Genetic linkage Flashcards
How can genes on the same chromosome appear to assort independently from each other during gamete formation?
When genes are on the same chromosome, but far apart, they assort independently due to crossing over
Define synteny
- Genes located on the same chromosomes.
- All loci on one chromosome are syntenic, regardless of their proximity or interactions in meiosis.
True or false? Linked genes can be located on different chromosomes.
False, linked genes are always syntenic genes (located on the same chromosome) that it is less likely for crossing over to occur.
What is the difference between complete linkage and linkage with recombination?
- Complete linkage
○ No crossing over occurs between linked genes because they are so close together (aka do not recombine)
○ All gametes are parental - Linkage with recombination
○ Crossing over is expected when distance < 50 cM
○ Mixture of parental and nonparental gametes
○ Both parental types are = in frequency, along with the two recombinant types
○ Proportion of parental to recombinant chromosomes varies between different pairs of genes
True or false? The frequency of genetic exchange between two genes depends on their chromosomal location.
True
Who constructed the first linkage map and what was the model organism used?
Sturtevant and used Drosophila (fruit flies)
Linkage analysis of genes A, B and C showed that recombination frequency between A and B is 10%; between B and C is 35%, and between A and C is 45%. Draw a linkage map for the 3 genes and show distances in linkage map units
Draw out yourself
How does the distance between two loci influence the crossing over frequency?
- Crossover frequency is proportional to the linear distance between two genes
- The further away two syntenic genes are, the more likely they will experience crossing over
What is the recombination frequency equation?
r= number of recombinants/total number of progeny
What are the differences between gametes of dihybrid cross of unlinked and linked genes?
- Unlinked dihybrid cross - undergoes independent assortment and produces four different gametes combinations with equal frequency
- Linked dihybrid cross - parental combinations will occur > 50% and nonparental combinations will occur < 50% of the time
Can you construct a genetic linkage map using male Drosophila? Explain.
No, because they exhibit complete linkage (no recombination)
What are the criteria for the construction of a genetic linkage map?
I. All loci under consideration must be heterozygous.
II. Genotypes of all gametes can be accurately determined by observing the phenotypes of the resulting offspring.
III. Large number of offspring must be produced in order to recover all crossover classes (need model organisms!).
Name the map unit of linkage maps and how does it correlate to recombination frequency
1 centimorgan (cM) = 1 map unit = 1% recombination
What types of markers should one use for generating linkage maps?
DNA markers (specific sequences) represent landmarks along the chromosome instead of traits
In relation to crossing over, what is interference?
- Phenomenon where a crossover even in one region of the chromosome inhibits a 2nd crossover nearby
- Identifies the proportion of double recombinants that are expected but are not produced in the experiment
○ Typically, the observed is less than the expected
○ negative interference is when I<0, genes were far apart
