. On the strand that pairs with it, how many thymine (T) bases must there be to pair with them?
Answer and explanation

40 thymine bases

Adenine pairs only with thymine, so 40 A bases require 40 T partners.

Problem 4

Explain why DNA replication is described as 'semiconservative.'
Answer and explanation

Each new DNA molecule is made of one original (conserved) strand and one newly built strand, so half of every copy is 'old.' That is what 'semiconservative' means.

Problem 5

Why does complementary base pairing make DNA easy to copy accurately?
  1. Each strand acts as a template that determines exactly which bases must be added to build its partner
  2. The bases pair together completely at random
  3. Both strands are identical, so one can be thrown away
  4. DNA does not need to be copied before a cell divides
Answer and explanation

Each strand acts as a template that determines exactly which bases must be added to build its partner

Because A only pairs with T and C only with G, one strand's sequence dictates its partner's, so each old strand is a perfect template for building a new one.

Problem 6

An original DNA strand has bases A-C-G-T. Give its complementary sequence and explain the pairing rule. Explain the reasoning in your own words.
Answer and explanation

T-G-C-A in corresponding paired positions; actual strands have opposite orientation.

A pairs with T and C with G.

Apply the rule to each position in the supplied order.

Response criteria

  • Use the stated conditions.
  • Show the evidence or calculation.
  • Explain the conclusion and any stated limitation.

Problem 7

Write the complement of TTGC using base-pair rules. Show the reasoning, not just the conclusion.
Hints

Pairing rules provide a template mechanism; sequence order still matters.

Answer and explanation

AACG, pairing T with A and G with C.

Problem 8

Why can an unrepaired replication error become a mutation?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

A sequence change can persist and be copied into later DNA molecules.

Problem 9

Each copied DNA duplex retains one original strandEach copied DNA duplex retains one original strandATTAGCCGATTAGCCGATTAGCCGSemiconservative copying retains one old strand per productBlue: original strandsGreen: newly synthesized strands
Original strand-origin and pairing comparison. Colors identify strand history, not different base chemicals or all replication-fork proteins.
Interpret the supplied visual. After one ideal round, does either product contain two entirely old strands?
Answer and explanation

No. Each has one old and one new strand under the semiconservative model.

Semiconservative replication produces two duplexes, each with an original template strand and a newly synthesized partner in the basic model. Strand separation exposes templates, and pairing constrains new sequence. DNA polymerase builds in the 5′→3′ direction, with additional enzymes and mechanisms supporting copying and ligation. Replication is not splitting each base in half or preserving an entirely old double helix beside an entirely new one. One old strand per product is the central inheritance pattern; actual forks involve more detailed leading/lagging behavior.

No. Each has one old and one new strand under the semiconservative model.

Response criteria

  • Read the structures, axes, directions and units correctly.
  • Connect the evidence with the biological mechanism.
  • Respect stated model assumptions and distinguish an observation from a further inference.

Problem 10

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 11

During transcription, a gene's DNA is copied into
  1. a lipid
  2. messenger RNA (mRNA)
  3. a finished protein
  4. a new chromosome
Answer and explanation

messenger RNA (mRNA)

Transcription produces an mRNA copy of the gene; the protein is built later, during translation.

Problem 12

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

ribosome

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

Problem 13

Each amino acid is coded by a codon of 3 mRNA bases. How many mRNA bases are needed to code for a protein that is 15 amino acids long?
Answer and explanation

45 bases

3 bases per codon x 15 amino acids = 45 bases.

Problem 14

Explain the difference between transcription and translation.
Answer and explanation

Transcription is copying a gene's DNA into mRNA inside the nucleus; translation is reading that mRNA's codons at a ribosome to build a protein from amino acids. Transcription makes the message, and translation reads it.

Problem 15

A single-base mutation changes a codon, but because the genetic code is redundant, the new codon still specifies the same amino acid. This kind of mutation is described as
  1. nonsense — it creates an early stop signal
  2. a frameshift that shifts every later codon
  3. always fatal to the cell
  4. silent — the protein is unchanged
Answer and explanation

silent — the protein is unchanged

When a changed codon still codes for the same amino acid, the protein is unchanged, so the mutation is 'silent' — possible because several codons can specify the same amino acid.

