Question

In a cross between two varieties of corn, \(g l_{1} g l_{1} W s_{3} W s_{3}\) (egg parent) \(\times G l_{1} G l_{1} w s_{3} w s_{3}\) (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.

Step-by-step solution

Identify the parent's genetic constitution

The genetic constitution of the parents is given as: Egg parent: \(g l_{1} g l_{1} W s_{3} W s_{3}\) Pollen parent: \(G l_{1} G l_{1} w s_{3} w s_{3}\)

Identify the offspring's genetic constitution

The genetic constitution of the offspring is given as: \(G l_{1} G l_{1} g l_{1} W s_{3} w s_{3} w s_{3}\)

Compare the offspring's genetics with the parent plants

Now, let's compare the offspring's genetics to that of the parent plants. In the offspring, for the \(G l_{1}\) and \(g l_{1}\) genes, there are two copies of the dominant gene (\(G l_{1}\)) and one copy of the recessive gene (\(g l_{1}\)). The pollen parent has two copies of the dominant gene, while the egg parent has only recessive genes for this trait. This suggests that the 2n gamete with the extra copy originated from the pollen parent for this gene. For the \(W s_{3}\) and \(w s_{3}\) genes, the offspring has one copy of the dominant gene (\(W s_{3}\)) and two copies of the recessive gene (\(w s_{3}\)). In this case, the egg parent has two copies of the dominant gene, while the pollen parent has only recessive genes for this trait. This suggests that the 2n gamete with the extra copy originated from the pollen parent for this gene as well.

Determine the origin of the 2n gamete

Since the offspring has extra copies of both dominant and recessive genes found in the pollen parent, we can conclude that the 2n gamete originated from the pollen parent.

Consider other possible explanations

The given information explains the genetic constitution of the offspring, so there is no need to consider other explanations. The conclusion that the 2n gamete originated from the pollen parent is consistent with the given information.

Key concepts

Triploidy

In genetics, triploidy refers to an organism that possesses three sets of chromosomes instead of the usual two. This unusual chromosome number can occur due to errors in cell division or through the fusion of an abnormal gamete.
A triploid organism arises when a standard haploid gamete (n) fuses with a diploid gamete (2n). The result is a zygote with three chromosome sets, or 3n.
**Causes of Triploidy**

• Error in meiosis: Sometimes, during gamete formation, cells fail to divide properly, producing a diploid gamete.
• Fusion with an unreduced gamete: A diploid gamete might fuse with a normal one, leading to triploidy.

In the given problem, the triploid offspring results from a cross-pollination event involving gametic abnormalities. Identifying the origins of the 2n gamete helps determine which parent contributed to the unexpected chromosome number in the offspring.

Gamete Formation

Gamete formation is a critical process in sexual reproduction where cells undergo meiosis to produce gametes—sperm in males and eggs in females. These cells are essential in the transfer of genetic material to offspring.
**Meiosis and Gamete Development**

• Diploid to haploid: Meiosis reduces the chromosome number by half, ensuring that offspring have a complete set after fertilization.
• Genetic variation: Brought about through processes like crossing over and independent assortment during meiosis, resulting in genetically unique gametes.

In cases like the exercise, an error during this process can lead to the formation of a 2n gamete instead of the typical n gamete. This occurs if the chromosomes fail to separate correctly, causing an excess chromosome copy in the gamete. Identifying the origin of such gametes helps solve genetic inheritance puzzles.

Dominant and Recessive Genes

Genes come in different versions known as alleles, which can be either dominant or recessive. Dominant alleles manifest in the phenotype even if only one copy is present, while recessive alleles require two copies to show their effect.
**Genetic Inheritance Patterns**

• Dominant traits: A single dominant allele is enough to express a trait.
• Recessive traits: Both alleles need to be recessive for their effects to appear.
• Homozygous and heterozygous: Homozygous refers to having two identical alleles, while heterozygous means having one dominant and one recessive allele.

In the provided exercise, the triploid offspring presents an interesting case for analyzing dominant and recessive inheritance. By examining the genetic makeup, where dominant and recessive alleles appear in unusual quantities, it aids in identifying the origin of the 2n gamete. For instance, the presence of two dominant alleles for a trait in the offspring indicates an origin from the parent with dominant genes in that trait.