10. Genetics and evolution
10. Genetics and evolution
10.1 Meiosis
Nature of science:
Making careful observations—careful observation and record keeping turned up anomalous data that Mendel’s law of independent assortment could not account for. Thomas Hunt Morgan developed the notion of linked genes to account for the anomalies.
Understandings:
Chromosomes replicate in interphase before meiosis.
Crossing over is the exchange of DNA material between non-sister homologous chromatids.
Crossing over produces new combinations of alleles on the chromosomes of the haploid cells.
Chiasmata formation between non-sister chromatids can result in an exchange of alleles.
Homologous chromosomes separate in meiosis I.
Sister chromatids separate in meiosis II.
Independent assortment of genes is due to the random orientation of pairs of homologous chromosomes in meiosis I.
Applications and skills:
Skill: Drawing diagrams to show chiasmata formed by crossing over.
10.2 Inheritance
Nature of science:
Looking for patterns, trends and discrepancies - Mendel used observations of the natural world to find and explain patterns and trends. Since then, scientists have looked for discrepancies and asked questions based on further observations to show exceptions to the rules. For example, Morgan discovered non-Mendelian ratios in his experiments with Drosophila.
Understandings:
Gene loci are said to be linked if on the same chromosome.
Unlinked genes segregate independently as a result of meiosis.
Variation can be discrete or continuous.
The phenotypes of polygenic characteristics tend to show continuous variation.
Chi-squared tests are used to determine whether the difference between an observed and expected frequency distribution is statistically significant.
Applications and skills:
Application: Morgan’s discovery of non-Mendelian ratios in Drosophila.
Application: Completion and analysis of Punnett squares for dihybrid traits.
Application: Polygenic traits such as human height may also be influenced by environmental factors.
Skill: Calculation of the predicted genotypic and phenotypic ratio of offspring of dihybrid crosses involving unlinked autosomal genes.
Skill: Identification of recombinants in crosses involving two linked genes.
Skill: Use of a chi-squared test on data from dihybrid crosses.
10.3 Gene pools and speciation
Nature of science:
Looking for patterns, trends and discrepancies - patterns of chromosome number in some genera can be explained by speciation due to polyploidy.
Understandings:
A gene pool consists of all the genes and their different alleles, present in an interbreeding population.
Evolution requires that allele frequencies change with time in populations.
Reproductive isolation of populations can be temporal, behavioural or geographic.
Speciation due to divergence of isolated populations can be gradual.
Speciation can occur abruptly.
Applications and skills:
Application: Identifying examples of directional, stabilizing and disruptive selection.
Application: Speciation in the genus Allium by polyploidy.
Skill: Comparison of allele frequencies of geographically isolated populations.
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