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Genetics problems- punnett squares

  • Biology
  • Thread starter colton4286
  • Start date
13
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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.
 

Answers and Replies

Moonbear
Staff Emeritus
Science Advisor
Gold Member
11,349
51
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?
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.

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.
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.
 

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