Question

In mouse, genes X, Y and Z are on chromosome 2. The map of genes X, Y and Z is:

Answer 1

Answer:

36 individuals xxyyzz

Explanation:

Available data:

• X-Y = 20MU
• Y-Z = 30MU
• Cross: XXYYZZ   x    xyyzz
• F1= XxYyZz
• I = 40%
• 2nd Cross: XxYyZz  x  xxYyzz
• F2 = 1000 individuals

We need to know how many individuals from the F2 are going to be xxyyzz

xxyyzz genotype is the result of a double recombinant event.

First, we need to analyze the given information:

→ Distances between genes

The genetic distance result from multiplying the recombination frequency of each region by 100 and expressing it in map units (MU). So, if we have the genetic distance we can divide it by 100 and get the recombination frequency:

Let us say that region I is the distance between X and Y. And region II is the distance between Y and Z.

GD = genetic distance

RF = Recombination frequency

• GD (X-Y) = RF x 100

20MU = RF x 100

20MU/100 = RF

0.2 = RF (Region I)

• GD (Y-Z) = RF x 100

30MU = RF x 100

30MU/100 = RF

0.3 = RF (Region II)      

Now we have the recombination frequency of Region I and Region II.

→ Interference    

Knowing the interference value will let us know the value of the coefficient of coincidence, which will also lead us to the number of individuals xxyyzz.

I = 40%

I = 1 - CC   ⇒ CC = coefficient of coincidence

Clearing this equation gives us the value of CC

CC = 1 - I

CC = 1 - 40% = 1 - 0.4

CC = 0.6

To calculate the coefficient of coincidence, CC, we can also use the next formula:

CC = observed double recombinant frequency/expected double recombinant frequency

Now, we are interested in knowing the value of the observed double recombinant frequency.

Note:  

• observed double recombinant frequency = total number of observed double recombinant individuals/total number of individuals
• expected double recombinant frequency = recombination frequency in region I x recombination frequency in region II.

So, to get the value of the observed double recombinant frequency, first, we need to get the value of the expected double recombinant frequency. And we can do it because we already have the recombination frequencies in both regions.

expected double recombinant frequency = recombination frequency in region I x recombination frequency in region II

- Recombination Frequency - RI = 0.2

- Recombination Frequency  - RII = 0.3

expected double recombinant frequency = 0.2 x 0.3

expected double recombinant frequency = 0.06 = EDRF

Now that we know the value of the CC and the expected DRF, we can get the observed double recombinant frequency.

CC = observed double recombinant frequency/expected double recombinant frequency

- CC = 0.6

- EDRF = 0.06

CC x expected double recombinant frequency = observed double recombinant frequency

0.6 x 0.06 = observed double recombinant frequency

0.036 = observed double recombinant frequency = ODRF

Finally, as this is a frequency, we need to multiply it by the total number of individuals in the F2 to get the number of observed double recombinant individuals.

- ODRF = 0.036

- N = 1000

Double recombinant individuals = 0.036 x 1000 = 36 individuals xxyyzz.