GENETICS
GENETICS
3.1 Chromosomes, genes and alleles
3.1.1 State that an eukaryote chromosome is made of DNA and protein.
The names of the proteins are not required, nor is the structural relationship between the DNA and the proteins.
3.1.2 State that chromosomes can be stained to show banding.
3.1.3 State that the chromosome structure and banding can be used to arrange the chromosomes in their pairs.The staining technique is not required.
Karyotyping can be done by using enlarged photocopies of chromosomes.
A Human Karyotype
3.1.4 Describe one application of karyotyping.
3.1.5 Define gene.
3.1.6 Define allele.
3.1.7 Define genome.
3.2 Gene mutation
3.2.1 Define gene mutation.
Mutation is central to the theme ‘Evolution’. Some discussion could come in here about the overlap between frequency of the sickle cell allele and the distribution of malaria.
The terms ‘point mutation’ or ‘frameshift mutation’ will not be used.
3.2.2 Outline the difference between an insertion and a deletion.
No mention of the causes of such events required.
3.2.3 Explain the consequence of a base substitution mutation in relation to the process of transcription and translation, using the example of sickle cell anaemia.
3.3 Meiosis
3.3.1 State that meiosis is a reduction division in terms of diploid and haploid numbers of chromosomes.
The terms haploid and diploid are acceptable at both SSC and AHL
3.3.2 Outline the process of meiosis including pairing of chromosomes followed by two divisions which result in four haploid cells.
The names of the phases are not required nor are any terms already excluded under 1.5 (Mitosis).
Crossing over is not required. Using simple examples of 2 and 3 pairs of chromosomes students may be able to deduce the formula 2n (number of different combinations of chromosomes is gametes is 2n, where n is the haploid number). The formula can then be applied to humans for interest. Students will not be required to know the formula.
The recognition of Down’s syndrome in a person is not required. Translocation of part of chromosome 21 possibly resulting in Down’s syndrome is not required.
3.3.5 State Mendel’s Law of Segregation.
3.3.6 Explain the relationship between Mendel’s Law of Segregation and meiosis.
3.4 Theoretical Genetics
3.4.1 Define genotype.
3.4.2 Define phenotype.
3.4.3 Define dominant allele.
3.4.4 Define recessive allele.
3.4.5 Define codominant allele.
3.4.6 Define locus.
3.4.7 Define homozygous.
3.4.8 Define heterozygous.
3.4.9 Define carrier.
3.4.10 Define test cross.
3.4.11 Draw a Punnett Grid.
3.4.12 Draw a pedigree chart.
3.4.13 State that some genes have more than two alleles (multiple alleles).
3.4.14 Describe ABO blood groups as an example of codominance and multiple alleles.
3.4.15 Outline how the sex chromosomes determine gender, by referring to the inheritance of X and Y chromosomes of humans.
3.4.16 State that some genes are present on the X chromosome and absent from the shorter Y chromosome in humans.
3.4.17 Define sex linkage.
3.4.18 State two examples of sex linkage.
Any species where the female is the homogametic sex can be used, although humans will probably be referred to most commonly.
3.4.19 State that a human female can be homozygous or heterozygous, with respect to sex-linked genes.
3.4.20 Explain that female carriers are heterozygous for X-linked alleles.
3.4.21 Calculate and predict the genotypic and phenotypic ratios of offspring of monohybrid crosses involving any of the above patterns of inheritance.
3.4.22 Deduce the genotypes of individuals in pedigree charts.
3.5 Applied geneticsPast examination questions should be a valuable teaching resource. A simple monohybrid cross can be used in the explanation of 3.3.6 (Law of Segregation and meiosis).
Students should be able to draw and use the Punett Grid in order to predict genotype and phenotype ratios. Examination questions will use the horizontal form of the grid (not the diamond form). Furthermore questions will test a general understanding of monohybrid crosses and so the organisms used maybe unfamiliar to the candidates.
The notation which will be used in examination questions is given below:
Sickle Cell: HbA = normal HbS = sickle cell
Colour blindness and haemophilia: both these conditions are produced by a recessive sex-linked allele on the X chromosome.
Xb and Xh is the notation for the alleles concerned. The corresponding dominant ones being XB and XH.
Blood groups : Phenotype Genotype
O ii
A IAIA or IAi
B IBIB or IBi
AB IAIB
Rh+ and Rh-
Codominance: The main letter should relate to the gene and the suffix should relate to the allele (both uppercase letters) e.g., red and white codominant flower colours they should be represented as CR and CW respectively.
Drosophila and other organisms: a+ for dominant allele and a for recessive allele. For example, vg+ for normal wing and vg for vestigial wing.
Students should be directed to choose letters representing alleles with care to avoid possible confusion between upper and lower case.
3.5.1 Define genetic screening.
3.5.2 Discuss three advantages and/or disadvantages of genetic screening.
Three advantages or three disadvantages or any combination between the two. This may include ethnical issues. Pre-natal diagnosis of genetic diseases, immigration disputes, confirming animal pedigrees, etc.
3.5.3 State that the Human Genome Project is an international cooperative venture to sequence the complete human genome.
3.5.4 Describe two possible advantageous outcome of this project.
It should lead to an understanding of many genetic diseases, to genome libraries and the production of gene probes to detect sufferers and carriers of genetic diseases (e.g., Duchenne muscular dystrophy). It also leads to production of pharmaceuticals based on DNA sequences.
3.5.5 Define clone.
3.5.6 Outline a technique used in the cloning of farm animals.
Early divisions of a fertilised egg produce 8 cells each of which could give rise to an embryo (totipotency). After in vitro processes the 8 resultant separated embryos can be transferred to surrogate mothers (e.g., cattle or sheep) to continue using selected prime animals for the production of more gametes. At present it is used for cloning genetically manipulated animals to produce pharmaceutical biochemicals.
3.5.7 Discuss the ethical issues of cloning human embryos.
Cloning happens normally – monozygotic twins. Some may regard the in vitro production of two embryos from one, to be acceptable – others would see this as leading to the selection of those ‘fit to be cloned’ and visions of ‘eugenics and super-race’.
Many examples can be found of results of breeding domesticated animals and crop plants over hundreds of years. Note also the modern agricultural biological techniques of breeding for disease resistance, increased food production, high yield of milk/wool/protein/etc., breeding plants to select those that can spread to external the range of species.