Problem 16

An mRNA contains AUG followed by GCU. Using a supplied table showing AUG=Met and GCU=Ala, identify the first two amino acids. Explain the reasoning in your own words.
Answer and explanation

Methionine followed by alanine in this simplified translation example.

Read the mRNA codons as supplied, not their complements.

Match AUG to methionine and GCU to alanine.

Response criteria

  • Use the stated conditions.
  • Show the evidence or calculation.
  • Explain the conclusion and any stated limitation.

Problem 17

An RNA codon changes but still specifies the same amino acid. What kind of protein-sequence effect is possible? Show the reasoning, not just the conclusion.
Hints

Specify template versus coding strand before converting a sequence.

Answer and explanation

It can be synonymous, leaving that amino acid unchanged; other regulatory effects are still possible.

Problem 18

Why must a codon table be read using RNA rather than a DNA template triplet directly?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

Translation reads mRNA; the template must first be converted to complementary RNA with correct orientation.

Problem 19

Different sequence changes can have different downstream consequencesDifferent sequence changes can have different downstream consequencesOriginal: AUG GCA UAA → Met–Ala–stopSubstitution: AUG GCU UAA → Met–Ala–stopAnother change: AUG GUA UAA → Met–Val–stopAn insertion can shift downstream reading-frame groups.A sequence change and a functional effect are different claims.
Original supplied-code sequence comparisons, not pathogenic-variant identification or the full range of molecular effects.
Interpret the supplied visual. Why does GCA→GCU not change this amino acid, while GCA→GUA does?
Answer and explanation

The supplied code maps GCA/GCU to alanine and GUA to valine; different codons can share or differ in meaning.

A substitution changes a base; an insertion or deletion changes sequence length. A reading frame groups consecutive bases into codons, so a non-three-base insertion within a translated region can alter downstream grouping. Code redundancy allows a substitution to retain the same amino acid, as GCA and GCU both specify alanine in the supplied entries. Other changes alter an amino acid or termination. Effects depend on location, regulation and protein context; not every mutation is visible or every substitution harmless.

The supplied code maps GCA/GCU to alanine and GUA to valine; different codons can share or differ in meaning.

Response criteria

  • Read the structures, axes, directions and units correctly.
  • Connect the evidence with the biological mechanism.
  • Respect stated model assumptions and distinguish an observation from a further inference.

Problem 20

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 21

An organism that is homozygous for a trait has
  1. no alleles for that trait
  2. two identical alleles (for example BB or bb)
  3. two different alleles (Bb)
  4. only one allele in total
Answer and explanation

two identical alleles (for example BB or bb)

Homozygous means the two alleles match (BB or bb); heterozygous (Bb) means they differ.

Problem 22

If B (brown) is dominant over b (white), an organism with genotype Bb will have which phenotype?
  1. A blend of brown and white
  2. It cannot be determined
  3. Brown
  4. White
Answer and explanation

Brown

One dominant B is enough to produce the brown phenotype, so Bb is brown.

Problem 23

In a cross between two Bb parents (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 the 4 boxes (25%) is bb, the recessive phenotype.

Problem 24

In a hypothetical complete-dominance model, an organism with dominant phenotype could be BB or Bb. Explain a model test cross and the limit of a small sample.
Answer and explanation

Cross with bb under the model. Any bb offspring implies the tested parent supplied b. All dominant offspring in a small sample do not prove BB because Bb can produce such a sample by chance.

Problem 25

Two Tt parents (T dominant). The probability that two of their offspring in a row are both tt equals 1 out of what number?
Answer and explanation

16 (1 out of this many)

Each tt has probability 1/4; multiplying independent events, 1/4 x 1/4 = 1/16, so 1 out of 16.

Problem 26

In a hypothetical complete-dominance model, cross Aa with Aa. Find genotype probabilities. Explain the reasoning in your own words.
Answer and explanation

¼ AA, ½ Aa and ¼ aa; each offspring is a new probability event.

Each parent contributes A or a with probability one half.

The four equally likely combinations are AA, Aa, aA and aa.

