4.1 Chromosomes, genes, alleles and mutations
4.1.1State that eukaryote chromosomes are made of DNA and proteins.
4.1.2 Define gene, allele and genome.
● Gene: a heritable factor that controls a specific characteristic. (The differences between structural genes, regulator genes and genes coding for tRNA and rRNA are not expected at SL).
● Allele: one specific form of a gene, differing from other alleles by one or a few bases only and occupying the same gene locus as other alleles of the gene.
● Genome: the whole of the genetic information of an organism.
4.1.3 Define gene mutation.
● A gene mutation is a change to the base sequence of a gene.
4.1.4 Explain the consequence of a base substitution mutation in relation to the processes of transcription and translation, using the example of sickle-cell anemia.
● The gene that codes for haemoglobin (HbA) can undergo a form of gene mutation called base pair substitution which results in an altered form of the gene that causes sickle cell anemia (HbS).
● Mutation occurs because one base is substituted for another on the sixth amino acid of the gene, so that GAG is mutated to GTG on the sense strand and CTC is mutated to CAC on the antisense strand.
● During transcription, the mRNA strand carries GUG instead of GAG as a result.
● This causes causing glutamic acid to be replaced by valine during translation.
● Since valine has a different shape and different properies from glutamic acid, the shape of the resulting polypeptide is modified. Therefore red blood cells containing the altered haemoglobin becomes sickle-shaped, hence sickle-cell anemia occurs.
● Sickle cells may carry oxygen less efficiently but can give resistance to malaria.
4.2 Meiosis
4.2.1 State that meiosis is a reduction division of a diploid nucleus to form haploid nuclei.
4.2.2 Define homologous chromosomes.
● Homologous chromosomes are a pair of chromosomes that are similar in shape and size, have the same sequence of genes, however one is maternal and one is paternal (and therefore they do not necessarily have the same alleles of a gene).
4.2.3 Outline the process of meiosis, including pairing of homologous chromosomes and crossing over, followed by two divisions, which results in four haploid cells.
● Prophase I
○ Chromosomes (existing as sister chromatids) supercoil, and become shorter and thicker.
○ Homologous chromosomes organize into bivalents
○ Centrioles begin to duplicate and eventually move to opposite poles ( in animal cells).
○ Spindle fibres made from microtubules form.
○ Crossing-over occurs, which is the exchange of genetic material between non-sister chromatids in a bivalent during prohase I.
○ Nucleoli breaks down.
○ At the end of prophase I the nuclear membrane breaks down.
● Metaphase I
○ Crossing over is terminated.
○ The bivalents line up randomly along the equator. (This is called random orientation)
○ Spindle mictrotubules attach to the centromeres of each chromosome.,
● Anaphase I
○ Homologous chromosomes separate and are pulled to opposite poles of the cell by spindle microtubules. The result is the independent assortment of genes which are not linked.
■The chromosome number is halved
■Because of crossing over, the two chromatids of each chromosome are not identical
● Telophase I
○ Spindle and spindle fibres disintegrate.
○ Some cells (usually plant) do not have a telophase I stage. In others (usually animal), a nuclear membrane forms around the groups of chromosomes at each pole and chromosomes may uncoil to some degree.
○ The cell divides to form two haploid cells, however each chromosome still consists of two chromatids.
● Prophase II
○ Chromosomes supercoil
○ Centrioles move to opposite poles (in animal cells)
○ New spindle fibres are produced
○ Nuclear membrane breaks down.
● Metaphase II
○ Chromosomes line at the equator in no specific order --this is called random orientation.
○ Spindle fibres from each pole attach to the chromosomes.
● Anaphase II
○ Centromeres of chromosomes are split, releasing each sister chromatid as an individual chromosome.
○ Spindle fibres pull individual chromatids to opposite ends of the cell.
■Again because of random orientation, chromatids could be pulled towards either side of the cell
● Telophase II
○ Nuclear membranes form around the groups of chromatids at each pole. Each chromatid is now considered to be a chromosome.
○ The two cells divide to form four cells in total.
○ The chromosomes uncoil.
○ Nucleoli appear.
4.2.4 Explain that non-disjunction can lead to changes in chromosome number, illustrated by reference to Down syndrome (trisomy 21).
