i am not able to find easy explanation of this question . Some people says , this is because only one sister chromatid of homologous chromosomes cross over with each other.
The recombination fraction, usually designated as θ (Greek theta), is a measure of the distance separating two loci, or more precisely an indication of the likelihood that a cross-over will occur between them. If two loci are not linked then θ equals
0.5 because, on average, genes at unlinked loci will segregate together during 50% of all meioses. If θ equals 0.05, this means that on average the syntenic alleles will segregate together 19 times out of 20 (i.e., a crossover will occur between them during, on average, only 1 in 20 meioses).
The unit of measurement for genetic linkage is known as a map unit or centimorgan (cM). If two loci are 1 cM apart, a crossover occurs between them, on average, only once in every 100 meioses (i.e., θ = 0.01). Centimorgans are a measure of the genetic, or linkage, distance between two loci. This is not the same as physical distance, which is measured in base pairs (kb – kilobases: 1000 base pairs; Mb – megabases: 1,000,000 base pairs).
A genetic distance of 1 cM is the distance between two genes that show 1% recombination, that is, in 1% of meioses the genes will not be co-inherited and is equivalent to approximately 1 Mb (1 million bases).
When studying the segregation of alleles at two loci that could be linked, a series of likelihood ratios is calculated for different values of the recombination fraction (θ), ranging from θ = 0 to θ = 0.5. The likelihood ratio at a given value of θ equals the likelihood of the observed data, if the loci are linked at recombination value of θ, divided by the likelihood of the observed data if the loci are not linked (θ = 0.5). The logarithm to the base 10 of this ratio is known as the LOD score (Z)—that
is, LOD (θ) = log10 [Lθ/L(0.5)]. Logarithms are used because they allow results from different families to be added together.
For example, a LOD score (Z) of 4 at recombination fraction (θ) 0.05 means that the results, in the families studied, indicate that it is 10,000 (104) times more likely that the disease and marker loci are closely linked (i.e., 5 cM apart)
than that they are not linked. A LOD score of +3 or more would be confirmation of linkage, yielding a ratio of 1000 to 1 in favor of linkage; however, because there is a prior probability of only 1 in 50 that any two given loci are linked, a LOD score
of +3 means that the overall probability that the loci are linked is approximately 20 to 1—that is, 1000 1 1 [ × 50]: . The importance of taking prior probabilities into account in probability theory is discussed in the section on Bayes’ theorem.
Shubham Bhardwaj
In short, the recombination frequency between two genes is equal to the proportion of offspring in which a recombination event occurred between the two genes during meiosis.
The recombination frequency between two genes cannot be greater than 50% because random assortment of genes generates 50% recombination (non-linked genes produce 1:1 parental to non-parental. Thus, the recombination frequency would be non-parental/total --> 1/(1+1) = 50%).
Hi there,
If two loci a/A and b/B are beared on two different chromosomes, alleles will segregate randomly during meiosis which means allele a will segregate equally with allele b and allele B (which will be 50% for ab and aB, and as a consequence the same for AB and Ab). If the 2 loci are on the same chromosome but away enough from each other then segregation will be also random. If the distance gets close then parental combination will be prevalent as the event of recombination gets less frequent with the decrease of intergenic distance.
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