ANSWERS 1 (52 Marks)



ANSWERS 1 (52 Marks)

IB Standard and Higher level Biology Dulwich College Shanghai

Topic 4 (SL) and 10 (HL): Genetics

1. Chromosomes, genes, alleles and mutations (SL)

1. State that eukaryote chromosomes are made of DNA and proteins.

2. Define gene, allele and genome.

3. Define gene mutation.

4. Explain the consequences of a base substitution mutation in relation to the processes of transcription and translation, using the example of sickle-cell anaemia.

2. Meiosis (SL)

1. State that meiosis is a reductive division of a diploid nucleus to form a haploid nuclei.

2. Define homologous chromosomes.

3. Outline the process of meiosis, including pairing of homologous chromosomes and crossing over, followed by two divisions, which results in four haploid cells.

4. Explain that non-disjunction can lead to changes in chromosomes number, illustrated by references to Down syndrome (trisomy 21).

5. State that, in karyotyping, chromosomes are arranged in pairs according to their size and structure.

6. State that karyotyping is performed using cells collected by chorionic villus sampling or amniocentesis, for pre-natal diagnosis of chromosome abnormalities.

7. Analyse a human karyotype to determine gender and whether non-disjunction has occurred.

1. Meiosis (HL)

1. Describe the behaviour of the chromosomes in the phases of meiosis.

2. Describe the behaviour of the chromosomes in the phases of meiosis.

3. Explain how meiosis results in an effectively infinite genetic variety in gametes through crossing over in prophase 1 and random orientation in metaphase 1.

4. State Mendel’s law of independent assortment.

5. Explain the relationship between Mendel’s law of independent assortment and meiosis

3. Theoretical Genetics (SL)

1. Define genotype, phenotype, dominant allele, recessive allele, codominant alleles, locus, homologous, heterozygous, carrier and test cross.

2. Determine the genotypes and phenotypes of the offspring of a monohybrid cross using a Punnett grid.

3. State that some genes have more than two alleles (multiple alleles).

4. Describe ABO blood groups as an example of codominance and multiple alleles.

5. Explain how the sex chromosomes control gender by referring to the inheritance of X and Y chromosomes in humans.

6. State that some genes are present on the X-chromosome and absent from the shorter Y chromosome in humans.

7. Define sex linkage.

8. Describe the inheritance of colour blindness and haemophilia as examples of sex linkage.

9. State that a human female can be homozygous or heterozygous with respect to sex-linked genes.

10. Explain that female carriers are heterozygous for X-linked recessive alleles.

11. Predict the genotypic and phenotypic ratios of offspring of monohybrid crosses involving any of the above patterns of inheritance.

12. Deduce the geneotypes and phenotypes of individuals in pedigree charts.

2. Dihybrid Crosses and Gene Linkage (HL)

1. Calculate and predict the genotypic and phenotypic ratio of offspring of dihybrid crosses involving unlinked autosomal genes

2. Distinguish between autosomes and sex chromosomes

3. Explain how crossing over between non-sister chromatids of a homologous pair in prophase I can result in an exchange of alleles.

4. Define linkage group

5. Explain an example of a cross between two linked genes. Alleles are usually shown side by side in dihybrid crosses, for example TtBb. In representing crosses involving linkage, it is more common to show them as vertical pairs for example:

[pic]

6. Identify which of the offspring are recombinants in a dihybrid cross involving linked genes.

[pic]

3. Polygenic Inheritance

1. Define polygenic inheritance

2. Explain that polygenic inheritance can contribute to continuous variation using two examples, one of which must be human skin colour

4. Genetic Engineering and Biotechnology (SL)

1. Outline the use of polymerase chain reaction (PCR) to copy and amplify minute quantities of DNA.

2. State that, in gel electrophoresis, fragments of DNA move in an electric field and are separated according to their size.

3. State that gel electrophoresis of DNA is used in DNA profiling.

4. Describe the application of DNA profiling to determine paternity and also in forensic investigations.

5. Analyse DNA profiles to draw conclusions about paternity or forensic investigations.

6. Outline three outcomes of the sequencing of the complete human genome.

7. State that, when genes are transferred between species, the amino acid sequence of polypeptides translated from them is unchanged because the genetic code is universal.

8. Outline a basic technique used for gene transfer involving plasmids, a host cell (bacterium, yeast or other cell), restriction enzymes (endonucleases) and DNA ligase.

