chapter 5 Flashcards
what are syntenic genes and how can they produce recombinant chromosomes
syntenic genes are genes on the same chromosome
alleles of syntenic genes can be reshuffled when crossing over occurs to produce recombinant chromosomes
linked genes and how do their alleles sort
syntenic genes that are close together, there alleles cannot sort independently
genetic linkage mapping and how can linkage be estimated
plots the positions of genes and their relative distances from each other on chromosomes
estimated by considering phenotypic ratios in offspring
what does independent assortment (dihybrid cross) predict
each type of gamete will have a frequency of 25%
how do linked genes assort
they do not assort independently
-parental phenotypes are more common than recombinant phenotypes when genes are linked
complete vs incomplete genetic linkage
complete: all gametes contain parental chromosomes (50% and 50%)
incomplete: parental gametes are 60% and recombinant gametes are 40% for these genes
recombination frequency formula
r = (# of recombinants)/(total # of progeny)
what is recombination frequency positively correlated to
physical distance between 2 genes on a chromosome (the larger the distance, the larger the recombination frequency)
recombination frequency max value and why
max: 0.5 (50%)
why? 50% frequency of non parental phenotypes represents inheritance through mendel’s laws (if each phenotype is equal = looking at unlinked genes)
How to tell/quantify if there is chance for genetic linkage?
-if genes are close together on chromosome = rare chance of recombination, short genetic distance
-if genes are far apart on chromosomes, large genetic distance, lots of recombination/large amount of recombinant offspring
crossing over timeline (Morgans Crossing over hypothesis)
- beginning of prophase 1: Homologous chromosomes normal
- early prophase 1: synaptonemal complex formation (ready to cross over)
- end of metaphase 1: crossover completed
- end of telophase 2: gamete formation, 4 gametes, creates parental and non parental gametes after segregation
Two-point test-cross analysis
gene linkage of autosomal genes can be interpreted this way, since homozygous recessive parent contributes only recessive alleles - the alleles from the dihybrid parent can be examined
what is genetic linkage
a physical relationship among genes located near one another on a chromosome
how often does recombination occur in linked genes
less than 50% of the time, more than 50% of the gametes contain parental allele combination
how does recombination frequency vary among linked genes
in proportion to distance between them
First genetic linkage map and units
Alfred Sturtevant realised variations in recombination frequency could be used to determine genetic maps for genes
map units (m.u)
% recombination
centimorgan (cM)
in most experiments, the number of observed double crossovers is _ than expected
how to find the expected double crossover frequency
less
it is the product of 2 single-crossover frequencies (multiply both lengths)
what is interference and how do you find it
what is c
the reduction in observed double recombinants compared to expected
interference = 1 - c (coefficient of coincidence)
c= observed double recombinants (add both smallest values from given data)/expected double recombinants
this interference value tells you the number of double recombinants is (I) lower than expected
negative interference (I)
I<0 because some genomic regions can produce more double recombinants than expected
genetic markers (used for human genetic analysis)
human genomic sequences that are polymorphic and used for genetic analysis
- we are able to locate genetic markers that are linked to alleles that determine phenotypic variation
-genetic markers can easily be analyzed from a DNA sample
-they link new gene mutations to already known markers
does recombination frequency accurately show actual distance between genes
Underestimates Actual
Distance Between Genes
ways/reasons that recombination rates vary
hotspots/coldspots
Recombination happens more frequently in recombination
hotspots
Recombination happens less frequently in recombination cold spots
influenced by environmental factors
* In Drosophila, recombination occurs more often in younger females
* Temperature also affects recombination rate
Mutations:
* Chromosomal inversions often suppress recombination due to lack of homology
Natural selection:
* Some life forms have higher rates of recombination than others
* High rates of recombination may be adaptive
exception: Recombination rates are not always proportional to
physical distance between genes
YEAST Physical genetic distances of genes in yeast based on sequencing
compared to traditional genetic distances through recombination mapping
spo7 and cdc15 appear further apart physically than their genetic distance suggests, implying a recombination cold spot.
cdc15 and FLO1 appear physically closer than their genetic distance suggests, implying a recombination hot spot.
Implications: Some regions undergo more recombination than expected (hot spots), while others recombine less frequently (cold spots), affecting genetic mapping accuracy.
in HUMANS, genome wide recombination rate is lower in males than females
* This results in a smaller genetic map in males, even for autosomal chromosomes where physical distances are the same
where are genetic markers found in humans? and what are they used for?
non-coding regions of the genome
Used as markers to tag genomic regions of interest for association studies
