Question

The presence of an extra chromosome in a zygote such as in case of Edward syndrome which is the result of an additional 18${}^{th}$ chromosome is due to

• A. Crossing over in meiosis I
• B. Nondisjunction in meiosis II
• C. A deletion during meiosis II
• D. A deletion during meiosis I
• E. Replication error in meiosis II

Answer 1

Answer: (B)

Nondisjunction in meiosis II

Crossing over does not increase the number of chromosomes. NNondisjunction of sister chromatids during meiosis II resulted in the formation of aneuploid gametes. Fertilization of gamete having one extra chromosome with a normal gamete result in the formation of a zygote with 47 chromosomes where one chromosome is added to the 18${}^{th}$ chromosome and causes Edward syndrome. A deletion is a type of mutation where the removal of one or more bases takes place, not the whole chromosome. Replication is the process of duplication of DNA strands it does not take place in meiosis II. Thus, the correct answer is option B.