chromosomes
are the unit of transmission in meiosis, not genes
- genes are on chromosomes
linked genes
can not undergo independent assortment
crossing over frequency
frequency of crossing over on a single chromosome is proportional to distance between them
recombination
crossing over result
chromosome maps
indicate relative location of genes on chromosomes
meiotic sequences - independent assortment
no linkage exhibited
complete linkage
linkage without crossing over
- no crossing over between two genes
- produces parental (non-crossover) gametes
recombinant crossover
linkage with crossing over
crossing over
- occurs between 2 non-sister chromatids
- both parental and recombinant (crossover) gametes are produced
- occurs between homologous pairs
linkage ratio
- complete linkage between two genes due to proximity (the closer the more of a link)
- unique F2 phenotypic ratio resutls
linkage group
- genes on the same chromosome are part of linkage group
- number of linkage groups should correspond to haploid number of chromosomes
chiasmata
- synapsed chromosomes in meiosis wrap around each other
- area of crossing over
- X-shaped intersections with points of overlap (points of genetic exchange)
chiasma
- percentage offspring resulting from recombinant gametes depends on distance between 2 genes on same chromosomes
- two genes located close to each other along a chromosome are less likely to have chiasma
sturtevant
- complied data from crosses
- recombination frequencies between linked genes are additive
because a chiasma occurs…
crossing over is the event
map unit (mu)
- 1% recombination rate between two genes on chromosomes
- also called centi-Morgans (CM)
- relative distances, not exact ones
- can help to decide the likely hood that there would be a recombinant event between 2 genes
single crossover (SCO)
- occurs between 2 non-sister chromatids
- recombination is observed in 50%
- in genes 50 map units apart, crossing over can be expected between 100% of tetrads
closer genes…
do not split during recombination
farther apart genes…
are likely to split during recombination
single exchange between 2 non-sister chromatids in tetrad stage
- 2 non-crossover (parental) gametes produced
- 2 crossover (recombinant) gametes produced
single crossovers…
used to determine distance between 2 linked genes
double crossover…
- double exchanges on genetic material
- used to determine distance between three linked genes
- genes must be heterozygous for 2 alleles
3 criteria of 3-point mapping
1) parent must be heterozygous for all three genes under consideration
2) phenotypic class must reflect genotype of gametes of parents
3) sufficient number of offspring must be produced for representative sample
non-crossover F2 phenotypes
- occur in greatest proportion of offspring
