ch 4 mapping eukaryote chromosomes by recombination Flashcards
linkage
the association of two genes on the same chromosome
coupling configuration
arrangement of linked alleles in a dihybrid such that the two dominant alleles are on a chromosome, the two recessive alleles are on the other chromosome (AB/ab)
repulsion configuration
arrangement of linked alleles in a dihybrid such that a dominant and a recessive allele are together on each chromosome (Ab/aB)
cis dyhybrid
a dihybrid in which the alleles are coupled (AB/ab)
trans dihybrid
a dihybrid in which the alleles are in repulsion (Ab/Ba)
two process that generate recombinant products
- independent assortment of genes on different chromosomes
- crossing-over between genes on the same chromosome
what regions have higher recombinant frequencies
longer regions
map unit or cM
a recombinant frequency of 1%
why map the relative position of genes
calculated map distance is correlated to physical distance on a DNA molecule
- knowing the position of a gene on a chromosome allows us to find its sequence
- leads to an understanding of its structure and function
map distance formula
precent recombinant progeny
= # recombinant progeny/total progeny x 100%
recombination frequency never exceeds
50%
3-point test cross
mapping 3 genes relative to one another
rules for a 3 point test cross
- there should be 8 phenotypic classes
- progeny classes are grouped in pairs = reciprocal cross over events
- largest classes are the parentals
- smallest classes are the double cross over classes
interference
a measurement of the independence of two cross over events (does one cross over interfere with the possibility of another cross over)
interference formula
interference = 1 - (observed DCO/(p(SCO I) x p(SCO II) x total number of progeny))
negative interference
more double cross overs than expected (cross over in one region promotes cross overs in another region)
positive interference
(usually)
fewer double cross overs than expected (cross over in one region inhibits cross overs in another region)
spores: division I - separation of
homologues
spores: division II - separation of
sister chromatids
distance between 2 points =
% meiotic products showing recombination between the 2 points
the frequency of crossing over between any two points on a chromosome is proportional to the distance between those two points
1/2 MII octads / total x 100%
octad
an ascus containing eight ascospores, produced in species in which the tetrad normally undergoes a postmeiotic mitotic division
first division segregation
different alleles go into different nuclei at the first meiotic division producing an M1 division pattern of ascospores (4:4)
second division segregation
different alleles go into different nuclei at the second meiotic division producing an MII division pattern of ascospores (2:2:2:2)
molecular marker
a site of DNA heterozygosity (difference), not necessarily associated with phenotypic variation, used as a tag for a particular chromosomal locus