Essential idea: Genes may be linked or unlinked and are inherited accordingly.
The diagram shows a gene map of a fruit fly (Drosophila melanogaster) chromosome. All the gene loci occur on the same chromosome and so all will be inherited together, unless crossing over occurs during meiosis: the closer together the loci, the stronger the gene linkage.
Understandings, applications and skills
|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.
|Morgan’s discovery of non-Mendelian ratios in Drosophila.
|Completion and analysis of Punnett squares for dihybrid traits. [Alleles are usually shown side by side in dihybrid crosses, for example, TtBb.]
|Polygenic traits such as human height may also be influenced by environmental factors.
|Calculation of the predicted genotypic and phenotypic ratio of offspring of dihybrid crosses involving unlinked autosomal genes.
|Identification of recombinants in crosses involving two linked genes. [In representing crosses involving linkage, show genotypes as vertical pairs seperated by horizontal lines repesenting the chromosomes.]
|Use of a chi-squared test on data from dihybrid crosses.
[Text in square brackets indicates guidance notes]
Presentation and Notes
The presentation is designed to help your understanding. The notes outline is intended to be used as a framework for the development of student notes to aid revision.
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. (3.1)
Theory of knowledge
The law of independent assortment was soon found to have exceptions when looking at linked genes. What is the difference between a law and a theory in science?