Question

The ovule of a diploid species with $2n = 8$ undergoes double fertilization. If the pollen is contributed by an individual with meiotic nondisjunction, the chromosome number of the zygote will be _______.

Hint:

In meiotic nondisjunction, gametes may carry the full diploid set (2n). When such gametes fertilize normal haploid gametes, the zygote’s chromosome number becomes $n + 2n$.

Answer 1

Step 1: Normal gamete formation. Diploid $2n=8 ⇒ n=4$. Normally, gametes have 4 chromosomes. Step 2: Nondisjunction in pollen donor. Meiotic nondisjunction can produce diploid gametes ($2n=8$ instead of $n=4$). Step 3: Fertilization. Female gamete = $n=4$, male gamete (abnormal) = $2n=8$.
Zygote = $4 + 8 = 12$ chromosomes. Therefore, chromosome number of the zygote = 12.