## Mendel Genetics

Genetics as it applies to evolution, molecular biology, and medical aspects.

biology_06er
Coral
Posts: 260
Joined: Tue Mar 21, 2006 9:33 am

### Mendel Genetics

Greetings

I have a question in my text book which gives the working out of how to do it but I still cant work out how to do it though...I have tried punnett squares and keep mucking them up..Could someone please explain how to go about answering the question sorta by step by step if possible

...Suppose we cross a trihybrid with purple flowers and yellow round
seeds (hetrozygous for all 3 genes) with a plant with purple flowers and green, wrinkled seeds(hetrozygous for flower colour but homozygous recessive for the other 2 characters). Using mendelian symbols our cross is PpYyRr X Ppyyrr. What fraction of offspring from this cross would be predicted to exhibit the recessive phenotypes for at least 2 of the 3 characters??

...So they start of by listing all the genotypes that fulfill this condition: ppyyRr, ppYyrr, Ppyyrr, PPyyrr and ppyyrr..this is the only part I am able to do ...and then have no clue what to do after...they talk about calculating the probability by xing together the individual probabilities for allele pairs..I understood the dihybrid example so don't know why I don't get this!!!!

...or better yet for the ppyyRr they have written 1/4(probability of pp)x1/2(yy)x1/2(Rr)...can you explain how to do just that one so I can work out the others

Thanks
biology_06er

Cloak & Dagger
Garter
Posts: 5
Joined: Mon Apr 10, 2006 2:43 pm
Hi

For the flower colour you have 4 possible products from the cross. These are PP, Pp, pP and pp so the probability of getting a pp is 1/4. Thats where the probability of pp comes from in the last bit you wrote.

For the other two genes there are only 2 products (Yy or yy and Rr or rr). So the probability of any of these is 1/2.

So to get ppyyRr there is a 1/4 probability of getting pp, a 1/2 probability of yy and a 1/2 probability of getting Rr. Multiplying these together gives 1/16 which is the probability of getting the whole genotype.

Do this for all 5 genotypes and add up the probabilities to get the proportion with at least two recessive traits.

biology_06er
Coral
Posts: 260
Joined: Tue Mar 21, 2006 9:33 am
Hi cloak and dagger

Thank you SOOOO much...ur a life saver!!!!!!..so simpleish now...

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