A disease caused by an autosomal primary non-disjunction is :

(A)

Klinefelter's Syndrome

(B)

Turner's Syndrome

(C)

Sickle Cell Anemia

(D)

Down's Syndrome

NEET - Medical UG-2017 Hard

Answer & Solution:

The correct answer is Down's syndrome.

After splitting of centromere one or more chromosomes fail to migrate properly due to abnormal function of achromatic spindle. This results in one daughter with trisomy and one with monosomy. It may occur both in mitosis and meiosis and may involve sex chromosomes or autosomes. Autosomal nondisjunction is less viable. During Mitosis: Nondisjunction in first cleavage division of zygote leads to mosaicism. Trisomy occurs when during gametogenesis: In Meiosis I: The 2 chromosomes of a pair go to same pole and in Meiosis II: A pair of sister chromatids go to same pole. This results in a gamete having 24 chromosomes. At fertilization this Abnormal gamete + Normal gamete equals Trisomy, i.e. 24 chromosomes + 23 chromosomes = 47 chromosomes in zygote. Down's syndrome results due to presence of an additional copy of chromosome number 21 (trisomy 21) which occurs due to non disjunction of the homologous pair of chromosome 21.

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