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

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

Review the key ideas

DNA Structure & Replication

DNA is a double helix of nucleotides whose bases pair by a strict rule (A-T, C-G); this complementary pairing lets DNA copy itself semiconservatively, so each new molecule keeps one original strand and one new one.

Protein Synthesis

A protein-coding gene is transcribed into RNA and its processed mRNA can be translated into a polypeptide. Some genes make functional RNA instead. Strand direction, reading frame, start/stop signals and subsequent processing matter when predicting a protein.

Mendelian Genetics

Punnett squares predict probabilities under specified inheritance assumptions; samples can vary and many traits need more complex models.

Inheritance Beyond One-Gene Models

Choose an inheritance model that fits the trait; probabilities and model limits belong in the explanation.

Gene Regulation, Mutation & Biotechnology

Follow the evidence from DNA or regulation to product and phenotype; not every sequence change has the same consequence.

Problem 1

Write the complementary DNA strand for the sequence A-T-G-G-C-A.
Answer and explanation

T-A-C-C-G-T

Apply the base-pairing rules to each base (A-T, C-G): A to T, T to A, G to C, G to C, C to G, A to T. Reading in order gives the complementary strand T-A-C-C-G-T.

Problem 2

State the complementary base-pairing rules of DNA, and explain why they let one strand act as a template for the other.
Answer and explanation

A pairs with T and C pairs with G; because each base has only one partner, one strand's sequence determines the other's exactly.

In DNA, adenine always pairs with thymine (A-T) and cytosine always pairs with guanine (C-G). Because each base bonds with only one specific partner, the order of bases on one strand automatically fixes the order on the other — so a single strand carries all the information needed to rebuild its complement.

Problem 3

Where in the cell does transcription happen, and where does translation happen?
Answer and explanation

Transcription happens in the nucleus; translation happens at a ribosome in the cytoplasm.

Transcription copies DNA into mRNA inside the nucleus, where the DNA is kept. The mRNA then leaves the nucleus and travels to a ribosome in the cytoplasm, where translation reads it to build a protein.

Problem 4

An mRNA is 30 bases long. Assuming it contains no stop signal, how many amino acids does it code for?
Answer and explanation

10 amino acids

The ribosome reads mRNA in codons of 3 bases, and each codon codes for one amino acid. So 30 bases / 3 bases per codon = 10 codons = 10 amino acids.

Problem 5

In rabbits, black fur (B) is dominant over white (b). Two Bb rabbits are crossed. Give the expected genotype ratio and phenotype ratio of the offspring.
Answer and explanation

Genotype 1 BB : 2 Bb : 1 bb; phenotype 3 black : 1 white.

The Bb x Bb Punnett square gives the boxes BB, Bb, Bb, bb. That is a 1:2:1 genotype ratio. Since B is dominant, BB, Bb, and Bb are all black and only bb is white, giving a 3:1 (black:white) phenotype ratio.

Problem 6

A tall pea plant (T is dominant) could be TT or Tt. Explain how a single cross with a short (tt) plant can reveal which genotype it has.
Answer and explanation

Recessive descendants establish Tt under the stated model; a small all-tall sample does not prove TT.

Cross with tt. A Tt parent has a chance of all tall descendants in a small sample, so more outcomes or genotype evidence may be needed.

Problem 7

Two heterozygous (Bb) parents plan to have three children. What is the probability that all three children show the recessive (bb) phenotype?
Answer and explanation

1/64

For each child, a Bb x Bb cross gives a 1/4 chance of bb. Each birth is independent, so multiply: 1/4 x 1/4 x 1/4 = 1/64. (That is about 1.6%.)

Problem 8

In a hypothetical complete-dominance model, two dominant-phenotype Aa parents have an aa descendant. Explain the allele pathway and one limit of applying this model to real traits.
Answer and explanation

Each parent can supply a. Their combination produces aa under the model.

Meiosis separates the modeled alleles into gametes; fertilization can combine a from each parent. This is a hypothetical single-locus example, not an accurate literal model of human eye color or every trait.

Problem 9

A double-stranded DNA sample contains 31 percent thymine. What percentage is guanine?
Answer and explanation

19 %

T = A = 31 percent, so A and T together are 62 percent, leaving 38 percent for G and C, which are equal: 19 percent each.

