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.