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Review Question If two genes are 13 map units apart on a linkage map, what proportion of recombinant offspring will be seen in a testcross? What proportion of meioses experienced crossing over between to the two genes?. Two X-linked genes. Female Male w m w+ m+

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Slide1 l.jpg

Review Question

If two genes are 13 map units apart on a linkage map, what proportion of recombinant offspring will be seen in a testcross?

What proportion of meioses experienced crossing over between to the two genes?


Two x linked genes l.jpg
Two X-linked genes

Female Male

w m w+ m+

—•——————————•—————————

P:—•——————————•————/

w m

Gametes: Egg (A B) Sperm (a b)

w m w+ m+

—•———————— —•————————

—•———/


Slide3 l.jpg

w+ m+

—•—————————

F1:—•—————————

w m

w m

—•—————————

—•————/

X

w+ m+

—•—————————

w m

—•—————————

w m

—•—————————

Par

Par

—•————/

Rec

Rec

Offspring (male or female)

314 Wild type

314 white, miniature

186 miniature

186 white

w+ m

—•—————————

w m+

—•—————————


Non recombinant offspring recombinant offspring l.jpg

___%non-recombinant offspring,____% recombinant offspring

Genes are how far apart?

___ map units


If two genes are 50 map units apart what proportion of recombinant offspring would you observe l.jpg

If two genes are 50 map units apart, what proportion of recombinant offspring would you observe?

What would you conclude about these two genes?



Three point mapping l.jpg

A B C time

—•—————————

—•—————————

a b c

A b C

a B c

Three-point mapping

A BC

—•—————————

—•—————————

a b c

A B C

a b c


3 recessive phenotypes in maize corn coded by three linked genes l.jpg
3 recessive phenotypes in maize (corn), coded by three linked genes

l l lazy or prostrate growth

g g glossy leaves

s s sugary endosperm


To map the genes mate a triple heterozygote to triple recessive homozygote l.jpg
To map the genes, mate a triple heterozygote to triple recessive homozygote

Ll Gg Ss x ll gg ss

Gene order is not known,

so the order shown here is arbitrary.

Linkage phase is not known


How many different kinds of gametes can you get from triple heterozygote l.jpg
How many different kinds of gametes can you get from triple heterozygote?

Ll Gg Ss

L or l G or g S or s

2 * 2 * 2


Slide12 l.jpg

Recomb. heterozygote?

Wildtpe for all lazy, gloss, sugary

L G S // l g s x l g s // l g s


Where to begin l.jpg

Where to begin? heterozygote?

Parental types will constitute ≥ 50% of all progeny, so…


Rule 1 l.jpg
Rule 1 heterozygote?

  • Two most-frequent gametes types are the __________types

  • Tells us the ___________________ of heterozygous parent:

  • L G S or L g S or l g S or L g s

  • l g s l G s L G s l G S


Slide15 l.jpg

L G S // l g s heterozygote? x l g s // l g s


Linkage phase in heterozygous parent l.jpg
Linkage phase in heterozygous parent? heterozygote?

  • L G S or L g S or l g S or L g s

  • l g s l G s L G s l G S


Rule 2 l.jpg
Rule 2 heterozygote?

  • The double-recombinant gametes will be the two least frequent types

A B C

a b c


Slide18 l.jpg

L G S / l g s heterozygote? x l g s / l g s


Rule 3 l.jpg
Rule 3 heterozygote?

  • Effect of double crossovers is to interchange the members of the middle pair of alleles between the chromosomes

A B C

AbC

a b c

a B c


Double crossover types l.jpg

Parental types: heterozygote?

Double-crossover types:

L G S and l g s

  • L G s and l g S

Which gene is in the middle?

LSG

Ls G

l sg

l S g


Now you know linkage phase of heterozygous parent and gene order l.jpg
Now you know heterozygote? linkage phase of heterozygous parent and gene order

  • L S G

  • l s g

How far apart are the genes?


Count the crossovers between adjacent genes l.jpg

L S G heterozygote?

l s g

Count the crossovers between adjacent genes

  • In parents, L allele on same homolog as S and l on same homolog as s. So if these get broken up ---> recombination between L and S loci

  • In parents, S on same homolog as G and s on same homolog as g. If these get broken up --> recombination between S and G loci


Slide23 l.jpg

L S G heterozygote?

l s g


Rule 4 reciprocal products expected to occur in approximately equal numbers l.jpg
Rule 4: heterozygote? Reciprocal products expected to occur in approximately equal numbers

  • LGS ≈ lgs (286 ≈ 272)

  • LgS ≈ lGs(59 ≈ 44)

  • Lgs≈ lGS(40 ≈ 33)

  • LGs ≈ lgS(4 ≈ 2)


Rule 5 l.jpg
Rule 5 heterozygote?

  • Don't forget to include the double recombinants when calculating recombination frequency!


Slide26 l.jpg

  • l G S heterozygote? 33

  • L g s 40

  • L G s 4

  • l g S 2

  • 79

Rec Freq S-G

Rec Freq L-S

L g S 59

l G s 44

L G s 4

l g S 2

109


Slide27 l.jpg

Rec Freq L-S heterozygote?

79/740 or 10.7% of gametes recombinant between L & S. So, map distance between L & S = ___ map units

l G S 33

L g s 40

L G s 4

l g S 2

79

Rec Freq S-G

109/740 or 14.8 % of gametes recombinant between S & G. So, map distance between S & G=____ map units

L g S 59

l G s 44

L G s 4

l g S 2

109


Genetic map l.jpg
Genetic Map heterozygote?

10.7 mu 14.8 mu

_____________________________

L S G


Interference l.jpg
Interference heterozygote?

  • Assuming independence, expected probability of double crossovers is the probability of recombination in one region times the probability of recombination in other (__________).


Maize example l.jpg
Maize example heterozygote?

  • Probability of recombination between L and S is 10.7%

  • Probability of recombination between S and G is 14.8%

  • If crossovers independent, probability of double crossover should then be

  • 0.107 * 0.148 = 0.0158

  • In 740 events, the double crossover class should occur


Slide31 l.jpg

L G s heterozygote? 4

l g S 2

6

  • Expected DCO = 12

  • Observed DCO = 6

  • Typical Result: O < E

  • Conclusion: Crossing over in one region reduces probability of crossing over in adjacent regions

  • This is Interference


Slide32 l.jpg
Why? heterozygote?

  • Physical constraints that prevent two chiasmata in close proximity during meiosis


Quantifying interference l.jpg
Quantifying Interference heterozygote?

  • Coefficient of coincidence = Obs DCO

  • Exp DCO

  • cc = _____

  • Interference = (1 - cc) =


Recombination is not independent at small distances l.jpg
Recombination is not independent at small distances heterozygote?

  • If distance between genes is small (<10 map units in Drosophila) no double crossovers occur (interference is complete, I=1)

  • At large distances (> 45 map units, Interference disappears, Obs = Exp and I=0


Slide35 l.jpg

In heterozygote? Drosophila, the allele b gives black body (wild type is tan); at a separate gene, the allele wx gives waxy wings (nonwaxy is wild type); and at a third gene, the allele cn gives cinnabar eyes (red is wild type). A female that is heterozygous for these three genes is testcrossed, and 980 progeny are classified as follows for body color, wing phenotype, and eye color:


Slide36 l.jpg

a) What is the linkage phase of the heterozygous female parent?b) What is the order of the three genes?c) Construct a linkage map with the genes in their correct order and indicate the map distances between the genes. d) Calculate the Interference.