Genetic Problem

May 2, 2010 at 12:16 pm #13245

Participant

Before i start, i would like to say Hi and hope to enjoy my time here 🙂

I have always been interested in Boiolgy since an early age and am always on the look out for new problems to solve.

I recently came across this genetic problem and cant find the answer for it, any help would be welcome and i hope to repay you back in the future by helping you 😳

In maize, the development of a purple pigment in the seed depends on the presence of dominant alleles at two unlinked loci. These alleles are called T and R, and a seed which is homozygous for the recessive allele at either locus is white. Two plants, each grown from a white seed were crossed together and all the F1 seed were purple. Give the genotypes of the parents and describe the F2 seeds.

Thanks

Alex

May 2, 2010 at 2:17 pm #99516

JackBean

Participant

That is typical exam on how to determine, whether is phenotype of two plants determined by mutations in the same gene or not.
Simply, both parents are homozygous for the recessive allele, but one of gene T and the other of gene R. Thus, if you cross them, you will get heterozygots in both genes and thus will be the seeds purple.

May 2, 2010 at 4:19 pm #99517

Alexander22

Participant

Thanks for the reply Jack,

How would the F2 look?

May 3, 2010 at 7:58 am #99525

JackBean

Participant

Well, one of your parents must be tt and the other rr, the other gene could be heterozygous or homozygous, but as all F2 are the same, they must be homozygous, thus RRtt and rrTT, so make the Punnett square 😉

May 3, 2010 at 6:20 pm #99529

Alexander22

Participant

Hi again, thanks for the help, but im really struggling, i cant seem to work out the answer, Genetics are my weakness 😥

For the F1, both phenotypes are white right? so the genotype would be tt and rr ? Gametes being t and r therefore genotype being tr which gives purple?

Phenotype Purple Purple

Genotype RRtt rrTT

Gametes RR,Rt,tt,Rt rT,rr,TT,rT

Thats the F2 i get, this right?

that would give me a ratio of 11 Purple : 5 white?????

Thanks

May 3, 2010 at 7:33 pm #99530

JackBean

Participant

R and T are separate genes, so you MUST have BOTH in plants AND in gametes!
So, you can’t have plants like rr or tt and so you can’t have gametes like RR.

As I wrote above, your parents must be RRtt and rrTT and their gametes are thus Rt and rT respectively. So, the offspring can be only RrTt and these are purple, because are heterozygous in both genes 😉

May 3, 2010 at 7:46 pm #99531

Alexander22

Participant

So F1 and F2 looks like this:

Parents
Parent Phenotype White White
Parent Genotype RRtt rrTT
Gametes Rt rT
F1 Genotype RrTt
F1 Phenotype Purple

F2 Generation
Parents
Phenotype Purple Purple
Genotype RrTt RrTt
Gametes RT,Rt,rT,rt RT,Rt,rT,rt

If this is right then i finally understand,

This would give me a ratio of 10 Purple : 6 White

Thanks again Jack, i really appreciate the help my friend 😳

May 3, 2010 at 8:28 pm #99532

JackBean

Participant

I think, it should be 9:7
try it here
http://www.changbioscience.com/genetics/punnett.html

May 3, 2010 at 8:45 pm #99534

Alexander22

Participant

yup, your right jack 🙂

I had Put a Capital R when it should have been small, it was a typo,

Thanks again, that punnet square calculator is very good 🙂

May 3, 2010 at 9:06 pm #99535

JackBean

Participant

Yeah, if you want to confirm your results or not to think, but not if you want to learn it 😉

May 3, 2010 at 9:27 pm #99536

Alexander22

Participant

quote JackBean:

Yeah, if you want to confirm your results or not to think, but not if you want to learn it 😉

so true, i will only be using it to check, i want to learn not cheat my way to the result 😀

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