Help with Science Fair Thoughts (urgent!!!)

[glridgel7]

Hello. This is my first time using the site. I want to know if you can help me with my lab report. Here's what i have so far:

Research Question (I know my wording for my question is off. IDK how to get my topic off in the correct format.)
What is the effect of genetic crossing, homotrophic dominant and homotrophic recessive, on the height (cm) of the offspring of Raphanus sativus?

Purpose
The purpose for doing this project was to see if I could eliminate the recessive gene (short) from the genotype of the successive offspring. If this could occur in radishes, could it occur in humans?

Hypothesis (I can't figure out how to format this, either.)
If the genetics are crossed, homotrophic dominant with homotrophic recessive, then the height (cm) of the Raphanus sativus will be tall, reflecting the dominant trait.

Rationale (I havent written down my research yet)
According to

Variables (I'm confused about negative and positive control groups)
Independent Variable is Genetic Crossing
Control Group (negative)is Radishes without Genetic Cross
Number of Trials is 1
Number of Test Subjects is 10
Dependent Variable is Height (cm)
Constants are Type of Radishes; Light; Water

Materials (There aren't many similar projects that grow generations of radishes, so I dont know exactly what i need)
Name Function
10 radish seeds (Raphanus sativus) The radish plants are the test subjects. 2 of the radish plants are functioning as the negative control group.
Planting Tray or Pots

Please help me. Thanks.

[heatherL]

Hello and welcome to Science Buddies!

It's nice to see a student with an interest in genetics. I want to clarify a few things with you that will hopefully help you.

First, I'd like to clear up the terminology. If you are referring to the genetic makeup for the height gene, I think you mean "homozygous" rather than "homotrophic." "Homozygous" means there are two alleles that are the same at the "locus" (location) for that gene. "Heterozygous" would mean there are two different alleles.

A couple things to consider before doing your crosses:
(1) Do you know that your plants are homozygous? If not, you are performing something called a "test cross," which would allow you to determine whether your tall plants are homozygous or heterozygous. You can read more about test crosses here: http://en.wikipedia.org/wiki/Test_cross
(2) Do you know that there is only one gene for height in radishes? This is true for pea plants, but you may need to check in your research whether there is only one gene for height in radishes.

For your purpose, I want to clarify that you will not be eliminating the recessive gene with just one cross. We know that a homozygous dominant (TT) cross with a homozygous recessive (tt) will generate all heterozygous offspring (Tt). Those heterozygotes will all display the dominant trait (tall), but they will carry the recessive allele. If you cross the offspring with one another (Tt x Tt), 1/4 of their offspring will be homozygous dominant (TT), 1/2 will be heterozygous (Tt), and 1/4 will be homozygous recessive (tt). In the homozygous dominant plants, you will have eliminated the recessive gene. But it takes two crosses to do this, and you will not be able to distinguish the homozygous tall plants from the heterozygous tall plants just by looking at them. To do that, you would need another test cross!

I am having trouble helping you develop a control group because I'm not sure what you are testing. If you want to do a test cross to see if radishes follow a single-gene pattern for height, then you could do a control by crossing two short plants to ensure that they produce only short offspring. If short plants produce offspring of multiple heights, you may be dealing with a trait that is controlled by more than one gene.

I don't want to overwhelm you with information, so let's start there. Take a look at what I've said, and post again (in this thread) with more questions. What exactly do you want to test? Do you want to see whether radishes follow the Mendelian rules of genetics (http://en.wikipedia.org/wiki/Mendelian_genetics)? Let me know, and I'll work with you to get your project going!

Heather

[glridgel7]

Thank you so much for replying. I'm sorry that I haven't been able to respond back earlier, but I have been swamped with homework. Here are some things that I have revised.

Research Question
What is the effect of genetic crossing, artificially selected homozygous dominant versus naturally selected, on the height (cm) of the offspring of Raphanus raphanistrum?

Purpose
The purpose for doing this project was to see if dominant traits could appear more reliably and evidently if the genes were altered.

Hypothesis
If the genetics are crossed, homozygous dominant with homozygous dominant, then the height (cm) of the Raphanus raphanistrum will be taller than a Raphanus raphanistrum that has been naturally selected.

