ESAT Biology: Genetics, DNA and inheritance, set 2

• Answer all questions. No calculator.
• Each question has exactly one correct answer.
• Aim to spend about 90 seconds on each question.

1. 11 mark

   Forensic scientists extract DNA from cells swabbed from the inside of a suspect's cheek to build a DNA profile. In which sub-cellular structure is the vast majority of this DNA located?

   1. A The mitochondrion, since mitochondrial DNA makes up most of a human cell's total genetic material.
   2. B The nucleus, since this is the main site of the genetic material in a eukaryotic cell.
   3. C The cell membrane, since it controls the passage of the DNA into and out of the cell.
   4. D The cytoplasm, floating freely and not enclosed within any organelle.
2. 21 mark

   Humans have 46 chromosomes arranged in 23 pairs. One of these pairs differs in size and shape between males and females. Which term describes the other 22 pairs, which are identical in type between the two sexes?

   1. A Alleles
   2. B Genotypes
   3. C Gametes
   4. D Autosomes
3. 31 mark

   The Human Genome Project aimed to determine the complete DNA sequence of a human being. Which of these statements about the term 'genome' is correct?

   1. A An organism's genome is the entire set of its genetic material (DNA), which is organised into and contained within its chromosomes.
   2. B The genome refers only to the DNA found in mitochondria, since this is inherited separately from the rest of an organism's DNA.
   3. C An organism's genome changes completely every time one of its cells divides by mitosis.
   4. D The genome is another word for 'phenotype', since both describe the complete set of an organism's observable characteristics.
4. 41 mark

   Two parents, neither of whom has albinism, have a child who is born with albinism. Which conclusion about the allele that causes albinism follows from this family pattern?

   1. A The allele must be dominant, since it appeared in the child even though neither parent shows the condition.
   2. B Neither parent can have inherited any copy of the allele, since the condition has only appeared in this generation.
   3. C The allele must be recessive, and both parents must be heterozygous carriers, each masking one copy of the allele with a dominant, unaffected allele.
   4. D The child's albinism must be caused by a new mutation that arose during that child's own development, since neither parent is affected.
5. 51 mark

   Huntington's disease is an inherited condition caused by a dominant allele (H) of a gene on an autosome; the recessive allele (h) does not cause the condition. A man who is heterozygous (Hh) and has Huntington's disease has children with a woman who does not carry the allele (hh). What is the probability that a given child of this couple will have Huntington's disease?

   1. A 1/4
   2. B 1/2
   3. C 3/4
   4. D 0, since Huntington's disease is rare and unlikely to be passed on.
6. 61 mark

   Which statement correctly reflects the specification's description of how phenotypes are usually inherited?

   1. A Most phenotypes are the result of multiple genes acting together, and only some phenotypes, including certain inherited conditions, result from single-gene inheritance.
   2. B Almost all human phenotypes, including height and skin colour, are the result of single-gene inheritance, in the same way as cystic fibrosis.
   3. C Multiple-gene inheritance only ever produces continuous variation such as height, so any condition caused by a single gene must instead be caused entirely by environmental factors.
   4. D A species uses either single-gene inheritance or multiple-gene inheritance for all of its characteristics, never both.
7. 71 mark

   Some viruses have genetic material made of single-stranded DNA (ssDNA), while most organisms have double-stranded DNA (dsDNA). Which statement correctly distinguishes these two structures?

   1. A ssDNA and dsDNA differ only in which sugar is used in their nucleotides, not in the number of strands present.
   2. B ssDNA contains only adenine and guanine bases, while dsDNA contains only thymine and cytosine bases.
   3. C dsDNA forms when two separate ssDNA molecules bond directly through their sugar-phosphate backbones, with no bases involved in holding the strands together.
   4. D ssDNA is a polymer of nucleotides forming one strand, while dsDNA is a polymer of two strands forming a double helix.
8. 81 mark

   The genetic code is read as a series of triplets: each group of three DNA bases (a codon) specifies one amino acid. Each of the three positions in a codon can independently be any of the four bases A, T, C or G. How many different triplet codes does this make possible in total?

   1. A 12
   2. B 64
   3. C 43
   4. D 20
9. 91 mark

   Insulin is a protein made of two separate polypeptide chains linked together. Which statement is consistent with the specification's account of protein synthesis?

