Ch7 Pt.2: Linkage & Chromosome mapping (Oct 6th)

Part II. What is linkage and how can we test for it?

Define Linkage

what is it the result of?

Linkage: The non-random correlation of alleles of different loci in a population
-Most often the result of physical connection between loci on a single molecule of DNA.

Define Linkage Group

a set of alleles that are nonrandomly correlated and thus inherited together with greater probability than as if they were
independently assorting

what does a sufficiently inclusive linkage group constitute

a chromosome

Define

Crossing over

chiasma

recombination

Crossing over: reciprocal exchange of chromatid segments
during prophase I of Meiosis
Chiasma: site of crossing over
Recombination: the result of crossing over

what does recombination disrupt

linkage

what is sturtevants insight

probability of recombination is proportional to physical distance between two genes

Detecting linkage through a test-cross and chi-square test

1. Parental: how

provide the example

what is the dihybrid cross genotype

Cross two pure breeding strains (one for each mutant)
-Black bodied mutant: autosomal recessive bb
(wild-type here called “grey”)
-Vestigial wing mutant: autosomal recessive yy

bbYY x BByy

Describe Linkage vs Unlinked

in Po generation to F1 generation

Linked: Po generation has genes on same chromosome, so F1 generation will have genes on same chromosome as well

Unlinked: Po generation has genes on different chromosomes, so F1 generation will have its genes on different chromosomes as well

Detecting linkage through a test-cross and chi-square test

2. what do you cross together, for crossing over to not occur in what gender?

what does is the F2 generation, how does this help determine linked or unlinked?

2. Backcross female F1 (BbVv) with male double mutants
(bbvv). Crossing over does not occur in male Drosophila. In linked and Unlinked

If 50% to 50% ratio result: this is linked bc crossing occurs in same chromosome.

If 25% x4 ratio result: This is unlinked bc crossing over occurs in different chromosomes, parental and recombination is present here.

Detecting linkage through a test-cross and chi-square test

3. Expected outcome without linkage

When crossing unlinked female and unlinked male

Parental: 1 black normal: 1 grey vestigial

Recombinant: 1 grey normal: 1 black vestigial

=50% parental and 50% recombinant in F2 generation or 25% x4 total

Why use a testcross rather than a dihybrid cross to
develop null expectation?

what can u now do?

1. Limits recombination to a single parent

2. A 1:1 distribution provides a more powerful statistical test
than a 9:3:3:1 distribution

can now perform chi-square test

What percentage does recombinant not exceed and why?

Does not exceed 50%, bc it is independent assortment at that point