In fruit flies, eye color is determined by a single gene with two different versions or alleles: a dominant allele for red eyes (R) and a recessive allele for white eyes (r). If both parents have red eyes, there are a few ways that they could produce a purple-eyed offspring.
One possibility is that each parent is heterozygous, meaning they have one copy of the dominant R allele and one copy of the recessive r allele. When they produce offspring, each parent has a 50% chance of passing on either the R or r allele. If both parents pass on an R allele, the offspring will have red eyes. However, if each parent passes on an r allele, the offspring will have white eyes. If one parent passes on an R allele and the other passes on an r allele, the offspring will be heterozygous like their parents and will have red eyes. This is because the R allele is dominant over the r allele, meaning that even if an individual has only one copy of the R allele, it will still produce red eyes.
However, if the purple-eyed offspring is produced, it must mean that one of the parents had a mutation or a new allele that caused the appearance of a purple eye color. This can happen due to a variety of factors, such as spontaneous mutations or genetic recombination. In this case, the purple-eyed allele would be dominant over both the R and r alleles, meaning that if an individual has even one copy of the purple-eyed allele, it will produce a purple eye color. Therefore, if both parents are heterozygous for the purple-eyed allele, they have a 25% chance of producing a purple-eyed offspring.
In summary, two red-eyed fruit flies can produce a purple-eyed baby if one or both of the parents carry a dominant allele for purple eyes, which is not the typical genetic makeup of fruit flies. However, this scenario illustrates the basic principles of dominant and recessive alleles, which determine how traits are passed down from generation to generation.