   1. A One or more polypeptides can combine to form a functional protein, so a protein made of two chains, like insulin, is one way this can happen.
   2. B A single polypeptide alone can never form a functional protein; every functional protein must contain at least two separate polypeptide chains.
   3. C Each of insulin's two polypeptide chains must be produced by a different type of cell, since one gene can only ever be expressed in one type of cell.
   4. D The two polypeptide chains of insulin must have identical amino acid sequences, since they both originate from the same organism's genome.
10. 101 mark

    A mutation occurs in a gene that codes for an enzyme, changing one base in the middle of the gene's sequence. In the majority of such cases, what is the most likely effect of this single mutation on the organism's phenotype?

    1. A The mutation will always be lethal, since any change to an enzyme-coding gene destroys the enzyme's function.
    2. B The mutation will always improve the enzyme's function, since mutations are the source of enzyme evolution.
    3. C The mutation cannot affect phenotype unless it occurs in a reproductive cell, since only inherited mutations do anything at all.
    4. D The mutation will most likely have no effect on phenotype, since most gene mutations have no effect.
11. 111 mark

    Bacteria have been genetically engineered to produce human insulin for use by people with diabetes. Which of these is a genuine advantage of producing insulin this way, compared with extracting it from animal pancreases?

    1. A Bacterially produced insulin is chemically different from human insulin, so it works for a wider range of patients.
    2. B Bacterially produced insulin can be manufactured in large, controlled quantities and avoids the risk of transmitting animal diseases to patients.
    3. C Genetically engineered bacteria automatically evolve to produce increasingly effective forms of insulin every generation.
    4. D Using bacteria to produce insulin removes any possibility of the modified gene ever passing to other organisms in the environment.
12. 121 mark

    Which of the following is a recognised risk associated with using embryonic stem cells in medical treatments?

    1. A Embryonic stem cells cannot divide, so they cannot be used to grow replacement tissue.
    2. B Embryonic stem cells are multipotent, so they can only ever be used to treat blood-related conditions.
    3. C Because embryonic stem cells are pluripotent, if they are not fully differentiated before use they may continue dividing uncontrollably, potentially forming tumours.
    4. D Using embryonic stem cells always causes the patient's immune system to accept the new tissue without any risk of rejection.
13. 131 mark

    A herd of dairy cows is selectively bred over many generations, with only the highest milk-yielding cows chosen as parents each generation. Which of these is a likely long-term impact of this process on the genetic variation within the herd?

    1. A The genetic variation within the herd is likely to increase, since selective breeding introduces new alleles into the population.
    2. B Selective breeding has no effect on genetic variation, since only natural selection can change the frequency of alleles in a population.
    3. C The herd's variation can only change if a new mutation occurs, since selection on its own cannot alter how common an allele is.
    4. D The genetic variation within the herd is likely to decrease, since repeatedly breeding from a narrow set of high-yield individuals reduces the range of alleles present.
14. 141 mark

    Biologists need one main criterion to decide whether two groups of organisms belong to the same species. Which of these correctly states that criterion?

    1. A The two groups live in the same habitat and eat the same food.
    2. B Members of the two groups can interbreed with each other to produce fertile offspring.
    3. C The two groups have exactly the same number of chromosomes as every other species on Earth.
    4. D The two groups were classified together historically, and this classification can never be revised.
15. 151 mark

    A biologist measures the hand span of every student in a large school year group. The measurements form a smooth range from smallest to largest, with no natural gaps separating distinct groups. Which type of variation does hand span in this population best illustrate?

    1. A Discontinuous variation, since hand span can be sorted into a small number of distinct categories such as 'small' and 'large'.
    2. B Continuous variation, since individuals show a full range of values with no distinct categories separating them.
    3. C Environmental variation only, since hand span is entirely determined by diet and has no genetic component at all.
    4. D Genetic variation only, since hand span is entirely determined by inherited alleles and is not affected by environment at all.

Worked solutions

Every question below carries the reasoning, not just the answer. The official material for this test publishes a correct option letter and nothing else.

