Calculating Genetic Distance Between Loci Of Genes

How do I calculate distance between two linked genes?

When genes are close together on the same chromosome, they are said to be linked. That means the alleles, or gene versions, already together on one chromosome will be inherited as a unit more frequently than not.We can see if two genes are linked, and how tightly, by using data from genetic crosses to calculate the recombination frequency.By finding recombination frequencies for many gene pairs, we can make linkage maps that show the order and relative distances of the genes on the chromosome.linkage map (also known as a genetic map) is a table for a species or experimental population that shows the position of its known genes or genetic markers relative to each other in terms of recombination frequency, rather than a specific physical distance along each chromosome. Linkage maps were first developed by Alfred Sturtevant, a student of Thomas Hunt Morgan.A genetic map is a map based on the frequencies of recombination between markers during crossover of homologous chromosomes. The greater the frequency of recombination (segregation) between two genetic markers, the further apart they are assumed to be. Conversely, the lower the frequency of recombination between the markers, the smaller the physical distance between them

How is distance mapped in genetics?

Genetics map can be built on:Gene mapping ( based on crossing-over frequency): you re , for example, interested in knowing the distance of two genes from each other. You may calculate the rate of their co-segregation. What I mean by that? If two genes are on the same chromosome and are quite close or very close, they re mo likely to segregate togheter when crossing-over occurs ( Cause, as you know, in the crossing over, there re some hot spot where a locus of the chromosome is swapped with a locus on the homologous chromosome during meiosis I ). If they re on the same chromosome but very far or even on two different chromosome, they would probably no segregate togheter. ( cause they re located in Teo different “ crossing over hot spot ). Of course, it may happen that a double-crossing over occurs on the same position ( so the area undergoing the crossing over is swapped on the homologous crossing over and than “ swapped back “ in its original position, this may lead to an underestimation of the distance of two genes. This event is taking into account when genetics map built by using this method, are applied. In this process, the distance is reffered as centimorgan. That is distance between chromosome positions (also termed loci or markers) for which the expected average number of intervening chromosomal crossovers in a single generation is 0.01.Physical mapping: Genome is shattered in hierarchically smaller pieces. Characterization of each single pieces flowed by the riassempbling of each single pieces back, allows researcher to infere about the distance betweeneach single shattered spieces.Gene mapping - Wikipedia

How do I calculate no. of genotypes for X linked gene with 'n' alleles?

A human female has 2x chromosomes. So if you have n alleles of a gene. Then one chromosome would contain one of the n and the second would contain any of the n genes. (For e.g. if one X chr has A allele then the other can have A or B or C or...... ). So for allele A on first chr we have 'n' options for filling the second chromosome, for allele B on first chr we have 'n-1' options for filling the second chromosome (because BA = AB so we'll count only AB which we've done before and hence n-1 options) and subsequently for the rest alleles. Now if you know a little maths then n + n-1 + n-2 + ..... + 1 = n(n+1)/2 So the number of genotypes would be (n(n+1))/2let's try and verify.we know that for 2 alleles we have 3 genotypes i.e. AA Aa aa. Putting n=2 in the above question, we get 3 as answer. Similarly for 3 as well!!. Hope this clears your doubt!!Edit: I didn't consider males.. sorry guys.. ;P. For them though it's easy. As they'll have only one X chr, so the number of genotypes would be n only viz Ax Bx....... where x represents no gene present on Y chr.Basic mathematical proof is simpler. We've 2 places to filled by n objects (therefore: nC2). But as options of the form AA are also valid. So the total number comes out to be (nC2 + n).

How are recombination frequencies calculated?

In drosophila, recombination frequencies can easily be estimated using information from crosses involving many flies. A dihybrid Test cross like the one shown below can be done, in a test cross a double homozygote recessive is crossed with a double heterozygote. (The double heterozygote is obtained through the mating of a true breeding double recessive and a true breeding double dominant)If the two loci assorted independently, then we would expect to observe a 1:1:1:1 ratio between the 4 possible phenotypes. And if the two loci were totally linked, with no recombination occurring, we would see a 1:1:0:0 ratio. The recombination frequency is :Often in human disease studies, we wish to establish linkage between a marker (a SNP for example) and a disease locus. In this scenario we are not so fortunate as to have such large numbers of offspring and we often do not have fully “informative” matings. This can get very complex and I encourage you to read about parametric linkage analysis if you are interested.