Genetic Diagrams I - Eye Color of Fruit Fly

A genetic diagram is an useful tool to help us predict the probability of a characteristic of an organism. In the following example, the result of mating between a homozygous white eye fruit fly and a homozygous red eye fruit fly are studied. It is known that the allele for white eye is dominant over the allele for red eye. 

Let W be the dominant allele for white eye. Let w be the recessive allele for red eye. Parent phenotype White eyed Red eyed Parent genotype  WW X ww Gametes W w Possible offspring genotype Ww Offspring phenotype White eyed

Since the two fruit flies are homozygous, their homologous chromosomes has the same alleles. When the homologous chromosomes separate during meiosis, the white eyed fruit fly produces gametes containing alleles W and W, while the red eyed fruit fly produces w and w gametes only. When the two gametes, (one from mother and one from father), fuse during fertilisation, only one type of cell results. The daughter cell contains the allele Ww, and is heterozygous. But W is dominant over w, therefore all the offspring must be white eyed.  

Relating Topics
- Paternal and Maternal Chromosomes
- Random Assortment