Response criteria

  • Use the stated conditions.
  • Show the evidence or calculation.
  • Explain the conclusion and any stated limitation.

Problem 27

For Aa×Aa, find expected recessive aa probability. Show the reasoning, not just the conclusion.
Hints

Probabilities describe many outcomes, not a guaranteed pattern in every small family.

Answer and explanation

1/4=25%, combining a from each parent.

Problem 28

Can a dominant phenotype alone distinguish AA from Aa in this model?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

No. Both produce the dominant phenotype; more genotype or family evidence is required.

Problem 29

An expected ratio does not dictate each finite familyAn expected ratio does not dictate each finite familyRecessive-category count / 80 offspringTrial family A15Trial family B25Trial family C20Trial family D17Expected mean under p=1/4: 20; actual samples can vary.
Original simulated family counts, not real breeding data or personal reproductive predictions.
Interpret the supplied visual. After three dominant-category outcomes, is a recessive-category fourth outcome guaranteed?
Answer and explanation

No. Each modeled independent outcome retains its probability; earlier outcomes do not force a balancing result.

A probability describes a model of possible outcomes, not a promised number in every small sample. For a recessive-category probability of 1/4 and 80 independent modeled offspring, the expected count is 20. Actual samples vary through chance, and unexpected counts require evaluation of sample size, model conditions and measurement rather than immediate rejection or certainty. Independent births in the model do not acquire a memory that forces the next child to balance earlier outcomes.

No. Each modeled independent outcome retains its probability; earlier outcomes do not force a balancing result.

Response criteria

  • Read the structures, axes, directions and units correctly.
  • Connect the evidence with the biological mechanism.
  • Respect stated model assumptions and distinguish an observation from a further inference.

Problem 30

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 31

OpenSciEd chromosome symbols for Rob, the extra-muscular offspring in the cattle case; two copies have matching orange and blue symbols but different green allele-category shapes.
OpenSciEd’s selected chromosome-symbol model for Rob; shapes represent allele categories, not actual chromosome outlines or base sequences. Chromosome-symbol pair for Rob · OpenSciEd · 8.5 Genetics, Lesson 6 chromosome model, source, CC BY 4.0. Figure excerpt rendered from the PDF and resized for the web.
Compare the two homolog-symbol copies at their orange, green and blue marked positions. Which categories match and differ? Explain what a drawn category difference does not establish about nucleotide sequence, dominance or the cause of a trait.
Answer and explanation

The orange and blue categories match while the green categories differ. They represent selected allele versions in a supplied model. A shape difference is not a nucleotide sequence, a measured chromosome deformity or proof of a dominance/trait relationship; those require the stated mapping and relevant functional/context evidence.

Compare corresponding marked positions on both homolog symbols.

Keep allele-category symbols separate from physical chromosome shape.

Request sequence, phenotype-model and functional/environment evidence before broader conclusions.

Response criteria

  • Correctly compare matching orange/blue and differing green categories.
  • Identify the drawing as an allele-category model.
  • State the limits for sequence, dominance or causal trait claims.

Problem 32

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

50 %

Two of four equally likely combinations are RW.

Problem 33

“Dominant” means…
  1. expressed in the heterozygote under the stated model
  2. always more common
  3. always healthier
  4. more genes than recessive
Answer and explanation

expressed in the heterozygote under the stated model

Dominance describes the genotype–phenotype relationship.

Problem 34

Why is eye color a poor example of a single-gene classroom cross?
Answer and explanation

Several genes contribute to pigment and observed color; a two-allele model omits important biology.

Problem 35

A hypothetical flower has RR red, WW white and RW pink. Cross RW with RW. Explain the reasoning in your own words.
Answer and explanation

1:2:1 phenotype ratio under the stated incomplete-dominance model.

Each parent supplies R or W.

The combinations RR, RW, WR and WW give ¼ red, ½ pink and ¼ white.

Response criteria

  • Use the stated conditions.
  • Show the evidence or calculation.
  • Explain the conclusion and any stated limitation.

Problem 36

A heterozygote distinctly expresses both allele products. Which pattern fits? Show the reasoning, not just the conclusion.
Hints

Do not extend one trait's pattern to all traits.

Answer and explanation

Codominance, rather than an intermediate blended phenotype.

