Question

Non-disjunction errors during meiosis result in several human medical disorders. For each of the following possible disorder indicate if the non-disjunction event could have taken place during maternal meiosis I, maternal meiosis II, paternal meiosis I, or paternal meiosis II by placing a check mark in the appropriate boxes. Maternal Paternal Disorder Meiosis I Meiosis II Meiosis I Meiosis II Down’s Syndrome (Trisomy 21) Kleinfelter Syndrome (XXY) XYY Males

Answer 1

Ans. Meiotic disjunction occurs in two phases-

1. Meiosis I disjunction: Homologous chromates separate. That is the duplicated chromosome number 21 from father and the same chromosome from mother form a homologous pair during gametogenesis. Say, if father’s chromosomes are designated C21-C21 (duplicated, paired during S-phase- the paired structure is considered as 1 chromosome) and mother’s chromosomes are c21-c21, the two paired chromosomes (one from mother, another from father) pair to form homologous chromosome.

Homologues pair (C21-C21 // c21-c21) separates into “C21-C21” and “c21-c21” during meiosis I.

2. Meiosis II disjunction: Sister chromates separates into two independent chromosomes. That is, “C21-C21” separates into two independent C21 chromosomes. Similarly does “c21-c21”.

Part A: Down’s trisomy (trisomy 21)

Possible gamete combination = (C21-C21) from male x (C21) from female ; or vice-versa

C21-C21 gamete is a gamete consisting of two copies of chromosome 21. C1 is a gamete consisting of a single copy of chromosome 21.

Chromosome 21 is an autosome. So, 21 trisomy is equally probable to be due to paternal or maternal disjunction meiosis II event.

Case 1: Meiosis I non-disjunction followed by meiosis II disjunction.

(C21-C21 // c21-c21) =Meiosis I Non-disjunction=> (C21-C21 // c21-c21)

(C21-C21 // c21-c21) =Meiosis II disjunction=> (C21-C21 // c21) + C21 or C21 + (C21 // c21-c21)

That is, (C21-C21 // c21-c21) produces may produce two gametes (C21-C21 // c21) and C21. Or, it can also produce another pair of gametes C21 and (C21 // c21-c21).

Fusion of abnormal gametes (underlined with 3 copies C21) with a normal gamete results a zygote with 4 copies of chromosome 21. Thus, it can’t lead to trisomy.

Case 2: Meiosis II non-disjunction

(C21-C21) Meiosis II disjunction=> C21-C21

Fusion of abnormal gametes (underlined with 2 copies C21) with a normal gamete results a zygote with 2 copies of chromosome 21. Thus, it meiosis II non-disjunction lead to 21 trisomy.

Part B. Klinefelter Syndrome (XXY or XYY)

XXY = there are 2 copies of X chromosome, that is the female gamete must had contained 2 X-chromosomes. A gamete can have 2 copies of the same chromosome only due to meiosis II non-disjunction (see case II above).

Therefore, XXY is a result of maternal disjunction + Meiosis II non-disjunction.

XYY is also produced similarly to XXY dur to maternal, meiosis II non-disjunction.