# Lectures 10, 11: Three-point test crosses

(version 22 August 2005)

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## Three-Point Testcross

If we have three loci, how do we order them?

Drosophila examples

sc (scute = bristles), ec (echinus, eye shape mutant), vg (vestigal wing).

sc+ , ec+, vg+ are dominant alleles

## 2nd example

vermilion (v), crossveinless (cv), cut (ct)

v+, cv+, ct+ are dominant

Cross v+ cv ct / v cv+ ct+ to a v cv ct / v cv ct triple-recessive testcross strain. Let RF(A,B) denote the recombination frequency (or fraction) between loci A and B.

## Which locus is in the middle?

Parents are v+ cv ct / v cv+ ct+

rarest class ( v cv+ ct, v+ cv ct+ ) = double recombinants

Which gene order gives these as double recombinants?

## Key steps to gene ordering

1. Compute recombination frequencies between all three pairs

2. Gene map should be close to additive when genes are close

3. Rarest class is the double-recombinant

4. Try all three possible gene orders

## Interference

Presence of one crossover may influence the probability of nearby crossovers

First, what is the expected recombination fraction if no interfence?

## Coefficient of Coincidence (coc)

If no interference, Pr(double recomb) = c1 * c2