Question

In a cross between two varieties of corn, \(g l_{1} g l_{1} W s_{3} W s_{3}(\) egg parent) \(\times G I_{1} G l_{1} w s_{3} w s_{3}\) (pollen parent), a triploid offspring was produced with the genetic constitution $G l_{1} G l_{1} g l_{1} W s_{3} w s_{3} w s_{3}\( From which parent, egg or pollen, did the \)2 n$ gamete originate? Is another explanation possible? Explain.

Short answer

Answer: The \(2n\) gamete originated from the pollen parent. The alternative explanation, which is non-disjunction occurring in both parents simultaneously, is unlikely.

Step-by-step solution

Analyze the genetic makeup of the parents and the offspring

The egg parent has the genetic constitution: \(g l_{1} g l_{1} W s_{3} W s_{3}\). The pollen parent has the genetic constitution: \(G I_{1} G l_{1} w s_{3} w s_{3}\). The triploid offspring has the genetic constitution: \(G l_{1} G l_{1} g l_{1} W s_{3} w s_{3} w s_{3}\).

Compare genetic makeup of parents and offspring to determine origin of the \(2n\) gamete

From the genetic constitution of the offspring, we can see that it has two copies of \(G l_{1}\), one copy of \(g l_1\), three copies of \(W s_{3}\), and three copies of \(w s_{3}\). The genetic material from the egg parent cannot provide all of the genetic material needed to form the offspring, which means that it cannot be the source of \(2n\) gamete. This is because egg parent only has genes \(g l_{1}\) and \(W s_{3}\) in its genetic constitution. It has no copy of \(G l_{1}\) or \(w s_{3}\) in its genetic constitution. On the other hand, the genetic material from the pollen parent can provide two copies of \(G l_{1}\) and three copies of \(w s_{3}\), since it has both genes in its genetic constitution. Therefore, the \(2n\) gamete must have originated from the pollen parent.

Consider alternative explanations

The only alternative explanation could be an event called non-disjunction, which is the failure of homologous chromosomes or sister chromatids to separate properly during cell division. It can result in the incorrect distribution of genetic material, creating an abnormal number of chromosomes in offspring. However, the triploid offspring in this case has genetic material from both parents, so the non-disjunction during meiosis must have happened in both parents simultaneously. This scenario is unlikely, especially considering that the source of \(2n\) gamete was identified. Thus, the most plausible explanation is that the \(2n\) gamete originated from the pollen parent.