Problem 37

Why can height differ among people with similar inherited variants?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

Many genes and environmental factors such as nutrition influence a complex trait.

Problem 38

Sex-linked inheritance requires a specified chromosome and locus modelSex-linked inheritance requires a specified chromosome and locus modelXᴬYXᴬXᴬXᴬXᴬYXᵃXᵃXᴬXᵃYSelected X-linked locus; Y has no corresponding allele hereThe model concerns a locus, not every characteristic of sex or gender.
Original constrained X-linked inheritance model, not a diagnostic pedigree or a complete account of human sex development.
Interpret the supplied visual. Why is an autosomal AA/Aa/aa model not automatically applicable to the single-X condition here?
Answer and explanation

The relevant allele-copy arrangement differs; the locus and chromosome-system assumptions determine the genotype possibilities.

A sex-linked locus lies on a sex chromosome. In a selected X-linked model with no corresponding allele on Y, a single X-linked copy can have a different expression pattern from a two-copy genotype. The square follows the stated gamete contributions and trait relationship. It does not imply all traits are X-linked, every organism uses the same chromosome system, or sex/gender characteristics reduce to one allele. Linkage and copy count must be stated rather than borrowed from an autosomal complete-dominance example.

The relevant allele-copy arrangement differs; the locus and chromosome-system assumptions determine the genotype possibilities.

Response criteria

  • Read the structures, axes, directions and units correctly.
  • Connect the evidence with the biological mechanism.
  • Respect stated model assumptions and distinguish an observation from a further inference.

Problem 39

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 40

Which change can affect protein amount without changing coding sequence?
  1. Altered gene regulation
  2. A new species name
  3. A changed graph color
  4. Every cell losing water
Answer and explanation

Altered gene regulation

Expression controls can change RNA or protein levels.

Problem 41

Why is not every mutation inherited by an organism’s offspring?
Answer and explanation

A mutation may occur only in body cells and not enter the reproductive lineage.

Problem 42

Why should an engineered organism be compared with a suitable control?
Answer and explanation

Other genetic or environmental differences could explain its performance.

Problem 43

A liver cell increases transcription after a hormone signal without changing DNA sequence. Is this a mutation? Explain the reasoning in your own words.
Answer and explanation

No; it is regulation of expression in the supplied scenario.

The described change is in gene use.

A mutation would require a sequence change, which the prompt excludes.

Response criteria

  • Use the stated conditions.
  • Show the evidence or calculation.
  • Explain the conclusion and any stated limitation.

Problem 44

A DNA substitution has no amino-acid effect. Must it have no biological effect at all? Show the reasoning, not just the conclusion.
Hints

Expression amount and DNA sequence are different measurements.

Answer and explanation

No. It might affect regulation or RNA processing, though it can also be neutral; evaluate the specific evidence.

Problem 45

Why include untreated controls when testing a gene-editing intervention?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

They help distinguish the intervention's effect from background variation and procedure effects.

Problem 46

Biotechnology requires evidence beyond achieving one molecular changeBiotechnology requires evidence beyond achieving one molecular changeSequence / designDefined targetDelivery + expressionFunction + monitoringA successful molecular step is not proof of a safe whole-system outcome.
Original biotechnology-evaluation chain, not laboratory operating instructions or a safety approval.
Interpret the supplied visual. Does detection of an intended edit establish its desired function and all downstream effects?
Answer and explanation

No. Expression, function, unintended effects and relevant system outcomes need independent evaluation.

Biotechnology uses biological processes or components for a defined purpose. Sequencing, selective breeding and molecular editing are different approaches with different mechanisms. A designed sequence must be delivered, expressed and evaluated in the relevant organism/context. Off-target effects are unintended changes or interactions beyond the desired target, and broader ecological or ethical questions require additional evidence and values. A classroom model should compare purpose, method, measured function and limitations; it is not a home genetic-modification protocol.

No. Expression, function, unintended effects and relevant system outcomes need independent evaluation.

Response criteria

  • Read the structures, axes, directions and units correctly.
  • Connect the evidence with the biological mechanism.
  • Respect stated model assumptions and distinguish an observation from a further inference.

Problem 47

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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