Question

In horses, the basic color of the coat is governed by the E locus. EE and Ee horses can make black pigment, while ee horses are a reddish chestnut. A different locus, the R locus, can cause roan, a scattering of white hairs throughout the basic coat color. However, the roan allele has a serious drawback: RR embryos always die during fetal development. Rr embryos survive and are roan, while rr horses survive and are not roan. The E locus and the R locus are tightly linked. Suppose that several centuries ago, 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 (eeRr) and 20 nonroan homozygous blacks (EErr). On the island, they interbred with each other and established a wild population. The island environment exerts no direct selection on either locus.

a. What was D, the coefficient of linkage disequilibrium, in the initial population of 20 horses? Was the initial population in linkage equilibrium or not? If not, what chromosomal genotypes were underrepresented?
b. Do you expect the frequency of the chestnut allele, e, to increase or decrease in the first crop of foals? Would your answer be different if the founding population had been just 10 horses (5 of each color)? Explain your reasoning.
c. If you could travel to this island today, can you predict what D would be now? Do you have predictions about whether more horses will be roan versus non-roan, or chestnut versus black? If not, explain what further information you would need.

Answer 1

The initial population of horses was not in linkage equilibrium since ER and er combinations were missing, causing a non-zero coefficient of linkage disequilibrium (D). The chestnut allele's frequency would likely remain stable without selection, but genetic drift could affect it in smaller populations. Predicting current phenotypes on the island requires more data.

Step-by-step explanation:

In considering the genetics of the horse population established on the island, we can address each question with some genetics concepts. Linkage disequilibrium (D) quantifies the non-random association of alleles at different loci. In the initial population of 20 chestnut roan (eeRr) and 20 nonroan homozygous black (EErr) horses, D was not zero because the combination of E and R alleles (ER and er) is missing and the population is not in linkage equilibrium. This is because only two of the four possible gametic combinations are present.

The frequency of the chestnut allele (e) was expected to remain stable in the absence of selection. However, if the founding population had been just 10 horses, genetic drift could have had a more pronounced effect, possibly changing the allele’s frequency by chance alone.

It's challenging to predict what D would be now without further information about reproductive success rates, mating patterns, or additional genetic events in the horse population. Likewise, we need more data to predict the current population's phenotypic ratios of roan versus non-roan and chestnut versus black horses.