● Non-disjunction is a form of chromosome mutation that refers to the failure of a pair of chromatids to separate and go to opposite poles during division of the nucleus. In meiosis, this results in gametes with more than and less than the haploid number of chromosomes.
● If the gamete that contains an extra chromosome become fertilized, the zygote produces three chromosomes of one type instead of the normal two. This anomaly is called a trisomy, and the case of Down’s syndrome, non-disjunction happens in the 21st pair of chromosomes.
4.2.5 State that, in karyotyping, chromosomes are arranged in pairs according to their size and structure.
4.2.6 State that karyotyping is performed using cells collected by chorionic villus sampling or amniocentesis, for pre-natal diagnosis of chromosome abnormalities.
4.2.7 Analyse a human karyotype to determine gender and whether non-disjunction has occurred.
4.3 Theoretical genetics
4.3.1 Define genotype, phenotype, dominant allele, recessive allele, codominant alleles, locus, homozygous, heterozygous, carrier and test cross.
● Genotype: the alleles of an organism.
● Phenotype: the characteristics of an organism.
● Dominant allele: an allele that has the same effect on the phenotype whether it is present in the homozygous or heterozygous state.
● Recessive allele: an allele that only has an effect on the phenotype when present in the homozygous state.
● Codominant alleles: pairs of alleles that both affect the phenotype when present in a heterozygote. (The terms incomplete and partial dominance are no longer used.)
● Locus: the particular position on homologous chromosomes of a gene.
● Homozygous: having two identical alleles of a gene.
● Heterozygous: having two different alleles of a gene.
● Carrier: an individual that has one copy of a recessive allele that causes a genetic disease in individuals that are homozygous for this allele.
● Test cross: testing a suspected heterozygote by crossing it with a known homozygous recessive. (The term backcross is no longer used.)
4.3.2 Determine the genotypes and phenotypes of the offspring of a monohybrid cross using a Punnett grid.
The grid should be labelled to include parental genotypes, gametes, and both offspring genotype and phenotype.
4.3.3 State that some genes have more than two alleles (multiple alleles).
4.3.4 Describe ABO blood groups as an example of codominance and multiple alleles.
NOV 2005 PAPER 2 EXAM QUESTION
● The ABO blood groups are an example of both codominance as well as multiple alleles.
● There are three alleles that can occupy the same locus for the gene that controls blood type: IA, IB, and i.
● IA and IB are codominant alleles, however both are also dominant to i which is recessive.
● As a result, there are six possible genotypes and four possible phenotypes:
4.3.5 Explain how the sex chromosomes control gender by referring to the inheritance of X and Y chromosomes in humans.
● Two chromosomes determine the gender (male or female) of an offspring. These are called sex chromosomes and they are the 23rd chromosomes.
● The X chromosome is relatively large and carries many genes.
● The Y chromosome is much smaller and carries only a few genes.
● If the genotype is XX in a human embryo, the offspring develops into a girl.
● If the genotype is XY, it develops into a boy.
● Therefore when a woman reproduces, she will always pass on one X chromosome in her gamete (egg cell).
● When a male reproduces, he will pass on either one X chromosome or one Y chromosome in his gamete (sperm cell), so gender therefore depends on whether the sperm that fertilizes the egg is carrying an X or Y chromosome.
● As a result, there is a phenotypic ratio of “1 male: 1 female” for gender.
4.3.6 State that some genes are present on the X chromosome and absent from the shorter Y chromosome in humans.
4.3.7 Define sex linkage.
● Sex linkage is the association of a characteristic with gender, because the gene controlling the characteristic is located on a sex chromosome.
4.3.8 Describe the inheritance of colour blindness and haemophilia as examples of sex linkage.
NOV 2005 PAPER 2 EXAM QUESTION
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● Both colour blindness and hemophilia are produced by a recessive sex-linked allele on the X chromosome.
○ Notation: Xb and Xh is the notation for the alleles concerned. The corresponding dominant alleles are XB and XH.
● In males, an allele present on the X chromosome is likely to be apparent in the phenotype even if it is recessive because he only has one X chromosome.
● In females, a single recessive allele is often masked by a dominant allele on the other X chromosome and therefore it will not be present in the phenotype.
