Genetic inheritance in Brassica rapa
Summary of task
Students were asked to perform a multi-generational study on the genetic inheritance of two different traits in the species Brassica rapa, a variety of the canola plant. Students were provided with two sets of seeds that had resulted from crossbreeding two distinctly different parental types of this plant species. The two sets of seeds represented the first- and second-generation offspring of the parent plants (referred to as F1 and F2 hybrids).
Students were asked to investigate plant height and stem colour of the hybrid plants. They were given the following information about the genetic makeup of the parent plants:
- For the gene which controls plant height there are two alleles, (T) which is dominant and is expressed as a tall stem, and (t) which is recessive and is expressed as a shorter stem (dwarfism).
- For the gene which controls stem colour there are two alleles, (P) which is dominant and gives a purple stem, and (p) which is recessive and gives a green stem.
- The parents of the F1 hybrids are homozygous dominant for one trait, and homozygous recessive for the other.
- Plants growing from the F1 seeds can be considered the parents of those growing from the F2 seeds (the F2 hybrids are plants whose parents are genetically the same as the F1 hybrids).
Students were asked to first grow plants from the F1 seeds by following a set of instructions and observe and record their physical traits (phenotypes). They were then asked to determine their genotypes from these observations.
Based on their results for the F1 genotypes, students were then asked to predict the probable genotypes and phenotypes of the F2 generation. After growing and observing the plants grown from the F2 seeds, they were asked to compare their predictions with their actual results and submit a report of the experiment using a given template. Students were provided with the raw data from each class in the year and they were asked to consider the combined results in their analysis. The report template contained section headings and several guiding questions for the discussion.
This task was given at the end of a teaching and learning unit on genetics in which students had learned about the role of genes, chromosomes and DNA in the processes governing genetic inheritance. Students had studied Gregor Mendel’s experiments and they were explicitly taught how to use monohybrid and dihybrid Punnett squares to predict the characteristics of organisms. During the practical lesson they worked in teams of three or four to grow the plants and observe and measure their characteristics. The report was completed as an individual take-home assignment.
Achievement standard
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Annotations
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Annotation 1
Describes roles of genes, genome, chromosomes, histones and DNA in genetic inheritance -
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Describes the roles of gametes and zygotes in mammalian reproduction -
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Explains how Gregor Mendel discovered laws of inheritance through mathematical analysis of inheritance patterns -
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Explains in detail Mendel’s three laws of inheritance
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Explains in detail how Punnett squares are used to predict outcomes of a crossbreeding experiment -
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Formulates quantitative hypothesis supported by theoretical prediction from dihybrid Punnett square analysis -
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Annotation 3
Lists materials and experimental steps used in investigation
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Displays class and year group results in tables
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Shows photographic evidence of group results -
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Compares actual with theoretical results -
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Displays dihybrid Punnett square of F1 generation genotypes, including resulting phenotypes -
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Annotation 4
Explains notation used to represent dihybrid genotypes
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Annotations
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Annotation 1
Explains how dihybrid Punnett square predicts distribution of genotypes and phenotypes of F1 generation -
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Annotation 2
Compares theoretical with actual results of F1 generation -
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Annotation 3
Infers possible cause for discrepancy between actual and theoretical results through deductive reasoning -
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Annotation 4
Displays dihybrid Punnett square of F2 genotypes, including resulting phenotypes -
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Annotation 5
Explains theoretically predicted distribution of F2 phenotypes in terms of probabilities -
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Annotation 6
Compares year group distribution of F2 phenotypes with theoretical prediction -
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Indicates possible genotypes for each phenotype -
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Annotation 8
Concludes close agreement of year group results and theoretical results, acknowledging larger sample size than in class results -
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Suggests valid improvement and possible extensions of given experiment
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Draws general conclusion implicitly acknowledging validity of Mendelian genetics -
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Provides extensive list of sources used to research topic of investigation
