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 Solution 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..

1. 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
Solution
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

2. 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.