9. State two examples of the current uses of genetically modified crops or animals.

10. Discuss the potential benefits and possible harmful effects of one example of genetic modification.

11. Define clone.

12. Outline a technique for cloning using differentiated animal cells.

13. Discuss the ethical issues of therapeutic cloning in humans.

Paper 1

Multiple Choice (10 Marks)

1. Which enzymes are needed to produce recombinant plasmids that are used in gene transfer?

A. DNA polymerase and ligase

B. DNA polymerase and restriction enzymes

C. Restriction enzymes and ligase

D. Helicase and restriction enzymes

2. Which response describes the behaviour of chromosomes in metaphase I and anaphase II of meiosis?

| |Metaphase I |Anaphase II |

|A. |Chromosomes line up at the equator |Separation of homologous chromosomes |

|B. |Tetrads (bivalents) line up at the equator |Separation of homologous chromosomes |

|C. |Chromosomes line up at the equator |Separation of sister chromatids |

|D. |Tetrads (bivalents) line up at the equator |Separation of sister chromatids |

3. In garden peas, the pairs of alleles coding for seed shape and seed colour are unlinked. The allele for smooth seeds (S) is dominant over the allele for wrinkled seeds (s). The allele for yellow seeds (Y) is dominant over the allele for green seeds (y).

If a plant of genotype Ssyy is crossed with a plant of genotype ssYy, which offspring are recombinants?

A. SsYy and Ssyy

B. SsYy and ssYy

C. SsYy and ssyy

D. Ssyy and ssYy

4. What constitutes a linkage group?

A. Genes carried on the same chromosome

B. Genes whose loci are on different autosomes

C. Genes controlling a polygenic characteristic

D. Alleles for the inheritance of ABO blood groups

5. A cell with a diploid number of 12 chromosomes undergoes meiosis. What will be the product at the end of meiosis?

A. 2 cells each with 12 chromosomes

B. 4 cells each with 6 chromosomes

C. 2 cells each with 6 chromosomes

D. 4 cells each with 12 chromosomes

6. Which process results in the greatest genetic variation in a population?

A. Meiosis

B. Mitosis

C. Cytokinesis

D. Natural selection

7. The following is a DNA gel. The results are from a single probe showing a DNA profile for a man, a woman and their four children.

[pic]

[Source: The Biology Project, University of Arizona]

Which fragment of DNA is the smallest?

A. I

B. II

C. III

D. IV

8. The following is a DNA gel. The results are from a single probe showing a DNA profile for a man, a woman and their four children.

[pic]

Which child is least likely to be the biological offspring of the father?

A. Child 1

B. Child 2

C. Child 3

D. Child 4

9. A parent organism of unknown genotype is mated in a test cross. Half of the offspring have the same phenotype as the parent. What can be concluded from this result?

A. The parent is heterozygous for the trait.

B. The trait being inherited is polygenic.

C. The parent is homozygous dominant for the trait.

D. The parent is homozygous recessive for the trait.

10. The allele for red flower colour (R) in a certain plant is co-dominant with the allele for white flowers (R’). Thus a plant with the genotype RR’ has pink flowers. Tall (D) is dominant to dwarf (d). What would be the expected phenotypic ratio from a cross of RR’dd plants with R’R’Dd plants?

A. 9:3:3:1

B. 50% pink 50% white, and all tall

C. 1:1:1:1, in which 50% are tall, 50% dwarf, 50% pink and 50% white

D. 3:1

Paper 2

Section A

Data Analysis (6 marks)

1. Polygalacturonase (PG) plays an important role in fruit softening by making the pectin of the cell wall more soluble. It is synthesized only when the fruit is ripe.

In order to slow down the ripening of tomatoes (Lycopersicon esculentum), antisense RNA technology was used. Messenger RNA from untransformed and transformed fruit was hybridized to a radioactively labelled probe specific to the PG sense strand.

The results of a gel electrophoresis of mRNA are given below. (The size of the mRNA strands is expressed in kilobases, kb.) The histogram shows these results expressed as the percentage of PG mRNA in ripe untransformed fruit.

Lane 1: Ripe untransformed fruit

Lane 2: Unripe untransformed fruit

Lane 3: Ripe transformed fruit

Lane 4: Unripe transformed fruit

[pic]

[Source: Smith et al., Nature, (1988), 334, pages 724–726]

(a) State the percentage of PG mRNA in ripe transformed fruit. (1)

6 (± 2) (%);

(b) Compare the results obtained for ripe and unripe fruit. (2)

only ripe transcribe PG mRNA (1) / ripe tomatoes produce more PG mRNA

than unripe;

band at 1.77 kb only in ripe (1);

(c) Using the information provided, explain how the antisense technology affects transformed fruit. (3)

no effect on unripe fruit;

band at 1.77 kb much smaller in transformed / less PG mRNA produced in

transformed ripe fruit;

antisense mRNA combines with sense mRNA (1);

inactivating the translation (1) / less translation;

less PG (1) to solubilize pectin of wall;

fruit takes longer to ripen;

Paper 2

Section A

Short Structured (18 Marks)

1. (a) Define sex linkage. (1)

gene / allele / trait on a sex (-determining, X or Y) chromosome

(b) State one example of sex linkage. (1)

examples include:

Fabry’s disease / Hunter’s syndrome / Lesch-Nyhan syndrome / haemophilia /

forms of colour blindness / Menkes’ steely-hair syndrome /

ALD (adrenoleukodystrophy) Renpennings syndrome /

Duchenne muscular dystrophy / G-6-P dehydrogenase /

testicular determining factor (TDF on Y-chromosome) /

calico-tortoiseshell cat fur colour / white eye Drosophila;

