3. A population of field mice is observed for coat color. Surviving mice were ra
ID: 33024 • Letter: 3
Question
3. A population of field mice is observed for coat color. Surviving mice were randomly mated and at birth the distribution of coat color in the young mice is:
Phenotype
Genotype
Number
Dark Brown
DD
2,450
Light Brown
Dd
2,100
White
dd
450
Total Observed
5,000
At breeding age, the phenotype distribution among survivors is:
Phenotype
Genotype
Number
Dark Brown
DD
1,960
Light Brown
Dd
1,470
White
dd
180
a.What are the relative fitness values of each genotype? (Relative to the most successful genotype)?
b.What form of dominance does this represent?
c.What is the expected frequency of white mice in litters produced by random mating these survivors?
Phenotype
Genotype
Number
Dark Brown
DD
2,450
Light Brown
Dd
2,100
White
dd
450
Total Observed
5,000
Explanation / Answer
a.
The relative fitness (w) values, for the first population of mice, for different coat colors are as follows:
Dark Brown (DD) = 2,450/2,450 = 1.0
Light Brown (Dd) = 2,100/2,450 = 0.85
White (dd) = 450/2,450 = 0.18
This indicates that on average, every Dark Brown (DD) born produces 1 viable offspring, Light Brown (Dd) born produces viable 0.8 offspring, and White (dd) born produces viable 0.2 offspring.
The relative fitness (w) values, for the second surving population of mice, for different coat colors are as follows:
Dark Brown (DD) = 1,960/1,960 = 1.0
Light Brown (Dd) = 1,470/1,960 = 0.75
White (dd) = 180/1,960 = 0.091
This indicates that on average, every Dark Brown (DD) born produces 1 viable offspring, Light Brown (Dd) born produces approximately 0.8 viable offspring, and White (dd) born produces viable 0.1 offspring.
b.
This represents incomplete dominance type of dominance, since both brown and white are partially expressed in Dd form of phenotype.
c.
In random mating,
If suppose Dark Brown (DD) is mated with White (dd), then we cannot expect any white mice in liltter, since all the litter produced would contain the genotype Dd and exhibit the phenotype Light brown.
If suppose Dark Brown (DD) is mated with Light brown (Dd), then we cannot expect any white mice in liltter, since all the litter produced would contain the genotypes DD and Dd, exhibiting the phenotypes Dark brown and Light brown respectively.
If suppose Light Brown (Dd) is mated with White (dd), then we can expect 50% white mice in liltter, since half all the litter produced would contain the genotype Dd, exhibiting Light Brown phenotype and the other half would contain the genotype dd, exhibiting the phenotype white.
If suppose White (dd) is mated with White (dd), then we can expect 100% white mice in liltter, since all the litter produced would contain the genotype dd, exhibiting the phenotype white.
Thus, expected frequency of white mice varies depending on the type of genotypes involved in mating.
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