Year 11 Genetics AS - question in black, notes in red, example answers in blue. NOTE: this question will take longer to answer than the average single NCEA question - it shows you a wide range of population/variation concepts. NOTE all the definitions in the answers.
A population of butterflies lives in a rainy green area. The butterflies are predominantly green in colour, but some are yellow.
a. Explain what advantage being green may provide for the butterflies and how the existing phenotypes may relate to genotypes.
Notes: this question is asking if you know why phenotypes become more predominant (more common) under certain conditions (the green environment during rainy times).
It asks you to relate phenotypes and genotypes: to do this you must DEFINE these two terms and explain how they are linked USING the example (green and yellow butterflies) from the question.
Example answer: In this situation the environment is mainly green because there is plenty of rain for the plants. This means that green butterflies will have an advantage in terms of being hidden from predators by their colour (and therefore not eaten). Yellow butterflies will be at a disadvantage because they stand out against the background green colour. This means that whatever alleles result in the green phenotype will be passed on more frequently and those alleles will become more common in the population over time, meaning the green phenotype will become more common over time.
The genotype of an organism is the pair of alleles that are present for a particular trait.
The phenotype is the physical expression of the genotype (how the organism looks etc)
An allele is a different version of a gene; alleles will have slightly different base sequences but still code for proteins that effect the same trait.
Because there are two phenotypes in this population (green and yellow butterflies) there must be at least two different alleles for the ‘butterfly colour’ gene. For example, the possible dominant allele could be green (with yellow being recessive – only showing in the phenotype if two copies are present). This would give three possible genotypes for the butterflies of GG, Gg and gg, with only gg giving yellow butterflies and GG/Gg resulting in green butterflies.
NOTES: if you were in this class you would have noticed how I constantly flicked back up to the question to check if I had covered everything it asked. The underlining/circling helped me with this.
b. A drought occurs where the butterflies live. Explain what could happen to the frequency of phenotypes in the population.
In a drought the plants would become yellower on average. Thus the yellow phenotype butterflies would now have an advantage over the green butterflies – they would be less visible to predators and more likely to survive to pass on their genes to offspring who would inherit the alleles they carry. Over time (generations) the frequency of yellow butterflies in the population would increase.
c. These butterflies reproduce sexually. Discuss all the possible sources of variation in this population.
There are two sources of variation in sexually reproducing populations such as these butterflies: mutation, which is the source of new alleles, and meiosis, which shuffles existing alleles.
A mutation is a permanent slight change in the base sequence of a gene, which may result in the formation of a new allele. Even asexually reproducing populations have mutation as a source of variation. Mutation can be caused by radiation, certain chemicals or even random copying errors in during DNA replication.
In a sexually reproducing population each individual is the result of the fusion of two gametes, one from each parent. Gametes are produced via a type of cell division called meiosis. During meiosis two events occur that can shuffle existing genes randomly into one of the four gametes produced. The first is crossing over (where sister chromatids from the same homologous pair exchange sections) and the second is independent assortment (where the random alignment of chromosomes along the equator of the cell during the division results in random assignment of chromosomes to each gamete).