1. Question 1Answer: B

   1. Spec B4.1 states the nucleus is the site of genetic material in eukaryotic cells such as human cheek cells.
   2. Cheek cells are eukaryotic, so almost all of their DNA is found inside the nucleus, organised into chromosomes.
   3. A very small amount of DNA does exist in mitochondria, but this is not the main store the specification refers to, and it is not located in the cell membrane or loose in the cytoplasm.
   4. So the nucleus, option B, is where forensic DNA profiling recovers the vast majority of a cell's genetic material.
   • Why not A: Confuses mitochondrial DNA, present in only a small amount, with the main store of genetic material, which is nuclear (spec B4.1).
   • Why not C: Assigns a genetic storage role to the cell membrane, whose function is controlling what enters and leaves the cell, not storing DNA.
   • Why not D: Assumes DNA sits loose in the cytoplasm rather than being organised and enclosed within the nucleus.

2. Question 2Answer: D

   1. Spec B4.2 lists 'autosome' among the genetic terms candidates must know.
   2. An autosome is any chromosome that is not one of the sex chromosomes.
   3. In humans, 22 of the 23 pairs are autosomes, identical in type between males and females; the remaining pair (XX or XY) determines sex.
   4. So the term for these 22 shared pairs is autosomes, answer D.
   • Why not A: Names a version of a gene at a particular position, not a category of chromosome.
   • Why not B: Names an organism's genetic makeup at a gene or set of genes, not a type of chromosome.
   • Why not C: Names reproductive cells (sperm and egg), which is a different concept from a category of chromosome found in body cells.

3. Question 3Answer: A

   1. Spec B5.1 defines the genome as the full set of genetic material (DNA) of an organism, contained within its chromosomes.
   2. This is what the Human Genome Project sequenced: essentially all of the DNA in the nucleus of a human cell, not just the small mitochondrial contribution.
   3. Mitosis produces genetically identical daughter cells, so an organism's genome is preserved, not altered, by ordinary cell division.
   4. The genome (genetic material) is distinct from the phenotype (observable characteristics), so option A is correct.
   • Why not B: Restricts the genome to mitochondrial DNA only, when the genome is the full set of DNA, overwhelmingly the DNA held in the nucleus.
   • Why not C: Invents a complete change at every mitotic division; mitosis instead produces daughter cells genetically identical to the parent cell, so the genome does not change.
   • Why not D: Confuses the genome, the DNA itself, with the phenotype, the observable characteristics that the genome, interacting with the environment, helps to produce.

4. Question 4Answer: C

   1. Spec B4.3 asks candidates to interpret family trees / pedigrees and understand inherited conditions.
   2. If two unaffected parents produce an affected child, the allele cannot be dominant, since a dominant allele's effect would show in any parent who carries it.
   3. The only way two unaffected parents can have an affected child is if the allele is recessive and both parents are heterozygous carriers (Aa), each passing on the recessive allele.
   4. So the correct conclusion is C: the allele is recessive, and both parents are carriers.
   • Why not A: Reverses the logic of dominance; if the allele were dominant, at least one parent carrying it would be expected to show the condition themselves.
   • Why not B: Assumes an allele cannot be present in a parent unless that parent shows the condition, ignoring that a recessive allele can be masked in a heterozygous carrier.
   • Why not D: Invents a new mutation as the explanation when the standard and much simpler explanation, two unaffected carrier parents, accounts for the pattern without needing one.

5. Question 5Answer: B

   1. Spec B4.3 requires using and interpreting genetic diagrams and expressing outcomes as ratios or probabilities.
   2. A Punnett square for Hh x hh gives offspring genotypes Hh, Hh, hh and hh: two Hh and two hh, a 1:1 ratio.
   3. Since H is dominant, any child with at least one H allele (Hh) has Huntington's disease, and half of the predicted offspring are Hh.
   4. So the probability that a child is affected is 1/2, answer B.
   • Why not A: Applies the 1/4 ratio expected from a cross between two heterozygous carriers of a recessive condition (Aa x Aa), which does not match this cross.
   • Why not C: Applies the 3/4 ratio for the dominant phenotype expected from an Aa x Aa cross, rather than the Hh x hh cross actually described.
   • Why not D: Ignores the genetics entirely and substitutes a vague judgement about rarity for the calculation the cross actually predicts.

