Probability and Genetics
Fruit Fly Color and
Probability
Fruit Flies

The body color in fruit flies is controlled by two alleles
–
–

Grey is dominant and represented by G
Black is recessive and represented by g
The phenotypes of the flies follow the chart below.
Phenotype
(color)
Genotype
(gene makeup)
Grey
Black
GG, Gg
gg
Sample Punnett Square 1

Cross a homozygous grey fly (GG) with a
homozygous black fly (gg)
Sample Punnett Square 1

All offspring are phenotype grey and genotype Gg
4
 100  100 %
4


So 4 out of 4 possible outcomes
have genotype Gg and
phenotype grey
There is a 100% possibility that
the offspring will be Grey (Gg)
Sample Punnett Square 2

Cross a heterozygous grey fly (Gg) with a
homozygous black fly (gg).
Sample Punnett Square 1

Offspring have two possibilities in this case



Grey (Gg)
Black (gg)
Each outcomes happens 2 times
out of 4 = 50% chance
2
 100  50 %
4
Sample Punnett Square 3

Cross two heterozygous grey flies (Gg)
Sample Punnett Square 3

Offspring have several possibilities in this case



Grey (Gg or GG)
Black (gg)
First explore phenotypes
–
–

Grey happens 3 out of 4 times = 75%
Black happens 1 out of 4 times = 25%
There is a 75% possibility the
offspring will be grey and a 25%
possibility they will be black.
Sample Punnett Square 3

Now let’s explore the probability of genotypes

Gg happens 2 out of 4 times = 50%
GG happens 1 out 4 times = 25%
gg happens 1 out of 4 times = 25%

For the whole example:


–
–
–
25% chance of Black (gg) flies
25% chance of Grey (GG) flies
50% chance of Grey (Gg) flies
Probability



The concept of probability and genetics goes
outside of just flies
For your project you will analyze the
probability for eye color
Use the same techniques used for flies