Lecture 26: Linkage

When genes are on the same chromosome, they can show departures from independent assortment

If genes on the same chromosome are sufficiently far apart, they can segregrate independently

Construction of linkage map (genetic map)

In sweet pea,

P- = purple flower, pp = red flower

L- = long pollen, ll = round pollen

P-L- , ppll over represented

Lack of independent assortment

Suppose parent is PL / pl

the gametes PL, pl are called parental gametes

the gametes pL, Pl are called recombinant gametes

Segregation of chromosomes at meiosis

Crossing-over occurs between paired chromosomes

Chiasma -- visible structure of crossing over in metaphase chromosomes

Recombination frequency = percentage of gametes that are recombinant (i.e., non-parental)

Key: if Freq(recombinant gametes) < 1/2, loci are linked.

Let c = recombination frequency

Pr(recombinant gamete) = c

Pr(parental gamete) = 1-c

What is the recombination frequency for the Bateson-Punnet data?

freq (ppll) = 55 / (284+21+21+55) = 55 / 381 = 0.144

From above, P(ppll) = (1/4)(1-c)² = 0.144

OR: c = 1 - ( 4 * 0.144)^1/2 = 0.24

double heterozygotes can come in two varieties: Coupling and Replusion

These are defined with respect to two alleles at different loci:

Alleles A and B are in coupling if the heterozygote is AB/ab

They are in replusion if the heterozygote is Ab/aB

Hence, Ab/aB is a coupling heterozygote for Ab, but a replusion heterozygote for AB or ab.


How do we determine the phase of a parent?

Testcross to double recessive homozygote

PpLl (phase unknown) X ppll

Since one parent always gives a pl gamete, we can deduce the other gamete from the phenotype.

The Pl and pL gametes are in excess, and hence are the parental gametes.

The frequency of recombinant gametes is 0.11+ 0.12 = 0.23, giving c = 0.23