6. Question 6Answer: A

   1. Spec B4.3 states that most phenotypes are the result of multiple genes, and only some result from single-gene inheritance.
   2. Traits like height and skin colour are influenced by many genes and by environment, giving continuous variation, whereas conditions such as cystic fibrosis are caused by a single gene.
   3. Both types of inheritance can occur within the same species for different characteristics.
   4. So option A correctly matches the specification's description.
   • Why not B: Wrongly generalises single-gene inheritance to almost all traits, when the specification states only some phenotypes are single-gene.
   • Why not C: Invents a rule that single-gene conditions must be purely environmental, when a condition like cystic fibrosis is genetic and caused by one gene, not by environmental factors.
   • Why not D: Invents a false either/or restriction at the level of a whole species, when a single species can show both single-gene and multiple-gene inherited characteristics.

7. Question 7Answer: D

   1. Spec B5.2 states that single-stranded DNA is a polymer of nucleotides forming one strand, and double-stranded DNA is a polymer of two strands forming a double helix.
   2. Both forms use the same four nitrogenous bases and the same sugar-phosphate backbone; the difference is purely in whether one or two strands are present.
   3. In dsDNA, it is complementary base pairing (A with T, G with C) between the two strands, not a backbone-to-backbone bond, that holds the double helix together.
   4. So option D is the correct distinction between ssDNA and dsDNA.
   • Why not A: Invents a sugar difference between ssDNA and dsDNA, when the real distinction the specification gives is the number of strands, not the sugar used.
   • Why not B: Invents an artificial split of the four bases between the two structures; both ssDNA and dsDNA are built from all four bases (A, T, G, C).
   • Why not C: Misplaces the bond that holds the two strands together in the backbone rather than between complementary bases; the sugar-phosphate backbone in fact runs along the outside of the helix.

8. Question 8Answer: B

   1. Spec B5.3 states that DNA bases are read in triplets (codons), and each of the three positions in a codon can independently be any of the four bases.
   2. The total number of different codes possible is found by multiplying the number of choices at each position together: 4 x 4 x 4.
   3. 4 x 4 x 4 = 64, so there are 64 different possible triplet codes.
   4. This is more than the 20 standard amino acids, so more than one codon can specify the same amino acid; the answer is B, 64.
   • Why not A: Adds the four choices at each of the three positions together (4 + 4 + 4 = 12) instead of multiplying them, when each position's choice is made independently of the others.
   • Why not C: Combines the digits 4 and 3 as if writing '43' rather than calculating 4 x 4 x 4, treating the two numbers as concatenated instead of multiplied.
   • Why not D: Confuses the number of possible triplet codes with the number of standard amino acids they specify; 20 is the number of amino acids, not the number of codons available, since more than one codon can code for the same amino acid.

9. Question 9Answer: A

   1. Spec B5.3 states that one or more polypeptides can form a functional protein.
   2. Insulin illustrates the 'more than one' case: its two different polypeptide chains join together to form one functional protein.
   3. This does not mean every protein needs two chains, nor that the chains must come from different cell types or be identical in sequence.
   4. So option A correctly matches the specification's description of protein synthesis.
   • Why not B: Overgeneralises the two-chain example into a universal rule, denying that a protein can ever be a single polypeptide, when 'one or more' explicitly allows for just one.
   • Why not C: Invents an unsupported restriction that different polypeptides of one protein must come from different cell types.
   • Why not D: Assumes chains from the same organism must be identical, ignoring that different genes, or different parts of a gene's product, can code for different amino acid sequences within one organism, as with insulin's two distinct chains.

10. Question 10Answer: D

    1. Spec B5.4 states that a mutation changes the sequence of nucleotides in the DNA, and that most mutations have no effect on the phenotype, some have a small effect, and occasionally a mutation determines the phenotype.
    2. A single base change in an enzyme gene most commonly falls into the 'no effect' category, for example if it does not change the resulting amino acid or does not affect the enzyme's active site.
    3. This rules out 'always lethal' and 'always improves function' as typical outcomes.
    4. So the most likely effect described by the specification is no effect on phenotype, answer D.
    • Why not A: Assumes the worst-case outcome is the typical one, contradicting the specification's statement that most mutations have no effect.
    • Why not B: Assumes mutations are typically beneficial, when the specification does not claim improvement is the usual outcome.
    • Why not C: Wrongly restricts any phenotypic effect to inherited mutations, when a mutation can still affect the phenotype of the very organism in which it occurs, for example in a body cell.

