Question

A diploid plant has 14 chromosomes, but its egg cell has 6 chromosomes. Which one of the following is the most likely explanation of this?
A. Nondisjunction in meiosis-I and II
B. Nondisjunction in meiosis-I
C. Nondisjunction in meiosis-II
D. Normal meiosis

Answer 1

Hint: Chromosomes comes from the word Chroma which means color and soma which means body. Chromosomes can be defined as oval or rod or thread like DNA and protein complexes which function as hereditary vehicles.

Complete answer:
Haploid organisms have a single pair of chromosomes while diploid organisms have a double pair of chromosomes.
At the time of gamete formation in the diploid organisms, meiosis a reductional division occurs. This causes the chromosomes number to become half in the gametes formed.
So if the diploid plant had 14 chromosomes then its gamete or the egg cell must have 7 chromosomes but as per the question, the egg cell has only six chromosomes. This occurs because of nondisjunction of chromosomes in meiosis-I and meiosis-II. At the time of meiosis-I, due to non-disjunction the chromosomes do not separate in the secondary oocyte and first polar body. It reduces the chromosome number from seven to six in the egg cell and at the time of meiosis-II, the chromatids fail to separate and this results in six chromosomes in the egg cell.

So the correct answer for this is 4. Nondisjunction in meiosis-II.

Additional information:
> Meiosis occurs in diploid germ cells of plants and animals and produce haploid gametes. It is formed of two cell divisions: Meiosis-I and Meiosis-II with no interkinesis or of short period in which no replication occurs.
> Meiosis- I is divided into karyokinesis-I or cytokinesis-I.
> Meiosis-II is like Mitosis and involves separation of genetically modified sister chromatids so brings haploidy in DNA amount while meiosis-I brings reduction in chromosome number only.

Note: Homologous chromosomes break their connections and separate out at Anaphase-I of meiosis-I and this process is known as disjunction of chromosomes. When this disjunction does not occur then the two cells produced have abnormal numbers of chromosomes.