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Gene R is genetically linked to gene T with a recombination frequency of 0.3. Gene T and gene X assort independently from each other. Your colleague tells you that gene T and gene X reside on the same chromosome. How is this possible?

So I know that R and T are on the same chromosome. T and X assort independently, so they should be on different chromosome. And then due to recombination, X gene crosses over so that it's also on the same chromosome as T and R? Can someone please help me?

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  • $\begingroup$ remember that chromosomes can be quite large - especially if T and X are on opposite sides of the centromere, if they are far enough apart from each other they will appear to assort completely independently from one another. $\endgroup$
    – MattDMo
    Commented Nov 18, 2013 at 4:39
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    $\begingroup$ I don't understand. If two loci segregate independently, it means that the recombination frequency equals 0.5, isn't it? In which condition they might or not be on the sea chromosome. But as soon as the recombination frequency is lower than 0.5, then the don't segregate independently and therefore, they are necessarily on the same chromosome (except if there is a segregation distorter). Let me know if I'm wrong $\endgroup$
    – Remi.b
    Commented Nov 18, 2013 at 8:29
  • $\begingroup$ homework? If so, use the tag. $\endgroup$
    – fileunderwater
    Commented Nov 18, 2013 at 9:29

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The comments pretty much answer the main concern here. But just to sum up:-

"....T and X assort independently, so they should be on different chromosome...."

Not necessarily. They can be on the same chromosome at such a distance that there is usually atleast one chiasma formation between them, and will undergo independent assortment, i.e. segregation of one gene among the gametes will be independent of the other. They will pretty much act and segregate like mendelian factors. Specifically, if the recombination frequency is 0.5 (50cM or map units' distance on the chromosome), they will assort independently.

"....And then due to recombination, X gene crosses over so that it's also on the same chromosome as T and R...."

This part is a little bit unclear. I assume (correct me if I am wrong) is that you are assuming X,T,R to be alleles, which can be present on only one chromatid and hence recombination might change their distribution and positions relative to one another. But as the question states, they are genes (or specifically gene loci) which are present on both chromatids and their relative position is fixed. The recombination frequencies refer to the recombination of different alleles at this locus.

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