Why is gene mapping important?
- gene position is crucial info for building complex genotypes required for experimental purposes or commercial applications
- knowing the position occupied by a gene provides a way pf discovering its structure and function
- can deduce evolutionary genetic mechanisms where genomes diverged by comparing the genes present and the arrangement to closely related species
How is the arrangement of genes on chromosomes represented?
- as a unidimensional chromosome map showing gene positions (loci) and the distance between the loci based on a scale
What are the 2 different types of chromosome maps?
- recombination-based maps
- physical maps
What are recombination-based maps?
- a map of the loci of genes that have been identified by mutant phenotypes showing single-gene inheritance
What are physical maps?
- show the genes as segments arranged along the long DNA molecule that constitutes a chromosome
Why do genotypes deviate from Mendel’s 1:1:1:1 prediction in a dihybrid cross?
- genes can be linked, so a pair of allele combinations can be similar numbers and make up the majority because they are the same genotypes as the parents
- also recombinant genotypes will have similar numbers but make up the minority of the progeny
What genotypes typically make up the majority of the progeny genotypes and why?
- the parental genotypes
- because they are typically linked
What is the general pattern for dihybrid crosses and the number and type of genotype?
- 2 equally frequent nonrecombinant classes total >50% of the progeny
- 2 equally frequent recombinant classes total <50% of the progeny
Why do allele combinations from the parental generations stay together?
- the genes are physically attached by the segment of chromosome between them
Why do allele combinations from the recombinants occur?
- when the homologous chromosomes pair in meiosis sometimes the chromosomes break and exchange parts in a process called crossing over
What are the rules for symbolizing linkage?
1) alleles on the same homolog have no punctuation between them
2) a slash symbolically separates the 2 homologs
3) alleles are always written in the same prder on eac homolog
4) genes known to be on different chromosomes (aka unlinked) are shown seperated by a semicolon
5) unknown linkages are seperated by a dot
What is the importance of the chiasmata?
- they are the sites of exchange during crossing over
What is crossover the result from?
- the breakage and union of DNA
What is has a direct correlation with the appearance of recombinants?
- the chromosomal event of crossing over
What do crossovers occur between?
- sister chromatids
- if it occurred at the 2 chromosome stage then there would only be a maximum of 2 different genotypes in an individual tetrad, but there are 4
What are double crossovers?
- 2 crossover events that occur in a meiocyte
Can double crossovers occur between sister chromatids?
- it can happen, but it is very rare
How many chromatids are typically involved in a double crossover?
- usually between 3 or 4 chromatids
- but in one meiocyte
How many chromatids are involved in a single crossover?
- only between 2 chromatids
What is the key to chromosome mapping?
- the frequency of recombinants produced by crossing over
T/F: the further apart genes are the less likely a crossover will occur
- false
- the further apart genes are the more likely they are to crossover and the higher number of recombinants there will be
What is the range of recombinant frequency for linked genes?
- 0-50%
What’s the recombinant frequency for genes far apart from each other?
- the further apart genes are the closer to 50% recombinant it will be
- when it’s close to 50% we cannot decide if they’re linked or unlinked
When is there a recombinant frequency greater than 50%?
- this never happens