Rationale
The artificially selected homozygous dominant traits should be taller than the naturally selected genetic traits, because the genetics of the artificially selected radishes have been purposely selected to become dominant.

Variables
Independent Variable is Genetic Crossing
Control Group (negative) is Radishes without Genetic Cross
Number of Trials is 1
Number of Test Subjects is 10
Dependent Variable is Height (cm)
Constants are Type of Radishes; Light; Water

I have been looking for research about if there is only one gene pertaining to the height of radishes, but I haven't been able to find any. I have also changed my project some, so it would make more sense. I changed Raphanus sativus to Raphanus raphanistrum to emphasize the fact that I'm using wild radishes. I've changed the incorrect wording. I'm planning to do a couple test crosses to make sure that the homozygous dominant plants will actually be homozygous dominant. I'm still working on research, but I'm getting there. I'm still editing my procedure and collecting materials, so those will be uploaded shortly. Thank you so much for helping me with my project, because you are helping me alot with organizing my thoughts and ideas into wording that makes sense to me and everyone else.

-Gregory

[heatherL]

Hi Gregory,

Thanks for following up!

It looks like you are interested in comparing traits under natural selection compared to artificial selection, which is a fascinating topic. The problem is that I do not think you will be able to demonstrate natural selection (for comparison) in a short time. In order to observe natural selection, there must be a selective pressure on that trait. This means that certain versions of the trait lead to better survival than others. I am not sure that height is under any selective pressure in natural populations of radish plants.

However, do not lose hope! I have suggestions that will still allow you to do a project similar to what you have proposed. If you are truly interested in the power of artificial selection, you can still perform experiments that will select for taller radish plants, and compare the height of your selected offspring to the average height of the original plants (and to plants in the wild, if you are so inclined). This will require two important things: (1) that you perform multiple crosses - maybe three generations - with multiple plants (say, 5 plants per cross), and (2) that height is a quantitative character, that is, a trait controlled by multiple genes.

If height is only controlled by a single gene with two alleles (versions), then you will only see two phenotypes: tall and short , and you should not expect the tall plants to get any taller. If height is controlled by multiple genes in radishes (as it is in humans), then you can expect to get taller and taller plants when you select for height.

Before I address your hypothesis and experimental design (including your control), I would like to ensure that you are on the right track with your project. Here are some suggestions for directions in which you can take the project. Please post again with the choice that most interests you, and I can help you design a solid experiment.

1. Determine (experimentally) whether height is controlled by only one gene or by multiple genes in radishes.
2. Perform artificial selection to grow radishes that are taller than would be expected in the wild. (** This will only work if height is controlled by multiple genes. **)
3. Perform artificial selection on a different trait in radishes that researchers know to be quantitative.

To help you with finding alternative traits, I found a paper that looked at different quantitative traits in wild radishes: http://www.genetics.org/content/180/2/945.full.pdf
This is a scientific paper, so it may have some unfamiliar terms. But you can use it to see some of the different radish traits and how they were measured.

I am looking forward to seeing where you'd like to take this project!

Heather

[glridgel7]

Thank you so much for helping me with this project. This is a HUGE help.

2. Perform artificial selection to grow radishes that are taller than would be expected in the wild. (** This will only work if height is controlled by multiple genes. **)

This is the one I think I would like to do. I'm reading the article you suggested, but I'm still googling terms, so I can understand the article better.

I know I need to update a lot on my question, hypothesis, rationale, etc., but this is just a quick post. Thank you, again, for spending some of your time helping me.

[heatherL]

Hi Gregory,

Glad to be of help! Now that I know the direction you would like to go, I can be more specific with my suggestions.

Initial Measurements
One way to get a hint of whether height is a quantitative trait (controlled by multiple genes) in radishes is to measure the heights of the plants. Once they are full grown, you can measure the plants and see whether height shows continuous variation (you see a lot of heights along a continuum) or discrete variation (like "tall" vs. "short," without the heights in between). However, an experiment will really tell you what's happening.

Experimental Design
To get started, you will first want to do some test crosses. Recall that this means crossing a homozygous recessive (tt) short plant with some tall plants to find out the genotype (genetic makeup) of the tall plants. I suggest doing at least 6 test crosses, meaning you cross a short plant with at least 6 tall plants, so you can find out whether the tall plants are homozygous dominant (TT) or heterozygous (Tt).