Problem 10

Explain why base pairing makes accurate replication possible, and what still allows errors to occur.
Answer and explanation

Each base pairs with only one partner, so an existing strand specifies the sequence of the strand built against it; the template carries the information needed to reconstruct its complement exactly. Errors still occur because pairing is not perfect in practice: an incorrect base is occasionally inserted, and repair mechanisms catch most but not all of these. Those uncorrected changes are mutations, which is why replication is highly accurate rather than infallible.

Response criteria

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

Problem 11

Chapter review: 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 12

How many mRNA bases are needed to specify a polypeptide of 40 amino acids, including one stop codon?
Answer and explanation

123

40 amino acids need 40 codons, plus one stop codon gives 41 codons, and 41 × 3 = 123 bases.

Problem 13

Trace the path from a gene to a functioning protein, naming each molecule and location.
Answer and explanation

In the nucleus, the gene's DNA is transcribed into messenger RNA by complementary base pairing, with uracil replacing thymine. The messenger RNA moves to a ribosome in the cytoplasm, where translation reads it three bases at a time; transfer RNA molecules bearing matching anticodons deliver the specified amino acids, which are joined into a polypeptide chain. The chain folds into its three-dimensional shape, and in a secreted protein the endoplasmic reticulum and Golgi apparatus then modify and sort it. Function depends on the final folded shape, not on the sequence alone.

Response criteria

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

Problem 14

Chapter review: 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 15

A Tt × tt cross produces 84 offspring. How many are expected to show the recessive phenotype?
Answer and explanation

42

Half are expected to be tt: 0.5 × 84 = 42.

Problem 16

A couple has had three children, all with the recessive phenotype from a heterozygous × heterozygous cross. A student says the fourth is "due" to show the dominant phenotype. Correct this.
Answer and explanation

Each conception is independent, so the probability for the fourth child is the same three quarters dominant and one quarter recessive as for the first. Previous outcomes do not change which gametes happen to meet. The 3:1 ratio is an expectation for large numbers, and small families routinely depart from it; three recessive children in a row has a probability of about one in 64, which is unlikely but not evidence that the ratio is wrong.

Response criteria

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

Problem 17

Chapter review: 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 18

Give an example of why phenotype may depend on more than genotype at one locus.
Answer and explanation

A polygenic trait depends on several loci, and conditions such as nutrition can affect growth. One genotype does not specify every phenotypic detail.

Problem 19

Human height varies continuously rather than falling into two categories. Explain what this indicates about its inheritance and why a Punnett square is a poor tool for it.
Answer and explanation

Continuous variation indicates a polygenic trait: many genes each contribute a small amount, so their combined effects produce a graded range rather than discrete classes, and environmental factors such as nutrition add further variation. A Punnett square handles one or two genes with a few alleles and produces discrete categories, so it cannot represent the many small contributions or the environmental component. Describing the distribution of the trait in a population is more informative than predicting an individual's value.

Response criteria

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

Problem 20

A trait is controlled by 3 genes, each with 2 alleles that separate independently. How many different gamete allele combinations are possible?
Answer and explanation

8

2³ = 8 combinations, which is why even a modest number of contributing genes produces a wide range of phenotypes.

Problem 21

Chapter review: 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 22

Trace a testable chain from a regulatory change to a trait.
Answer and explanation

Regulatory change → changed RNA production → altered protein amount → changed cellular process → measured trait; test each link and alternatives.

Problem 23

A bacterium produces a sugar-digesting enzyme only when that sugar is present. Explain the advantage and the general principle it illustrates.
Answer and explanation

Making the enzyme requires materials and energy, so producing it when the sugar is absent would waste both. Regulating transcription in response to the sugar's presence means the cell invests only when the enzyme can be used. The general principle is that having a gene and expressing it are separate matters: a cell adjusts which genes are transcribed according to its conditions, which is why two cells with the same genome can behave differently.

Response criteria

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

Problem 24

Enzyme activity rises from 4 units to 96 units after an inducing sugar is added. By what factor did it increase?
Answer and explanation

24

96 / 4 = 24 times, the kind of induced change that indicates transcription was switched on rather than merely adjusted.

Problem 25

Chapter review: 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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