Consider a hypothetical scenario in which the genes that determine body color, eye color, and wing type in fruit flies occur on the same chromosome. To determine the order of the three genes on the chromosome, a researcher looks at hundreds of flies and makes these observations: 90% of flies with red eyes have curly wings. 75% of flies with red eyes have black bodies, whereas the remaining 25% have brown bodies. 1% of flies have curly wings, white eyes, and black bodies. Based on these observations, what is the most probable location of each gene on the given chromosome

To know if two genes are linked, we must observe the progeny distribution. In a tri-hybrid cross, If individuals, whos genes assort independently, are test crossed, they produce a progeny with equal phenotypic frequencies 1:1:1:1:1:1:1:1. If we observe a different distribution, that is that phenotypes appear in different proportions, we can assume that genes are linked in the heterozygote parent.

We can recognize the parental gametes in the descendants because their phenotypes are the most frequent, while the double recombinants are the less frequent. And simple recombinant gametes produced by the cross, which frequencies are intermediate.

By comparing the parental and the double recombinant we will realize which is the allele that changes in position. The position of this gene is in the middle of the other two genes because in a double recombinant only the central gene changes position in the chromatid.

So, according to this knowledge and the available information in the problem we can estimate the position of the genes in the chromosome.

Phenotypic proportions:

90% of flies with red eyes have curly wings (Parental)

75% of flies with red eyes have black bodies (Simple recombinant)

25% of flies with red eyes have brown bodies (Simple recombinant)

1% of flies have curly wings, white eyes, and black bodies (Double recombinant)

90% of individuals are red-eyed and curly-winged. This is a high frequency, which means that this is the parental phenotype. The other parental phenotype must be white eyes and straight wings.

Flies with curly wings, white eyes, and black bodies represent the lower proportion (1%) so this phenotype must be the double recombinant.

Flies with red eyes and black bodies, and flyes with red eyes and brown bodies are intermediate in proportion, so they must be simple recombinant phenotypes.

Parental)

Red, Curly

White, Straight

Double recombinant)

White, Curly, Black

Red, Straight, Brown

Simple recombinant)

Red, Black

Red, Brown

In the chromatid, there are two regions, Region 1 (from the first gene to the gene in the middle), and region 2 (from the gene in the middle to the gene of the other extreme).

The fact that both simple recombinants are red, tells us that the eye color gene and the body color gene are together, and the recombination occurred either in region 1 or 2.

We know that the double recombinant is White, Curly, Black. We also know that the eye color gene and the body color gene are together, so one of these two must be in the middle.

Parentals are red and curly, which means that the eye color gene and wing shape gene are also together. This leads us to assume that the eye color gene is in the middle.

Finally, the fact that the double recombinant is White, Curly, Black, and knowing that the eye color is in the middle and that both the extreme alleles change position, we can assume that the parental Red, Curly is also black, while the other parental is white, straight and brown. So:

Parental)

Curly ------ Red ------Black (90%)

Straight ----- White ----- Brown

Simple recombinant)

Curly -----White-----Brown

Straight-----Red -----Black (75%)

Curly----Red-----Brown (25%)

Straight ----White-----Black

Double recombinant)

Curly-----White------Black (1%)

Straight----Red------Brown