3. Genetics

3. Genetics

3.1 Genes

Nature of science:

  • Developments in scientific research follow improvements in technology - gene sequencers are used for the sequencing of genes.

Understandings:

  • A gene is a heritable factor that consists of a length of DNA and influences a specific characteristic.

  • A gene occupies a specific position on a chromosome.

  • The various specific forms of a gene are alleles.

  • Alleles differ from each other by one or only a few bases.

  • New alleles are formed by mutation.

  • The genome is the whole of the genetic information of an organism.

  • The entire base sequence of human genes was sequenced in the Human Genome Project.

Applications and skills:

  • Application: The causes of sickle cell anemia, including a base substitution mutation, a change to the base sequence of mRNA transcribed from it and a change to the sequence of a polypeptide in hemoglobin.

  • Application: Comparison of the number of genes in humans with other species.

  • Skill: Use of a database to determine differences in the base sequence of a gene in two species.

3.2 Chromosomes

Nature of science:

  • Developments in research follow improvements in techniques - autoradiography was used to establish the length of DNA molecules in chromosomes.

Understandings:

  • Prokaryotes have one chromosome consisting of a circular DNA molecule.

  • Some prokaryotes also have plasmids but eukaryotes do not.

  • Eukaryote chromosomes are linear DNA molecules associated with histone proteins.

  • In a eukaryote species there are different chromosomes that carry different genes.

  • Homologous chromosomes carry the same sequence of genes but not necessarily the same alleles of those genes.

  • Diploid nuclei have pairs of homologous chromosomes.

  • Haploid nuclei have one chromosome of each pair.

  • The number of chromosomes is a characteristic feature of members of a species.

  • A karyogram shows the chromosomes of an organism in homologous pairs of decreasing length.

  • Sex is determined by sex chromosomes and autosomes are chromosomes that do not determine sex.

Applications and skills:

  • Application: Cairns’ technique for measuring the length of DNA molecules by autoradiography.

  • Application: Comparison of genome size in T2 phage, Escherichia coli, Drosophila melanogaster, Homo sapiens and Paris japonica.

  • Application: Comparison of diploid chromosome numbers of Homo sapiens, Pan troglodytes, Canis familiaris, Oryza sativa, Parascaris equorum.

  • Application: Use of karyograms to deduce sex and diagnose Down syndrome in humans.

  • Skill: Use of databases to identify the locus of a human gene and its polypeptide product.

3.3 Meiosis

Nature of science:

  • Making careful observations - meiosis was discovered by microscope examination of dividing germ-line cells.

Understandings:

  • One diploid nucleus divides by meiosis to produce four haploid nuclei.

  • The halving of the chromosome number allows a sexual life cycle with fusion of gametes.

  • DNA is replicated before meiosis so that all chromosomes consist of two sister chromatids.

  • The early stages of meiosis involve pairing of homologous chromosomes and crossing over followed by condensation.

  • Orientation of pairs of homologous chromosomes prior to separation is random.

  • Separation of pairs of homologous chromosomes in the first division of meiosis halves the chromosome number.

  • Crossing over and random orientation promotes genetic variation.

  • Fusion of gametes from different parents promotes genetic variation.

Applications and skills:

  • Application: Non-disjunction can cause Down syndrome and other chromosome abnormalities.

  • Application: Studies showing age of parents influences chances of non-disjunction.

  • Application: Description of methods used to obtain cells for karyotype analysis e.g. chorionic villus sampling and amniocentesis and the associated risks.

  • Skill: Drawing diagrams to show the stages of meiosis resulting in the formation of four haploid cells.

3.4 Inheritance

Nature of science:

  • Making quantitative measurements with replicates to ensure reliability. Mendel’s genetic crosses with pea plants generated numerical data.

Understandings:

  • Mendel discovered the principles of inheritance with experiments in which large numbers of pea plants were crossed.

  • Gametes are haploid so contain only one allele of each gene.

  • The two alleles of each gene separate into different haploid daughter nuclei during meiosis.

  • Fusion of gametes results in diploid zygotes with two alleles of each gene that may be the same allele or different alleles.

  • Dominant alleles mask the effects of recessive alleles but co-dominant alleles have joint effects.

  • Many genetic diseases in humans are due to recessive alleles of autosomal genes, although some genetic diseases are due to dominant or co-dominant alleles.

  • Some genetic diseases are sex-linked. The pattern of inheritance is different with sex-linked genes due to their location on sex chromosomes.

  • Many genetic diseases have been identified in humans but most are very rare.

  • Radiation and mutagenic chemicals increase the mutation rate and can cause genetic diseases and cancer.

Applications and skills:

  • Application: Inheritance of ABO blood groups.

  • Application: Red-green colour blindness and hemophilia as examples of sex-linked inheritance.

  • Application: Inheritance of cystic fibrosis and Huntington’s disease.

  • Application: Consequences of radiation after nuclear bombing of Hiroshima and accident at Chernobyl.

  • Skill: Construction of Punnett grids for predicting the outcomes of monohybrid genetic crosses.

  • Skill: Comparison of predicted and actual outcomes of genetic crosses using real data.

  • Skill: Analysis of pedigree charts to deduce the pattern of inheritance of genetic diseases.

3.5 Genetic modification and biotechnology

Nature of science:

  • Assessing risks associated with scientific research - scientists attempt to assess the risks associated with genetically modified crops or livestock.

Understandings:

  • Gel electrophoresis is used to separate proteins or fragments of DNA according to size.

  • PCR can be used to amplify small amounts of DNA.

  • DNA profiling involves comparison of DNA.

  • Genetic modification is carried out by gene transfer between species.

  • Clones are groups of genetically identical organisms, derived from a single original parent cell.

  • Many plant species and some animal species have natural methods of cloning.

  • Animals can be cloned at the embryo stage by breaking up the embryo into more than one group of cells.

  • Methods have been developed for cloning adult animals using differentiated cells.

Applications and skills:

  • Application: Use of DNA profiling in paternity and forensic investigations.

  • Application: Gene transfer to bacteria using plasmids makes use of restriction endonucleases and DNA ligase.

  • Application: Assessment of the potential risks and benefits associated with genetic modification of crops.

  • Application: Production of cloned embryos produced by somatic-cell nuclear transfer.

  • Skill: Design of an experiment to assess one factor affecting the rooting of stem-cuttings.

  • Skill: Analysis of examples of DNA profiles.

  • Skill: Analysis of data on risks to monarch butterflies of Bt crops.

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