1. A hypothetical model for polygenic inheritance of flower lobe number in Linan
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Question
1. A hypothetical model for polygenic inheritance of flower lobe number in Linanthus was presented in class. According to this model, alleles at three separate loci (A,a; B,b; C,c) influenced lobe number. Genotype "aabbcc" had a baseline value of 5 lobes, and each capital-letter allele that an individual has at any locus adds one lobe to the baseline value. Suppose that 98% of plants in a Linanthus population have 5 lobes and 2% have 6 lobes. If artificial selection for increased lobe number was applied to this population for two generations, what is the most extreme phenotype that could be produced?
2. The amphipod Hyalella azteca (a small aquatic crustacean) is routinely used to study toxic effects of water pollution. In one experiment, H. azteca genotypes with different alleles at the PGM locus (which codes for the phosphoglucomutase enzyme) were exposed to polluted water containing 37 micrograms per liter of cadmium, a toxic heavy metal. The following table shows the number of animals tested, and the number that remained alive after 70 hours:
PGM genotype: number exposed: number still alive after 70 hours:
A1A1 109 52
A1A2 66 42
A2A2 153 112
Has exposure to cadmium in this experiment caused evolution to occur in H. azteca? If the survivors reproduced by mating at random, how would the genotype composition of their newborn offspring differ from the set of individuals that were present at the start of the experiment?
Explanation / Answer
1. Given that 2% plant population have 6 lobes, means they have one capital letter in their genotype such as Aabbcc or aAbbcc, aaBbcc or aabBcc, aabbCc or aabbCc. The possible number of genotypes is 3 (since Aabbcc is equal to aAbbcc, etc).
If artificial selection has to be done to increase lobe number, we can choose parents with six lobes and self fertilize. If we slect parents with six lobes, the generation offspring can have seven lobes (with maximum two capital letters).
Self fertilization of F1 generation (parents with seven lobes) gives the offspring with 9 lobes (they inherit maximum four capital letters).
Thus, if artificial selection for increased lobe number was applied to this population for two generations, the most extreme phenotype could be produced is flowers with 9 lobes (with four capital letters in their genotype).
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