(c) Draw a simple pedigree chart that clearly shows sex linkage in humans. Use conventional symbols. Start with an affected woman and an unaffected man. (4)

affected woman and unaffected man in first generation drawn correctly (1);

all sons in the 2nd generation affected (1);

all daughters 2nd generation unaffected (1);

at least one son (but no daughter in 3rd generation unless father was affected) of a carrier

daughter (in 2nd generation) must be affected;

drawing of pedigree chart (2 generations) with correct symbols and connecting

lines; (1)

example:

[pic]

2. The following diagram represents a two generation pedigree showing the blood groups of the individuals. The female has been married to two different individuals.

[pic]

(a) Define the term co-dominant alleles. (1)

alleles of gene / pairs of alleles which both affect the phenotype / both expressed

(when present together in an individual / in the heterozygote);

Reject: both dominant / both recessive

(b) Deduce with a reason the probable father of 2nd generation–1. (2)

1st generation–3 / father 3;

father 1 can only donate an O allele / B allele cannot come from O parent;

(c) If 2nd generation–3 marries a man with blood group AB, predict the possible genotypes of the children. (3)

IA IA / AA; (1)

IA IB / AB; (1)

IA IO and IB IO / AO and BO (1) / IA i and IB i;

Award marks for correct answers then deduct [1] for each incorrect genotype, eg including genotypes with only one allele. Minimum mark [0]. Do not accept phenotypes instead of genotypes.

3. (a) Define the term degenerate as it relates to the genetic code. (1)

more than one codon / base triplet codes for an amino acid

(b) Apart from international cooperation, outline two positive outcomes of the Human Genome Project. (2)

may lead to an understanding of genetic / inherited diseases / conditions;

may lead to the production of gene probes to detect carriers of genetic diseases;

may lead to the production of pharmaceuticals based on DNA sequences;

study of similarities / differences between human race / population;

find location of genes / produce a complete gene map;

study of human origins / migration / relationships with other species

(c) State the catalytic activity of reverse transcriptase. (1)

reverse transcriptase catalyzes the production of DNA from RNA

(d) State one use of monoclonal antibodies in diagnosis and one use in treatment. (2)

Award [1 max] for use in diagnosis and [1 max] for use in treatment.

diagnosis:

detection of (antibodies to) HIV;

detection of HCG / pregnancy test kits;

detection of cardiac enzyme in suspected heart attacks;

detection of tissue / blood type;

testing for (different strains of) malaria;

ELISA test;

treatment:

targeting cancer cells with attached drugs;

treatment of rabies / Ebola / lymphoma

destroying T-cells to reduce rejection of transplants;

Section B

Extended Response (18 Marks)

1. Describe, with the aid of a diagram, the behaviour of chromosomes in the different phases of meiosis. (5)

chromosomes condense / coil /

become shorter and fatter during prophase I; (1)

(homologous) chromosomes pair up in prophase I; (1)

crossing over / chiasmata formation in prophase I; (1)

movement of pairs of chromosomes / bivalents to the equator in metaphase I; (1)

movement of half of the chromosomes to each pole in anaphase I;

movement of chromatids to opposite poles in anaphase II; (1)

decondensation / uncoiling in telophase II;

[4 max] if no diagram is shown.

Do not award a mark for a statement if a diagram has been drawn that does not fit in with the statement. For example, if the candidate states that pairs of chromosomes move to the equator in metaphase I but shows single chromosomes, do not award that mark.

2. Explain how meiosis and fertilization can give rise to genetic variety. (6)

random orientation/ independent assortment (1) of bivalents / pairs of chromosomes;

maternal and paternal chromosome could go to either pole;

2n combinations; eg over 8 million in humans;

crossing over; (1)

exchange of material (1) between homologous chromosomes / non-sister chromatids;

segregation/ splitting of alleles in meiosis; splitting of homologous pairs

combinations of alleles are broken up;

fertilization brings together genes (1) / alleles from two different parents;

fertilization generates new combinations of genes (1)/ alleles;

random fertilization (1)/ many possible combinations of male and female gamete;

eg over 64 million million in humans (ignoring crossing over);

3. Describe the consequence of a base substitution mutation with regards to sickle cell anaemia. (7)

mutation is a change in DNA sequence (1);

changes the mRNA during transcription;

changes the amino acid sequence (1);

substitution mutation / changes to one codon (1);

glutamic acid is changed to valine (1) / GAG to GTG;

changes the shape of hemoglobin / hemoglobin becomes less soluble (1)

cannot carry oxygen (1) as well;

red blood cells sickle / impairs blood flow;

causes other health problems / anaemia (1)/ tiredness;

sickle cell anemia caused by two mutated recessive alleles;

DNA sequence ( amino acid sequence ( protein characteristics ( determines phenotype (explain the phenotype)

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