Linkage, Recombination and Eukaryotic Mapping

Outline Introduction Complete Linkage compared to independent assortment Crossing over with linked genes –“coupling and repulsion” gene arrangements Predicting outcomes with linked genes Gene mapping with recombination frequencies 2-point test cross

Alfred Sturtevant Observed: certain traits in Drosophila tended to inherited together. Suggested: these genes were on the same chromosome Thomas Hunt Morgan Also suggested: gene close together rarely shuffled

Let’s compare inheritance of 2 linked genes with the inheritance of 2 genes that assort independently (like what we have already studied according to Mendel’s principles.)

According to Mendel (non-linked genes—this is review, ch. 3) AaBb x AaBb AaBb aAbB AABB aabb Non-recombinants Recombinants Independent Assortment 3:1

Linked genes Genes located close together on the same chromosome

Linked Genes On the same chromosome genes “travel” together and arrive a the same destination. But how can you tell when a gene is linked or if it is independent?

Use the Test Cross Method x m d M D m d or m d ALL NON-RECOMBINANTS m d M D MmDd x mmdd MmDd or mmdd Mmdd or mmDd 50% non- recombinants 50% - recombinants 1:1:1:1 50% If Completely Linked: If Independent

Sometimes however the linkage is “broken” by a process called CROSSING OVER”. “Incomplete Linkage” recombination It is another way to

Crossing Over (meiosis—prophase 1) Intrachromosomal Recombinantion Physical exchange of DNA

How do we know if crossing over occurs between linked genes? Again use a test cross and look the offspring distribution. Let’s continue to use the same test cross: x m d M D

Calculation of Recombination Frequency X 100 = 12%

RF allows you to prediction proportion of expected offspring with linked genes If you know that RF = 16% T= warty d= dull color

Recombination Frequencies (RF) Can be used to determine the order of genes on a chromosome. Chromosome maps determined from RF are called“Genetic Maps”. Distances are given in map units (m.u.) or centimorgans (cM) 1 m.u.= 1% recombination 50% RF is expected with independent assortment (or distant genes)

2 point test crosses were done and the results follow First a few simple examples of displaying a “genetic map”

1. Genetic Map using RF A to B = 5 m.u. B to C= 10 m.u. A to C = 15 m.u. Then a simple genetic map using RF is A 5 m.u. B 10 m.u. C

2. Genetic Map using RF A to D = 8 m.u. B to D= 13 m.u. C to D = 23 m.u. Now include “D” to the previous map, what would the map look like now? A 5 m.u. B 10 m.u. C Another set of 2 pt test cross

2 point test cross map: data given from a series of test crosses » RF a and b50% a and c50% b and c 20 b and d10 c and d28

Genetic Map RF values from 2 point test cross b c. 20 m.u.

Genetic Map RF values from 2 point test cross d b c 10 m.u. 20 m.u. 30 m.u.

3 point test cross (to be applied to your problem)

But is the order of the genes correct?

Interference and Co-efficient of coincidence (CC) CC= # of observed double crossovers/# of expected double crossovers

Onto Ion Exchange