Artificial Selection Lab
Purpose:
The purpose of this lab was to attempt to artificially select genes to be expressed in plants. We used a variety of plants expressing different physical traits to be included in our parent generation, then we experimented with various offspring crosses in order to artificially select the traits that the plants would express.
Introduction:
The phenotypes expressed by plants all rely on the genes that the plant carries in its DNA sequence. The genotype (that is, the genes that the plant has in its DNA sequence) is determined by the specific alleles that the parent plants pass down through their gametes to the child organism. For example, if two plants heterozygous for the “tall” gene (which will be considered dominant—T—over the gene for small—t) the child plant will be homozygous tall (TT) one-fourth of the time, heterozygous tall (Tt) one-half of the time, and homozygous short (tt) one-fourth of the time. In this lab, similar questions of heritability were questioned, except concerning multiple phenotypes.
Methods:
Firstly, we began by setting up the plant pots that would house the seeds that would grow the first generation (F1) of plants. We threaded a wicking cord through a hole in the bottom of the plant pot, filled the pot with light planting mix and packed it in gently. In one pot, we planted six purple stem, hairy seeds, in another we planted six non-purple, hairy seeds, and in a third we planted six non-purple stem, yellow-green leaf seeds.
Fast Plants: The Beginninging
Once that was completed, the seeds were covered in a layer of planting mix, the wicks were threaded into the dilute fertilizer reservoir, and each pot was marked with a label indicating what plants were planted there.
Over the next several days, the plants were watered daily and observed for growth. Here’s what happened over those days...
(The first watering) Fast Plant seeds…
(The first watering) Fast Plant seeds…
(The first germination) Put ‘em in the soil and watch ‘em grow… HEY!
Sir, we have achieved growth in all species.
However, our victory was short lived. Non-purple stem, yellow-green leaves was the first to die. (Not to mention that only one seed from each seed type grew…)
But yellow-green soon made an attempt to rebound…
Only to fail once again. RIP in pastas our dear friend yellow-green.
Filled with determination, the hairy and the non-hairy continued to grow.
Only for the worst to happen… Hairless joined the yellow-green family in death. Buuuut on the bright side our purple-stemmed fellow blossomed!
Only the strong survive.
With the memory of our fallen kin fresh in the back of our minds, we had no choice but to continue the experiment with the sole remaining plant our group had… the purple-stemmed, hairy plant. However, in order for us to attempt cross pollination between different plant species to determine the phenotypes of the F2 generation, an outside plant would need to be used to produce seed pods.
As such, we used a living, non-purple, yellow-green leaf plant from another group to facilitate breeding. Using a cotton swab, we gathered pollen from the yellow-green leaved plant and deposited it onto the flowers of our purple-stemmed, hairy plant. After that was done, we waited for seed pods to grow.
Gathering plant sperm has never been more fun.
The soil labels from both plants.
Swab o’ sperm.
After several more days, seed pods were had! If our hypothesis was right, these seeds would have the potential to grow both yellow-leaved, non-purple stemmed plants and purple-stemmed, hairy plants. Depending on how the genes are organized on the chromosome, there would potentially be hairy, non purple-stemmed plants, yellow-green, purple stemmed plants, etc. After removing the seed pods from the plant, we euthanized the plant and allowed the seed pods time to dry out.
The fruits of our labor.
The seeds after a weekend of drying out.
RIP our sole survivor.
Once we gave the seeds time to dry, we repeated the earlier steps of preparing a plant pot, and planted six of the seeds from our seed pods in the prepared pot. After that, we gave our F2 generation time to grow and develop.
Preparing a new plant pot for a new generation of survivors!
Unfortunately, however, the seeds never grew, and we had no F2 generation to analyze. And thus, the reign of the purple, hairy-stemmed plant came to an end.
A graveyard of seeds.
Data:
Because the F2 generation of plants wasn’t able to germinate, there isn’t exactly much data to be gleaned from this lab. We cannot determine how the phenotypes relate to their position on the gene because we could not determine which plants would grow from a Purple Stemmed, Hairy x Non-Purple Stem, Yellow-Green Leaf testcross. Even within the first generation, the fact that only one plant grew curtails any comparison that could be made between F1 plants. As such, we can only really hypothesize as to what plants the aforementioned testcross would be able to produce. If each of the characteristics of the plants follows simple Mendelian dominance, whichever is dominant in height, leaf color, hairiness or lack thereof, and stem color would occur in a 3:1 ratio favoring the dominant trait, provided that the dominant traits were heterozygous. If all the dominant traits were homozygous, the F2 generation would show only show dominant traits. If the genes for specific traits were linked, however, most of the plants would look like the F1 generation, but a few would have characteristics of both parent plants. The offspring would appear in a 9:3:3:1 ratio, favoring plants that appear most similar to the F1 generation.
Discussion:
Altering the environmental conditions would affect the rate of survival for plants with certain characters. In our experiment the purple stem hairy plant was better equipped to survive in these conditions. For example, in harsh conditions hardier plants will survive. With artificial selection one can eliminate a trait. If we chose to remove the hairy characteristic of the plants we are able to do this by breeding the two non hairy plants and leaving the purple hairy stem out of the cross for several generation until we don't see that trait anymore.
Conclusion:
In the first generation of plants we planted three seeds of all three types of plants: purple stem hairy seeds, non-purple stem yellow green leaf, and non-purple hairless seeds. At first we started with dirt, then we planted the offspring of the F1 generation. For the F2 generation we planted 6 seeds and unfortunately none grew. We predicted that the reason why the F2 generation couldn't reproduce was because the lack of water, too much water, not enough light from the UV, too much light from the UV, mot enough nutrients in the soil, and competition between the 6 seeds. We also believe that the seed pots that were placed at the corner of the tray were at a disadvantage because they didn't get as much light to grow. However, our F2 generation of plants were in the middle of the tray, so whether or not that is an accurate hypothesis is indeterminate.
References:
Unless otherwise stated, all information related to artificial selection has been synthesized from Jane Reece’s Campbell Biology AP Edition Textbook and the BioFax! Artifical Selection Lab from Flinn Scientific Inc.
Paraphrased procedures as shown in the Methods section have been synthesized from procedures found in BioFax! Artifical Selection Lab from Flinn Scientific Inc.
- All pictures are original from the in-class experiment.