If a tall plant is homozygous dominant (TT), you can expect all of the offspring from the test cross to be tall plants (Tt).
If a tall plant is heterozygous (Tt), half of the offspring will be tall (Tt) and half of the offspring will be short (tt).
If your test crosses give you discrete heights, like tall vs. short, or tall vs. medium vs. short, then height is probably controlled by only one gene in radishes. If so, you can describe whether radishes exhibit complete dominance, in which heterozygotes look just like homozygous dominant plants, or incomplete dominance, in which heterozygotes have a height right in between tall and short. If your test crosses give you a range of heights, and especially if you get some that are taller than any of the parent plants, you may be working with a quantitative trait.

Once you find out you are working with a quantitative trait, you can breed tall plants with more tall plants for a few generations to see if you can get even taller plants through artificial selection. Make sure to breed short plants with other short plants as a control.

As I have described it, you may have two research questions:
1) Is height a discrete or quantitative trait in radish plants?
2) Will artificial selection allow the growth of taller radish plants than those found in the wild?

Your variables would be as follows...
Independent Variable: Type of genetic cross
Dependent Variable: Height of the (offspring) radish plants
Control Group: You could measure the height of wild radishes, or use your Short (x) Short cross as a control when selectively breeding tall plants.

I recommend that you do more than one trial, by doing at least three crosses per type of cross.

Please let me know if you have any questions, especially if anything I have said is confusing. Let me know when you are ready for your next step, and I'll be happy to help!

Heather

[glridgel7]

To get started, you will first want to do some test crosses. Recall that this means crossing a homozygous recessive (tt) short plant with some tall plants to find out the genotype (genetic makeup) of the tall plants. I suggest doing at least 6 test crosses, meaning you cross a short plant with at least 6 tall plants, so you can find out whether the tall plants are homozygous dominant (TT) or heterozygous (Tt).

I understand this part, but how will I know if I have homozygous recessive radishes? Do I just pick the shortest out of the first generation?

Gregory

[heatherL]

Hi Gregory,

how will I know if I have homozygous recessive radishes? Do I just pick the shortest out of the first generation?

This is a great question. If height is controlled by a single gene with classical dominance (meaning heterozygotes have the same phenotype as homozygous dominant plants), then you should only have "tall" plants and "short" plants. If that is the case, your "short" plants should be the homozygous recessive ones.

If you see a lot of variation, so that it is difficult to tell which plants are "tall" and which plants are "short," you may be dealing with a quantitative trait. In that case, you may want to try doing selective breeding in both directions: try breeding your tallest plants to see if you can grow them even taller, and try breeding your shortest plants to see if you can grow them even shorter. It would be interesting to see how far away from the natural variation you can get with selective breeding.

I hope this makes sense. Let me know if you have more questions.

Heather

[glridgel7]

Thank you for all of your help for this project. I want to know your last name and the organization you work with, so I may document you in my paper. Thank you so much.

[heatherL]

Hi Gregory,

I'm glad to help, and would love to hear an update on your project!

Because this is a public forum, we are prohibited from providing personal information. However, I am perfectly happy for you to give me credit as "HeatherL, an Expert for Science Buddies" in your project paper.

Good luck, and please let me know how everything turns out with your project.

Heather

[glridgel7]

Thank you for your help with this project. But, I have run into a major problem. I won't have enough time to plant a second generation. I started too late on the first generation. Could you help me figure out how to create another experiment with the heights that I already have collected?

[heatherL]

Hi Gregory,

You can definitely use the data you have already collected. One thing you can do is try to figure out the genotypes of your parent plants based on the heights of your first generation of offspring. This will still allow you to try to determine whether height is (1) a discrete trait controlled by one gene, which would give you either tall or short offspring, or (2) a continuous trait controlled by many genes, which would result in a range of heights in your offspring. Since you had a hard time finding out the genetic nature of height in radishes in your research, this would be an interesting experiment all on its own.

Does this make sense? If you share more details about what you did and what you measured, I can help you interpret your data.

Best,
Heather