Topic 4 Flashcards
What are the 3 reasons obtaining a map of gene on chromosomes is crucial information?
- Gene position is needed to build complex genotypes needed for experimental purposes
- Knowing position occupied by a gene provides a way of discovering its structure and function (genes position can be used to define it at the DNA level
- Genes present and their arrangement on chromosomes often slightly differ is related species
What is the chromosome map?
2 types of maps?
Shows gene positions (loci)
Distances between loci
Recombination-based map- map the loci of genes that have been identified by mutant phenotypes showing single gene inheritance
Physical maps- show genes as segments arranged along the long DNA molecule that constitutes a chromosome
How do we know if genes are linked?
If genes are linked, parental combinations are overrepresented
Two equally equally frequent non recombinant classes totalling excess of 50 percent
Two equally frequent recombinant classes totalling less than 50 percent
Also means the loci of these genes are on the same chromosome and the alleles on any one homolog are physically joined (linked)
Genes are close together on the same chromosome (linked)
Linked genes do not assort independently but produce a recombinant frequency of less than 50%
Slides 8-27 topic 4
Page 130-131
What is crossing over of genes?
Explains how recombinants are produced when genes are linked
When homologous chromosomes pair at meiosis, the chromosomes occasionally break and exchange parts (crossing over)
Chiasmata are the sites of crossing over
Coupling heterozygote is when normal parents cross over
Repulsion heterozygote is where recombinant parents cross over and make two normals
Multiple crossovers didn’t not impact the overall proportion of recombinants (2 chromatids carry parental info and 2 carry recombinant)
Page 132, 138
Slides 10-27
What is linkage symbolism and terminology?
Cis- 2 dominant, or wild type alleles are present on same homolog
Trans- they are on different homologs
Alleles on same homolog have no punctuation between them
Slash (/) separates 2 homologs
Alleles always written in same order
Genes on different chromosomes shown by semicolon
Page 132
What is the frequency of recombination (RF)?
RF measures the intensity of linkage
In the absence of linkage, RF~50%
Very tightly linked gene, RF~0%
The physical distance between alleles on chromosomes dictates the statistical chance a crossover event happening between the two loci
CO frequency will increase until is reaches 50%, the max genetic distance possible to calculate
RF=number of recombinant gametes/P+R gametes X100 (slide 81)
Slides 27-39
When does crossing over occur?
Takes place at the 4 chromatid stage of meiosis
Recombination is caused by a physical exchange between homologous chromosomes in prophase of the first meiotic division and after S phase
What is the genetic map unit?
Genetic map unit (mu) is defined as the distance between genes for which 1 product of meiosis in 100 is recombinant
Recombinant frequency (RF) of 10.7% is defined as 10.7 mu Mu can also mean cM
Genetic map unit is the percent chance of a crossover between 2 location on a chromosome
Picture top of page 137
Slide 60-61
What is a 3 point testcross?
Trihybrid (triple heterozygote) with a triple recessive tester
8 gamete genotypes possible (2x2x2)
These enable geneticists to evaluate linkage between 3 or more genes and determine gene order (all in one cross)
Gene that is switched in the double crossover classes compared to the parental information is the middle gene
Slides 62-99
How do you add genes on 3 point test crosses and establish gene order?
Count RF values for different recombinant values
Always count the 2 rarest classes TWICE because each represents double recombinants
Page 139-140
Slides 63-100
How can we deduce gene order by inspection?
Doesn’t need recombinant frequency analysis
Typically for linked genes, we have 8 genotypes:
2 at high frequency
2 at intermediate frequency
2 at a different intermediate frequency
2 rare
Only 3 gene orders are possible (each with different gene in middle position)
2 most common phenotypic classes are the parentals, 2 most rare represent double crossovers
Page 141 picture
Are the crossovers in adjacent chromosome regions independent events or does a crossover in one region affect the likelihood of a crossover in an adjacent region?
Crossovers inhibit each other somewhat in interaction called interference
If the cross overs in the 2 regions are independent, we can use produce rule to predict the frequency of double recombinants (that frequency would equal the product of the recombinant frequencies in the adjacent regions
Ex: if v-ct RF value is 0.132 and ct-cv RF value is 0.064, if there’s no interference, doubke recombinants might expect frequency of 0.132x0.064= 0.0084
What is the coefficient of coincidence (coc)?
How do you calculate this and interference?
Interference first quantified by calculating this
It is the ratio of observed to expected double recombinants
COC= observed frequency or number of double recombinants / expected frequency of double of recombinants
Interference = 1 - COC
If you don’t observe any double recombinants, COC=0 and interference is 1 (interference is complete)
What are the phenotypic ratios for: Monohybrid testcrossed Monohybrid selfed Dihybrid testcrossed (independent assortment) Dihybrid selfed (independent assortment) Dihybrid testcrossed (linked) Trihybrid testcrossed (independent assortment) Trihybrid testcrossed (all linked)
Page 144
Monohybrid testcrossed- 1:1
Monohybrid selfed- 3:1
Dihybrid testcrossed (independent assortment)- 1:1:1:1
Dihybrid selfed (independent assortment)- 9:3:3:1
Dihybrid testcrossed (linked)- P:R:R:P
Trihybrid testcrossed (independent assortment)- 1:1:1:1:1:1:1:1
Trihybrid testcrossed (all linked)- P:P:SCO:SCO:SCO:SCO:DCO:DCO
What are the steps in the practical guide to building a genetic map?
A) determining gene order
- Possible gene orders
- The 2 most common are the parentals
- The 2 rare classes are double crossovers
B) determining distance between genes
1. Calculate RF between each pair of genes
