In fruit flies, red eyes are dominant (R) and white eyes are recessive (r).

Question

– What is the probability that a cross between two heterozygous red-eyed flies will produce a homozygous white-eyed fly?

– What is the probability that a cross between a homozygous white-eyed fly and a heterozygous red-eyed fly will produce a homozygous white-eyed fly?

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Answer 1

To solve the questions, we can use a simple method called a Punnett square, which helps us see the possible combinations of genes from the parents.

1. Cross between two heterozygous red-eyed flies (Rr x Rr):

   • Parent 1: Rr
   • Parent 2: Rr

   The possible combinations of alleles from these parents are:

   • RR (homozygous red)
   • Rr (heterozygous red)
   • Rr (heterozygous red)
   • rr (homozygous white)

   The results can be summarized as follows:

   • 1 RR (red)
   • 2 Rr (red)
   • 1 rr (white)

   So, out of 4 total possible combinations, only 1 (the rr) is for the homozygous white-eyed flies.

   The probability of getting a homozygous white-eyed fly (rr) is: \[ \frac{1}{4} = 25% \]

2. Cross between a homozygous white-eyed fly and a heterozygous red-eyed fly (rr x Rr):

   • Parent 1: rr
   • Parent 2: Rr

   The possible combinations of alleles from these parents are:

   • Rr (heterozygous red)
   • rr (homozygous white)

   The results can be summarized as follows:

   • 1 Rr (red)
   • 1 rr (white)

   So, out of 2 total possible combinations, only 1 (the rr) is for the homozygous white-eyed flies.

   The probability of getting a homozygous white-eyed fly (rr) is: \[ \frac{1}{2} = 50% \]

Summary:

• The probability of two heterozygous red-eyed flies producing a homozygous white-eyed fly is 25%.
• The probability of a homozygous white-eyed fly and a heterozygous red-eyed fly producing a homozygous white-eyed fly is 50%.