If two genes are on the same chromosome, are they always “linked”? What if they are more than 50 cM apart? Then they would follow Mendalian pattern of inheritence. So are they still linked?


The centimorgan is not a measure of the physical distance between gene loci, but a measure of the linkage between loci.

Suppose that you have two genes located adjacent to each other, at the tip of a chromosome. The likelihood of their recombination is almost zero, since it is very unlikely that the two genes will be segregated in a crossover event during meiosis.

In contrast, if you have two genes as far apart from each other, but still on the same chromosome (i.e. they are on opposite tips), their likelihood of recombination is as close to 100%---this means a 1:1 ratio of parental type to recombinant type gamates.

So, if a 0% likelihood of recombination implies that the genes are right next to each other, and a 100% likelihood of recombination implies that they are as far away from each other, we will be able to approximate how far away a gene is from another gene by their likelihood of recombination.

Now, the likelihood of recombination is measured through the calculation of a recombination frequency. Genes that are unlinked give you a 1:1 parental to recombinant type in the offspring, which is a recombinant frequency of 50%. Genes that are 'perfectly linked', i.e. always segregated together, gives offspring of the parental type all the time, so they have a recombinant frequency of 0%.

Having said all that, we are able to associate,

$$\text{0% likelihood of recombination} \implies \text{0% recombinant frequency}\\ \text{100% likelihood of recombination} \implies \text{50% recombinant frequency}$$

And relating that to the loci of two genes on a chromosome,

$$\text{Adjacent genes} \implies \text{0% recombinant frequency}\\ \text{Perfectly Distant Genes} \implies \text{50% recombinant frequency}$$

And if we represent the length of a chromosome as 50 centimorgans, then

$$1\text{ centimorgan} \approx 1\%\text{ recombinant frequency} \approx 1\%\text{ likelihood of recombination}$$

That is to say, the theoretical maximum distance between two genes on a chromosome is the length of the chromosome itself, which is 50 centimorgans.

Note that there are many implicit assumptions in the explanations above. The first is that there are no random fluctuations in calculating the recombinant frequency. The second is that exactly one crossover event takes place during meiosis, and the loci that it acts on is uniformly random. When I say 'likelihood of recombination', I refer to the possibility of a single crossover event between two homologous chromosomes, resulting in a 1:1 parental to recombinant type gamete.


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