Question

Chromosomal abnormalities occur during meiosis, a process that produces gametes (eggs and sperm) within human bodies. Abnormalities occur when there is an uneven split of chromosomes into each gamete. The following table shows 6 of the most common syndromes associated with chromosomal abnormalities.
$\begin{array}{cccc}\text{Syndrome name}& \text{Chromosome Affected}& \text{Incidence per 10,000 births}& \text{Type of mutation}\\ \text{Polyploidy}& \text{Extra complete set of chromosomes}& \text{Fatal, doesn't reach full term}& \text{Addition}\\ \text{Patau syndrome}& \text{13}& \text{2}& \text{Addition}\\ \text{Edwardsn syndrome}& \text{18}& \text{3}& \text{Addition}\\ \text{Down syndrome}& \text{21}& \text{15}& \text{Addition}\\ \text{Klinefelter syndrome}& \text{23}& \text{10(affects only males))}& \text{Addition}\\ \text{Turner Syndrome}& \text{23}& \text{2(affects only females))}& \text{Deletion}\end{array}$

(I) Following 30,000 of all births, how many children would be predicted to have Turner syndrome?
(a) 0
(b) 3
(c) 4
(d) 6

(II) From above mentioned table, what is the sex chromosome makeup of a child with Klinefelter syndrome?
(a) XO
(b) XY
(c) XXY
(d) XX

(III) The incidence of Down’s syndrome increases with maternal age, and not significantly with paternal age. What may reasonably explain this trend?
(a) Failure of muscular contractions to correctly fuse the two gametes.
(b) Gradual accumulation in germ-line chromosomal mutations throughout reproductive age.
(c) Maternal oocytes have a longer period of meiosis than paternal spermatozoa, and thus, there are more opportunities for mutations to develop.
(d) Maternal mitochondrial DNA is mutated in Down’s syndrome.

• A. $\begin{array}{ccc}\text{(I)}& \text{(II)}& \text{(III)}\\ \text{a}& \text{b}& \text{c}\end{array}$
• B. $\begin{array}{ccc}\text{(I)}& \text{(II)}& \text{(III)}\\ \text{d}& \text{c}& \text{c}\end{array}$
• C. $\begin{array}{ccc}\text{(I)}& \text{(II)}& \text{(III)}\\ \text{b}& \text{c}& \text{a}\end{array}$
• D. $\begin{array}{ccc}\text{(I)}& \text{(II)}& \text{(III)}\\ \text{c}& \text{a}& \text{b}\end{array}$

Answer 1

The correct option is B $\begin{array}{ccc}\text{(I)}& \text{(II)}& \text{(III)}\\ \text{d}& \text{c}& \text{c}\end{array}$

Statement I:
Keeping in mind given info (2 individual affected /10,000 of all births) so per 30,000 of all births, 6 children would be predicted to have Turner syndrome. Hence, option d is correct.

Statement II:
The sex chromosome makeup of a child with Klinefelter syndrome is option c, i.e. XXY since it affects only male child with extra additional Y chromosome.


Statement III:
Down's syndrome is caused due to trisomy in chromosome 21 due to nondisjunction. Maternal oocytes have a longer period of meiosis than paternal spermatozoa, and thus there are more opportunities for mutations (nondisjunction in this case) to develop, is more appropriate.