Linkage and 2/3 point mapping Flashcards
(28 cards)
What are homologous chromosomes
These align during metaphase - pairs of chromosomes - one from father and one from mother - that share the same gene loci, differing only by alleles. AKA Homologues
What is a chiasma?
Homologous chromosomes become entangled, and as they are pulled apart in anaphase, they break at a point called a chiasma, and parts of the homologous chromosomes can be swapped
What are recombinants?
Recombinants are offspring whose geno/phenotype differs from both parents due to the reshuffling of genetic material during meiosis, typically by crossing over homologous chromosomes.
How does recombination indicate the chromosomal locations of genes?
The closer two alleles are on the chromosome, the rarer a chiasma between them. So if recombination is very rare between two alleles, they must be close together.
How is recombination frequency calculated and what does it indicate?
Recombination Frequency = Recombinants / Total = 44/400 = 0.11 (or 11%)
This means the relative map distance between the vg and pr loci is 11 cM (centi Morgans) or MU (Map Units)
What mutant drosophila phenotypes are used in the three point cross?
sc or scute - loss of thoracic bristles, ec or echinus - roughened eye, cv - crossveinless wing. F1 triple heterozygotes are backcrossed with parental triple mutants - expected 1:1:1:1:1:1:1:1
How do double crossovers allow loci order to be established?
Out of the double recombinants (the rarest phenotype observed), the middle allele is the one which can be switched between the two to restore parental phenotypes
What is the relevance of recombination to Mendel?
Relevance to Mendel. His second law assumed independent assortment of loci. Today we know that some of his loci were linked, but because they were so distantly separately on the chromosomes, they did not appear to be linked.
Why do relative distances of loci in a three-point cross add up?
Double recombinants’ frequencies need to be doubled.
What is crossover interference?
Formation of one chiasmata physically makes it less likely for another to appear nearby, reducing the no. of double recombinants below expected.
How is expected double recombinant frequency calculated?
the recombination frequencies between A-B and B-C are multiplied. Expected no. of double recombinants is found using double recombinant frequency * no. individuals.
What is the coefficient of coincidence
observed double recombinant frequency/expected double recombinant frequency
How is the coefficient of interference calculated?
= 1-coefficient of coincidence.
what are crossover hotspots?
regions of the genome a few 100 - 1000 base pairs long where crossovers and recombination events are more frequent. Ensure proper chromosome segregation and genetic diversity.
What causes crossover hotspots?
PRDM9 recognizes certain DNA motifs and modifies nearby histones to make the DNA more accessible for the recombination machinery. This targeted chromatin modification increases the probability of a double-strand break (DSB)
What is negative crossover interference?
In some species such as neurospora, when a crossover occurs, this increases the chance of another nearby crossover. However the general rule of inverse proportionality of proximity and recombination still applies between loci.
What does 1 centiMorgan really represent?
1% recombination frequency = 1cM or one MU
Why are linkage maps sex-averaged?
Recombination frequencies differ between sexes: e.g. male drosophila none at all and human females 1.6 times as much.
Why do multiple crossovers make linked genes which are far apart appear unlinked?
A double crossover between two loci looks like no crossover at all – and so do 4, 6, 8, 10 or any even number of X-overs. Loci a long way apart come up with about 50%
recombinants and 50% parentals even though they are linked.
Define free recombination
when two or more loci are unlinked. Gives 50% parental phenotypes and a map distance of 0.5 MU, any less shows linkage.
What does it mean for alleles to be in coupling or repulsion?
Alleles in coupling are all of the same chromosome of a homologous pair. In repulsion some mutant (recessive) and wildtype (dominant) alleles are found on both.
What are translocations?
Non-homologous exchanges of regions of different chromosomes, important in some human cancers. Can occur at the pseudo-autosomal regions at the Y chromosome tips.
What is the role of the synaptonemal complex?
Protein structure that forms between homologous chromosomes during meiosis - thought to mediate synapsis and recombination in prophase I of meiosis. It is currently thought that the SC functions primarily as a scaffold to allow interacting chromatids to complete their crossover activities as a tetrad.
Can recombination occur between the male sex chromosomes
Generally no, as X&Y aren’t homologous, but the two tips of the Y can recombine with the respective telomeres of the X - these regions of the Y are called Pseudo-Autosomal regions (PAR1 and PAR2). Thus about 5% chromosome can recombine - the most gene rich regions
