Linkage & Mapping Flashcards
Why was linkage discovered?
In certain crosses, a greater no. of offspring with ‘parental’ genotypes were generated than expected. Test cross with double recessive parent. No crossover occurs because the genes are close on the same chromosome.
Why do non-parental genotypes arise?
Due to crossing over in meiosis 1, called recombinants.
Why can recombination be used to produce a ‘linkage map’?
The frequency of recombination between two genes will depend on the distance between them.
Why does increased distance between genes = higher frequency of recombination?
Decreased linkage.
Give 2 features of genetic maps.
Linear and additive.
Give the units of genetic maps.
map units / m.u.
Describe how 3 point crosses map chromosomes.
- calculate distances
- consider 2 of the genes at a time
- calculate the recombinant frequencies for each pair
What does 2 classes of offspring tell you about linkage?
1 gene is unlinked.
What does 3 classes of offspring tell you about linkage?
All 3 genes are linked
Why might the numbers in a 3 point cross not add up?
A double recombinant gene is present, and must be included twice in calculations.
When ordering genes, if the gene in the middle must be the only double recombinant what can be assumed?
Double recombinants are found alongside alleles not found alongside in the parent genes.
Describe the process of determining linkage.
Look at patterns, are there 2 or 3 classes of offspring?
Determine parent & double recombinant types.
Deduce gene order.
Calculate distance between pairs of genes.
