1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Genetics problems- punnett squares

  1. Apr 13, 2008 #1
    I need help with these:

    1. Charles was married once before, & he and his first wife had a child who suffers from cystic fibrosis. His current wife Elaine's brother died of cystic fibrosis. What is the probability that Charles & Elaine will have a baby with cystic fibrosis?

    Let's say A= normal, a= has cystic fibrosis. If Elaine's brother's genotype is aa, that means both parents carry the recessive allele. From here, I am stuck. How do I know whether the parents are both heterozygous or if one parent is homozygous recessive & the other parent heterozygous. How do I know if Elaine is AA or Aa?

    2. T= tall, t= short; Y= yellow, y= green
    A green tall pea plant is crossed with a yellow short plant. Their offspring are as follows:

    77 green short
    81 yellow short
    79 yellow tall
    84 green tall

    What are the genotypes of the parent plants?

    Since there are 4 different phenotypes, I'm assuming that the offspring came from the selfing of a plant with genotype YyTt. I found the phenotypic ratios from this data and they all seem fairly close to each other, almost the same (24%, 25%, 25%, 26%) but don't know where to go from here.
  2. jcsd
  3. Apr 13, 2008 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    You don't know. You have to factor that into your determination of the probability. She could be a carrier or she might be completely unaffected, so you have to work out the probabilities with both choices.

    Unless there's more to the problem that you haven't shown, there's nowhere to go from here. You've found the answer already.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook