Basic genetics questions?

Okay, so this basically is the same with Mendel's initial experiemt with pea plants. As in the first generation(F1) you only have red flowers, it means that the gene for red flower colour is dominant to the gene for white flower color.

Let's use R for red flower colour and r for white fllower colour; Since in the second generation (F2) you get red:white=3:1, it means that the parent (the F1) was Rr, since this is the only way you can obtain such a result;

So your initial flowers were: red (RR) and white (rr), in F1: you only had Rr (only possible situation, and colour is red, since R is dominant to r) and in F2 you get: RR once (red flowers) Rr twice(red flowers) and rr once (white flowers)- for all possible combinations; Punnet square looks like this:

/ R r

R RR Rr

r rR rr

Now to the questions:

1. to determine the genome of the petunia you use what is classically called a test cross; a test cross is a hybridisation of an organism of unkown genome with a double recessive one; in our case our unknown flower is xy; we cross it with rr; we obtain:

/ x y

r xr yr

r rx ry

our unkown plant can only have 3 possible genomes: RR, Rr and rr;

a. if the result of our hybridisation is all red flowers, it means that x=y=R (same situation as the one we obtained in F1);

b. if the result of our hybridisation is red:white=3:1, it means that one allele must be white (as we obtain one white flower only when we have rr) and the other must be red; this is the same as F2 results discussed up; so x=r and y=R or viceversa, Rr=rR

c. if the result is only white flowers; it means that x=y=r, the only way we can only get white flowers; if either x or y were R, we would have got at least 1 red flower, as the gene is dominant;

it would be good if you memorise the classic monohybridisation results discussed here, they are really simple and handy as far as mendelian genetics is concerned;

2. Question 2 refers to molecular genetics. All genes in eukaryotic multicellular organisms are regulated both at translation and transciption. What I presume the questions asks is how gene expressions is stopped. So in our case, the colour of the flower would most likely be caused by a protein; protein A for red flowers and let's call it protein B for white flowers; in genomes where R is present: RR, Rr; protein A is obviously not synthesized as the flower with RR and Rr genomes are never white; so there must be a step in translation or transcription where expression is stopped;

skipping more general talk let's get back to the question; protein A has a sequence of amino acids; each amino acid is coded by a codon (nucleotide triplet); so in a laboratory you can actually trace the amino acid sequence of the protein and from there, the nucleotide sequence of the mRNA from which it is translated; now you can introduce the mRNA in white plant cell (rr), which does not normally synthesize protein A; after a while:,

a.if you find protein A in the cytoplasm it means that the gene is regulated as transcriptional level, as it encountered no problem going past the translational level;

b.if you do not find protein A in the cytoplasm, it means the gene is regulated at translational level, as the cell recognised the mRNA as aberrant and foreign and did not produce the protein the mRNA coded for;

3. This is really easy now, I anticipated the question at 2. So in red flowers, say you wiill find protein A to be abundant and protein B not at all; in white flowers, protein B- abundant, protein A- not at all; to check the flower colour- protein link, you just have to introduce protein A in a white flower, if it colours red, then there is the link!