Question

In horses, the basic coat color is controlled by the E locus. There are two alleles. E1 and E2. E1 is dominant so homozygous E1E1 or heterozygous E1E2 have black pigment. E2E2 homozygotes are a reddish chestnut color. A second gene locus R can cause roan, a scattering of white hairs throughout the basic coat color. The Roan allele has some serious drawbacks: R1 when homozygous (R1R1) results in death of the embryo. R1R2 embryos survive and the horse are roan in color. The R2R2 also survive, but are not roan. The two loci R and E are tightly linked.

Now suppose A Spanish galleon with a load of conquistadors’ horses was shipwrecked by a large grassy island. Just by chance, the horses that survived the shipwreck and swam to shore were 20 chestnut roans (E2E2 R1R2), 20 non-roan homozygous blacks (E1E1 R2R2). On the island the interbred with each other and established a wild population. The island environment exerts no direct selection on either locus.

What was the coefficient of linkage disequilibrium (D) in the initial population of 40 horses?

Is this population in linkage equilibrium or disequilibrium? Explain