11. Question 11Answer: B

    1. Spec B6.1 asks candidates to explain the benefits and risks of using genetic engineering in medical applications.
    2. A genuine benefit of bacterial insulin production is large-scale, controlled manufacture without relying on animal pancreases, and without the risk of transmitting animal-derived diseases.
    3. The bacteria are engineered to produce insulin identical to human insulin, not a different form, and genetic engineering does not cause the inserted gene to spontaneously improve through generations.
    4. The possibility of modified genes transferring to other organisms is a recognised risk, not something eliminated by using bacteria, so option B is correct.
    • Why not A: Invents a chemical difference from human insulin, when the purpose of inserting the human insulin gene is to produce insulin identical to the human form, not a different one.
    • Why not C: Confuses genetic engineering, which fixes a chosen gene in the bacteria, with natural selection; genetic engineering does not cause the inserted gene to spontaneously 'improve' over generations.
    • Why not D: Overstates safety by claiming a real, recognised risk, that engineered genes might transfer to other organisms, simply cannot happen.

12. Question 12Answer: C

    1. Spec B6.2 asks candidates to know the likely benefits and risks of using stem cells in medical applications.
    2. Most embryonic stem cells are pluripotent, able to differentiate into any cell type; this same ability is a risk if the cells are used before being fully differentiated, since they may keep dividing and form a tumour.
    3. This rules out claims that embryonic stem cells cannot divide, that they are narrowly multipotent, and that immune rejection is never a concern.
    4. So the recognised risk described is option C.
    • Why not A: Wrongly claims embryonic stem cells cannot divide, contradicting their defining ability to self-renew and proliferate.
    • Why not B: Misapplies the term 'multipotent' to embryonic stem cells, which the specification describes as pluripotent, and invents an unsupported blood-only restriction.
    • Why not D: Overstates a benefit as guaranteed and risk-free, when immune rejection is in fact a genuine concern with using stem cell-derived tissue.

13. Question 13Answer: D

    1. Spec B6.3 asks candidates to understand the impact of selective breeding on populations.
    2. Selectively breeding only from the highest-yielding cows repeatedly narrows the pool of parents contributing alleles to the next generation.
    3. Over many generations this tends to reduce the range of alleles present in the herd, decreasing genetic variation, rather than introducing new alleles or leaving variation unchanged.
    4. So option D correctly describes the likely long-term impact.
    • Why not A: Wrongly claims selective breeding introduces new alleles; it instead selects among alleles already present, which tends to narrow variation rather than increase it.
    • Why not B: Denies that human-directed selection can change allele frequencies, when both natural and artificial (selective) selection can do this.
    • Why not C: Also denies that selection alone can shift allele frequencies, overlooking that repeatedly choosing a narrower breeding stock changes which alleles are common without requiring any new mutation.

14. Question 14Answer: B

    1. Spec B7.1 defines a species as a group of organisms that can reproduce to produce fertile offspring.
    2. Sharing a habitat or a diet is an ecological relationship, not the biological test for species membership, and different species can live alongside one another.
    3. Chromosome number differs widely between species and does not define species membership, and scientific classification can be revised as new evidence is found.
    4. So the defining criterion is the ability to interbreed and produce fertile offspring, answer B.
    • Why not A: Describes sharing a habitat and diet, an ecological relationship that different species can also share, rather than the biological criterion used to define species membership.
    • Why not C: Invents an impossible universal rule; chromosome number varies enormously between different species and is not what defines whether two groups belong to the same species.
    • Why not D: Invents an unscientific claim that classification is fixed forever, when species classifications can be, and are, revised as new evidence emerges.

15. Question 15Answer: B

    1. Spec B7.2 distinguishes continuous variation, where individuals show a full range of values with no distinct categories, from discontinuous variation, where individuals fall into a small number of distinct categories.
    2. Hand span, like height, is controlled by many genes acting together as well as by environmental factors, and this combination typically produces a smooth range of values rather than a small number of distinct groups.
    3. This rules out sorting hand span into a small number of discrete categories, and rules out claiming it is determined purely by environment or purely by genetics.
    4. So the smooth, ungrouped spread of hand spans described here illustrates continuous variation, answer B.
    • Why not A: Wrongly imposes a small number of distinct categories on a trait that in fact varies smoothly across a continuous range, which is the opposite of what continuous variation means.
    • Why not C: Wrongly claims hand span has no genetic component at all, when it results from the combined effect of many genes as well as environmental factors.
    • Why not D: Wrongly claims hand span has no environmental component at all, when factors during growth are known to influence body measurements alongside genetics.