Two Drosophila with genotypes AABBCC and aabbcc are crossed to produce a heterozygous offspring, AaBbCc (the gene order is unknown). This AaBbCc Drosophila is crossed with an aabbcc Drosophila, resulting in the offspring below: Number: Phenotype: 600 ABC and abc 200 AbC and aBc 150 ABc and abC 50 Abc and aBC a. Which phenotype represents the double crossovers

The double recombinant individuals are less frequent ---> Abc and aBC

Explanation:

We can know if genes are linked, by looking at the progeny distribution. In a tri-hybrid cross, If individuals, whose 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, meaning that phenotypes appear in different proportions, we can assume that genes are linked in the heterozygote parent.

We can recognize the parental gametes because their phenotypes are the most frequent in the descendants, while the double recombinants are the less frequent. Finally, the simple recombinant gametes are the ones with intermediate phenotypes frequencies.  

In the present example, the phenotypic ratio shows different proportions than the expected ones if they were not linked.

We can recognize the parental gametes in the descendants because their phenotypes are the most frequent,

while the double recombinants are less frequent.

And simple recombinant gametes produced by the cross, which frequencies are intermediate.  

a. Which phenotype represents the double crossovers

The double recombinants are less frequent ---> Abc and aBC

the right side of the picture going down is:

polymerase

protein

transport vesicle

modified protein

the left side of the picture going down is:

dna helix

single-stranded rna

endoplasmic reticulum

ribosome

golgi apparatus

protein released from vesicle

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