● Therefore, colour blindness or haemophilia are more likely to be apparent in males than in females because a female can be homozygous for a normal condition (XB XB or XH XH) or heterozygous for a normal condition (XB Xb or XH Xh). To have the condition, a female must be homozygous recessive (Xb Xb or Xh Xh) and this is extremely rare. A male with a single recessive allele (XbY or XhY) will be affected.
● Males always inherit recessive sex-linked conditions from their mothers. Males with these conditions cannot pass it on to their sons because the Y-allele does not carry them, however they will always pass it on to their daughters. Their daughters will therefore always be either heterozygous or homozygous (depending on the mother) for the condition.
4.3.9 State that a human female can be homozygous or heterozygous with respect to sex-linked genes.
4.3.10 Explain that female carriers are heterozygous for X-linked recessive alleles.
● In males, an allele present on the X chromosome is likely to be apparent in the phenotype even if it is recessive because he only has one X chromosome. Therefore males can never be carriers.
● In females, a single recessive allele is often masked by a dominant allele on the other X chromosome and therefore it will not be present in the phenotype, and in this case they are carriers. A female can be homozygous for a normal condition (XB XB or XH XH) or heterozygous for a normal condition (XB Xb or XH Xh). To have the condition, a female must be homozygous recessive (Xb Xb or Xh Xh) and this is extremely rare. A male with a single recessive allele (XbY or XhY) will be affected.
4.3.11 Predict the genotypic and phenotypic ratios of offspring of monohybrid crosses involving any of the above patterns of inheritance.
4.3.12 Deduce the genotypes and phenotypes of individuals in pedigree charts.
For dominant and recessive alleles, upper-case and lower-case letters, respectively, should be used. Letters representing alleles should be chosen with care to avoid confusion between upper and lower case.
For codominance, the main letter should relate to the gene and the suffix to the allele, both upper case. For example, red and white codominant flower colours should be represented as CR and CW, respectively. For sickle-cell anemia, HbA is normal and Hbs is sickle cell.
4.4 Genetic engineering and biotechnology
4.4.1 Outline the use of polymerase chain reaction (PCR) to copy and amplify minute quantities of DNA.
The purpose of PCR is to repeatedly replicate DNA when only small samples of DNA are available. DNA is replicated until there are sufficient enough quantities to conduct analysis. PCR is carried out at high temperatures using a DNA polymerase enzyme from Thermus aquaticus, a bacterium that lives in hot springs. As a result of PCR, very small samples of DNA from crime scenes (e.g. semen, blood, or long-dead tissue) can be replicated and then used for analysis.
4.4.2 State that, in gel electrophoresis, fragments of DNA move in an electric field and are separated according to their size.
4.4.3 State that gel electrophoresis of DNA is used in DNA profiling.
4.4.4 Describe the application of DNA profiling to determine paternity and also in forensic investigations.
● In forensics, DNA profiling can be used to confirm the guilt of suspects. In cases such as suspected murders, burglaries, etc, biological specimens are left behind in the form of blood, semen, hair roots or body cells.
● DNA profiling can be used to determine parentage. A range of DNA from individuals suspected to be related is compared side by side. Since the child inherits half their DNA from their mother and half from their father, the bands of DNA that are not the mother’s must be the father’s.
4.4.5 Analyse DNA profiles to draw conclusions about paternity or forensic investigations.
The outcomes of this analysis could include knowledge of the number of human genes, the location of specific genes, discovery of proteins and their functions, and evolutionary relationships.
4.4.6 Outline three outcomes of the sequencing of the complete human genome.
1. Easier identification of genetic diseases: diseases can be compared with the genes in the HGP database in order to pinpoint mutations that have occurred.
2. Development of gene therapy: allow for the production of new drugs based on DNA base sequences of genes or she structures of proteins coded for by these genes.
3. Give us new insight into the origins, evolution, and migration of humans.
4. A greater understanding of how genes influence human development.
a. How is the structural, physiological and behavioural complexity of humans delivered by so relatively few genes?
b. How do experiences/responses to external factors regulate the expression of particular genes?
4.4.7 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.
