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bstone · Dec 11, 2004, 10:35 PM

I'm studying for a college biology exam. About 30% of it is going to be mapping genes. Giving questions such as:

A pea plant is crossed with a recessive dwarf pea plant. What will the phenotypic and genotypic ratio of offspring be:
a. if the tall plant was TT
b if the tall plant was Tt

I am rather stumped. I know TT is dominant and Tt is recessive, but being able to actually map it is confusing me. I have read the chapter over and over and the study guide, but I am at a loss. Anyone know of a simple, easy guide? I did some google searches buy came up empty.

Thanks.

Gene Jockey · Dec 11, 2004, 11:09 PM

Google "Punnett Square".

The "I feel lucky" link should do it.

If you're still confused, hit me with a PM and I'll walk you through it. I've got an MS in Molecular Biology, so the info ought to be floating around upstairs somewhere

Phat Bastard · Dec 11, 2004, 11:48 PM

You might have already gotten the answer to this, but here's what my 3/4 of a Ph.D. in Biochemistry says:

case a: tall plant = TT, dwarf plant = tt (recessive)

Here's your Punnett Square (the spacing is off, just move each small t to line up with a Tt)

t t
---------
T | Tt Tt
T | Tt Tt

Genotypic ratio- Tt:all others = 4:0
Phenotypic ratio- Tall:dwarf = 4:0, assuming one allele is sufficient for dominance (safe bet for intro genetics), so phenotype(Tt) = phenotype(TT)

case b: tall plant = Tt, dwarf plant = tt

t t
---------
T | Tt Tt
t | tt tt

Genotypic ratio- Tt:tt = 2:2 = 1:1
Phenotypica ratio- Tall:dwarf = 2:2 = 1:1

I hope that helps!

tinman831 · Dec 12, 2004, 12:00 AM

Your logic isnt quite right, TT is not dominant and Tt is not recessive. The 'T' allele is dominant over the 't' allele and the 't' allele is recessive to the 'T' allele. What phat B said about punnet squares is what you would do to solve your problem.

Komisar · Dec 12, 2004, 12:13 AM

Tinman is correct you can have a dominat person who has the genotype of Tt it does not have to be TT unless it is stated that the organism is heterozygous of homozygous

bstone · Dec 12, 2004, 11:46 PM

I did a google search on punnett squares and I got this web site:
baby steps to bio

I am very impressed with it and even tho geared for seventh grades it does a good job in explaining it to me, taking an intro to bio course. I am happy. Thanks everyone

Rev-O · Dec 13, 2004, 01:18 AM

Welcome to the real world! The first time (notice I said 'the first time') I took Calculus (the math majors version... hey, I was a stupid art major, I didn't know better), I drew a big smiley face on the tracing the curve problem. It was beautiful. And it was about one quarter of the test.

The lesson I learned from this: never take a calc class taught by a freakin' grad student from India! WTF is tan tayta?! Yate?! Yate?! WTF is yate?! Sure, if i called the psychic friends network, I woulda figured out that 'tan tayta' was tangent of theta, and yate was the number eight. Instead, I spent my freshman afternoons drinking.

In my defense, I re-took calc for math majors my senior year and did better than the math majors. The grad student teaching was somewhat perplexed... "So... you're an art major?"

Yah, so I can vaguely remember the first order derivative of freaking anything...

Phat Bastard · Dec 13, 2004, 09:28 AM

Thanks for the incredibly irrelevant story.

voyageur · Dec 13, 2004, 01:31 PM

Things must have changed a bit since I took genetics, back in the stone age. "Mapping genes" back then meant trying to map the relative positions of a set of genes on a chromosome, or whether they were on the same chromosome at all. This was detemined experimentally by breeding multiple generations of fruitflies with certain traits and observing the frequency with which each trait sorted from the other in the next generation.

f1000 · Dec 13, 2004, 01:43 PM

Voyageur's right -- the original question has little to do with gene mapping.

Rev-O · Dec 13, 2004, 11:25 PM

Originally posted by Phat Bastard:
Thanks for the incredibly irrelevant story.

Y'know, you're right. What the hell was I thinking? My post had zero to do with this thread. Even by my standards I was being way too tangental. It even sucked as a thread derail. I'm gonna blame it on my evil twin. Sorry. My bad.

Gene Jockey · Dec 14, 2004, 12:52 AM

Originally posted by f1000:
Voyageur's right -- the original question has little to do with gene mapping.

Yes, well, I figured we'd tackle one problem at a time.