The boss in your laboratory has just heard of a proposal by another laboratory that genes for eye color and the length of body bristles may be linked in Drosophila. Your lab has numerous pure-breeding stocks of Drosophila that could be used to verify or refute genetic linkage. In Drosophila, red eyes (c +) are dominant to brown eyes (c), and long bristles (d +) are dominant to short bristles (d).
Your lab boss asks you to design an experiment to test the genetic linkage of eye color and bristle-length genes, and to begin by crossing a pure-breeding line homozygous for red eyes and short bristles to a pure-breeding line that has brown eyes and long bristles.
Question: Assume the eye color and bristle-length genes are separated by 28 m.u.. What are the approximate frequencies of phenotypes expected from the cross you proposed in the previous answer?
Answer the approximate frequencies of phenotypes expected(in whole number):
cd+= %
c+d= %
cd= %
c+d+= %
Question: Assume the eye color and bristle-length genes are NOT linked. What are the approximate frequencies of phenotypes expected from the cross you proposed in the previous answer?
Answer the approximate frequencies of phenotypes expected (just use a whole number):
cd+ = %
c+d = %
cd = %
c+d+ = %
To determine the approximate frequencies of phenotypes expected from the cross, we need to understand the principles of genetic linkage and independent assortment.
1. When genes are linked:
If the eye color and bristle-length genes are linked, they are inherited together more often. In this case, we are given that the genes are separated by 28 m.u. (map units).
The approximate frequencies of phenotypes expected from the cross can be determined using a Punnett square:
[cd+ x c+d]
First, let's determine the gametes produced by each parent:
cd+ x c+d
The first parent produces gametes: cd+ and cd+
The second parent produces gametes: c+d and c+d
Now, let's fill in the Punnett square:
| c d+ | c d
----------------------------------------
cd+ | cd+ cd+d+ | cd+ cd+d
cd | cd cd+d+ | cd cd+d
From the Punnett square, we can see the following approximate frequencies of phenotypes:
cd+ = 2/4 = 50%
c+d = 1/4 = 25%
cd = 1/4 = 25%
c+d+ = 0/4 = 0%
Therefore, the approximate frequencies of phenotypes expected from the cross when the genes are linked are as follows:
cd+ = 50%
c+d = 25%
cd = 25%
c+d+ = 0%
2. When genes are not linked (independent assortment):
If the eye color and bristle-length genes are not linked, they will assort independently during meiosis.
The approximate frequencies of phenotypes expected from the cross can also be determined using a Punnett square, assuming independent assortment:
[cd+ x c+d]
Since the genes are independent, we can multiply the individual frequencies of each trait.
cd+ frequency = 2/2 = 100%
c+d frequency = 2/2 = 100%
cd frequency = 0/2 = 0%
c+d+ frequency = 0/2 = 0%
Therefore, the approximate frequencies of phenotypes expected from the cross when the genes are not linked are as follows:
cd+ = 100%
c+d = 100%
cd = 0%
c+d+ = 0%