Genetics Lecture 14: Gene Mapping Flashcards
how does the probability of recombination differ wrt genes
increase interlocus distance = increase probability of recombination due to the fact theyre further apart so more likely to crossover
what main principle do three point test crosses use
use principle of increase interlocus distance = increqase recombination, to map the distance and sequence of three linked gene diploid eukaryotes
what is mapping accuracy affected by
chromosome interference reducing the number of DCO and undetected recombinant progeny from even numbered crossovfers
what is ideal interlocus distance to produce most accurate maps
less than 15cM
what is the null hypothesis in two point test crosses
frequency of parental offspring = frequency of recombinant offspring and so expect a 1:1:1:1 ratio of each class.
when do you reject the null hypothesis for a two point test cross
reject if chi^2 critical value is exceeded, the genes do not assort independently and linkage is a possibility –> use p=0.05 for 3df
what does genetic mapping indicate
relative lovation of genes on chromosome
what do recombination frequencies show
they produce a genetic map in centiMorgans (cM) or map units (mu)
does recombination frequency depend on gene config
NO - the RF is the same whether the genes in the double heterozygote/dihybrid are in trans or cis
what gene configurations are there
cis or trans
what is cis gene config
COUPLING eg. wm/w+m+ produces wm and w+m+
what is trans gene config
REPULSION eg.w+m/wm+ produces w+m and wm+
what can you or can you not deduce from an RF value of 50
CANT tell whether the genes are syntenic or non syntenic from a single cross
it is the theoretical limit of observed recombination due to crossing over
what does RF value
syntenic lovi which are linked: recombination occurs by reciprocal crossing over at Pachytene 1 Meiosis
what does value 50 RF syntenic loci which are not linked suggest
genes are so far apart that crossing over causes them to assort independently
what does value 50 RF non syntenic loci suggest
recombination occurs by independent assortment
why does a cross over only result in 50% RF not higher than 50?
because only 2 of the 4 bivalent strands are involved
method of three point test crossing
- work out RF for each pair of loci
2. See which pairs are syntenic ie RF
what to remember in problem sets/exam questions
some loci may be in coupling, others in repulsion
also loci may not be in alphabetical order
which is the rarest recomb cateogry
DCO
discuss the accuracy of gene mapping
between 2 genes, mapping is accurate if distances are
what do mapping functions do
describe relationship between map units and %RF (corrects for effect of multiple crossovers and interference)
interference doesnt have major impact below 7cM where functions are linear
how genetic and physical maps differ
genetic map depends on recombination
recombination differs between sexes in some species but physical map is same for both sexes
recombination differs between parts of chrom
what is the coincidence of coefficience
observed no. DCO / expected no DCO
