What is the fraction of the offspring?

Question

What is the fraction of the offspring with parents AaBBCCddEE and AaBBccDdEE will have a genotype aaBBCcddEE?

I am trying to figure this out WITHOUT using the punnet square.... so what I know is:
The probability of aa from Aa and Aa = 0.25
The probability of BB from BB and BB = 1
The probability of Cc from CC and cc = 1
The probability of dd from dd and Dd = 0.5
The probability of EE from EE and EE = 1

The answer is 0.125 or 1/8, but I am having a hard time figuring out how to get that answer from the above information. I thought you would add up the probabilities and divide by 25 (since there are 5 probabilities from parent 1 and 5 probabilities from parent 2), but I am getting 0.15 instead of 0.125

Please help!!

Answer 1

The probabilities that you have are absolutely correct.  So you know that you will get BBccEE no matter what.  That probability is 100%.  So, what you really only need to worry about is the probability of the genotype being Aa and Dd.  This makes the problem a little simpler.  As you have determined, it is a 25% chance of the offspring having the genotype Aa, and a 50% chance of the offspring having the genotype Dd.

 

Lets step away from genetics for a moment and think of this as a simple probability problem.  I have two coins, and the probability that I will get heads on each coin is 50%.  The probability that I will get heads on both coins is 25% (given that the options are tails/tails, tails/heads, heads/tails, heads/heads).  To determine the probability of two independent events occuring, you multiply the probability of those two independent events.

 

So, with that in mind, this problem is a case of two independent events, since these two genotypes are not linked.  If you multiply .25 times .50, you will get to the correct answer.  If this problem involved three genotypes that were different, you would be multiplying three numbers.