Genetics Lecture 14: Gene Mapping

Question 1

how does the probability of recombination differ wrt genes

Answer 1

increase interlocus distance = increase probability of recombination due to the fact theyre further apart so more likely to crossover

Question 2

what main principle do three point test crosses use

Answer 2

use principle of increase interlocus distance = increqase recombination, to map the distance and sequence of three linked gene diploid eukaryotes

Question 3

what is mapping accuracy affected by

Answer 3

chromosome interference reducing the number of DCO and undetected recombinant progeny from even numbered crossovfers

Question 4

what is ideal interlocus distance to produce most accurate maps

Answer 4

less than 15cM

Question 5

what is the null hypothesis in two point test crosses

Answer 5

frequency of parental offspring = frequency of recombinant offspring and so expect a 1:1:1:1 ratio of each class.

Question 6

when do you reject the null hypothesis for a two point test cross

Answer 6

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

Question 7

what does genetic mapping indicate

Answer 7

relative lovation of genes on chromosome

Question 8

what do recombination frequencies show

Answer 8

they produce a genetic map in centiMorgans (cM) or map units (mu)

Question 9

does recombination frequency depend on gene config

Answer 9

NO - the RF is the same whether the genes in the double heterozygote/dihybrid are in trans or cis

Question 10

what gene configurations are there

Answer 10

cis or trans

Question 11

what is cis gene config

Answer 11

COUPLING eg. wm/w+m+ produces wm and w+m+

Question 12

what is trans gene config

Answer 12

REPULSION eg.w+m/wm+ produces w+m and wm+

Question 13

what can you or can you not deduce from an RF value of 50

Answer 13

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

Question 14

what does RF value

Answer 14

syntenic lovi which are linked: recombination occurs by reciprocal crossing over at Pachytene 1 Meiosis

Question 15

what does value 50 RF syntenic loci which are not linked suggest

Answer 15

genes are so far apart that crossing over causes them to assort independently

Question 16

what does value 50 RF non syntenic loci suggest

Answer 16

recombination occurs by independent assortment

Question 17

why does a cross over only result in 50% RF not higher than 50?

Answer 17

because only 2 of the 4 bivalent strands are involved

Question 18

method of three point test crossing

Answer 18

1. work out RF for each pair of loci

2. See which pairs are syntenic ie RF

Question 19

what to remember in problem sets/exam questions

Answer 19

some loci may be in coupling, others in repulsion

also loci may not be in alphabetical order

Question 20

which is the rarest recomb cateogry

Answer 20

DCO

Question 21

discuss the accuracy of gene mapping

Answer 21

between 2 genes, mapping is accurate if distances are

Question 22

what do mapping functions do

Answer 22

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

Question 23

how genetic and physical maps differ

Answer 23

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

Question 24

what is the coincidence of coefficience

Answer 24

observed no. DCO / expected no DCO

Question 25

how to work out interference

Answer 25

1-coefficient of interference = 1 - (DCO observed/DCO exoected)

Question 26

what does interference value of 0 mean

Answer 26

no interference; observed DCO = expected DCO