4.4.8 Outline a basic technique used for gene transfer involving plasmids, a host cell (bacterium, yeast or other cell), restriction enzymes (endonucleases) and DNA ligase.
The use of E. coli in gene technology is well documented. Most of its DNA is in one circular chromosome, but it also has plasmids (smaller circles of DNA). These plasmids can be removed and cleaved by restriction enzymes at target sequences. DNA fragments from another organism can also be cleaved by the same restriction enzyme, and these pieces can be added to the open plasmid and spliced together by ligase. The recombinant plasmids formed can be inserted into new host cells and cloned.
● The vectors used for the gene transfer are plasmids isolated from bacteria (eg. E.coli).
● The human gene of interest (e.g. the gene for insulin) is extracted in the form of mRNA and converted to DNA using reverse transcriptase.
● Restriction enzymes (e.g. BamHI) are used to cut the circular plasmid at one spot of a specific palindromic sequence.
● The same restriction enzyme is used to cut the human DNA carrying the gene of interest, and the gene is isolated using gel electrophoresis.
● The restriction enzyme used cuts DNA to produce sticky ends.
● The opened plasmids and isolated genes are spliced together using ligase, which creates phosphodiester bonds between the complementary nucleotides left by the sticky ends.
● The recombinant plasmids are then reinserted into a new bacterial host cell and cloned. The genetically modified bacteria will express the human gene to produce the desired protein, which is then extracted and purified for use.
4.4.9 State two examples of the current uses of genetically modified crops or animals.
Examples include salt tolerance in tomato plants, synthesis of beta-carotene (vitamin A precursor) in rice, herbicide resistance in crop plants and factor IX (human blood clotting) in sheep milk.
4.4.10 Discuss the potential benefits and possible harmful effects of one example of genetic modification.
Bt maize – genetically modified maize that contains a bacterial protein called BT toxin that kills corn borers feeding on the maize.
| Beneficial effects | Harmful effects |
| 1. There will be less pest damage and therefore a higher crop yield to help reduce food shortages. 2. There will be less use of insecticide sprays, which are expensive and can be harmful to farm workers/wildlife. 3. Less land is needed for the same amount of crop production, so some of the areas could be reserved for wildlife conservation | 1. The unknown impact of digestion of GM maize could be harmful, because it may result in illness from toxic effects in the body by Bt toxin. 2. There may be a chance of cross-breeding of Bt maize with weeds, resulting in pest-resistant weeds. 3. Insects that are not pests could be killed. Maize pollen could be blown onto wild plants near the Bt maize, and insects like the Monarch butterfly caterpillars might be killed even though they do not feed on the maize. |
4.4.11 Define clone.
Clone: a group of genetically identical organisms or a group of cells derived from a single parent cell.
4.4.12 Outline a technique for cloning using differentiated animal cells.
● In the case of Dolly the sheep:
● Udder cells were taken from a donor sheep. The cells were first cultured in a low nutrient medium in order to “switch off” the active genes and make them become dormant.
● Also, unfertilized egg cells were taken from another sheep and their nuclei were removed using a micropipette.
● The egg cells without nuclei were fused with the dormant udder cells using a pulse of electricity.
● The fused cells developed like zygotes and became embryos.
● The embryos were implanted into the uterus of another sheep who became the surrogate mother.
● After gestation, Dolly was born. Dolly was genetically identical to the sheep whose udder cells were used (because the nuclei of the udder cells were present).
4.4.13 Discuss the ethical issues of therapeutic cloning in humans.
Benefits
● Therapeutic cloning (cloning of stem cells) can be used for therapies that can save lives and reduce suffering.
○ For example, in the development of organs for transplants or treatment of genetic diseases.
● Infertile couples could have their own children.
● Cells can be removed from embryos that have stopped developing, thereby putting to use embryos that would have died anyways.
● Cloning techniques are as safe and reliable as other comparable medical procedures.
Risks
● Human beings may be cloned and produced for the sole intention of supplying “spare parts” for health problems in naturally born human beings. It is an ethical issue.
● Cloning could facilitate “improving” humans by designing and delivering a race of superior people with more favourable genes.
● Cloning techniques are experimental and therefore unreliable, resulting in the death of many embryos since there are still so many unknown factors operating.
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