The chromosomal disorders are caused due to absence, excess or abnormal arrangement of one or more chromosomes. Failure of segregation of chromatids during cell division results in the gain or loss of chromosome(s), called aneuploidy.
Which of the following statements are correct with respect to the humans suffering from the disorder?
(A) One in every 800 live births shows this karyotype
(B) The affected individual has 45 chromosomes.
(C) Non-disjunction of chromosomes at the time of gamete formation causes this disorder.
(D) In this disorder, individual has 45 autosomes
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (A), (C) and (D) Only
(A) One in every 800 live births shows this karyotype — True. Down’s syndrome occurs in about 1 in 800 births.
(B) The affected individual has 45 chromosomes. — False. There is an extra chromosome 21, giving 47 chromosomes.
(C) Non-disjunction of chromosomes at the time of gamete formation causes this disorder. — True. Failure of chromatids to separate gives a gamete with an extra chromosome.
(D) In this disorder, individual has 45 autosomes — True. 44 autosomes plus one extra chromosome 21 make 45 autosomes, besides the two sex chromosomes.
NCERT Extract:
Chromosomal Disorders – “… Down’s Syndrome : The cause of this genetic disorder is the presence of an additional copy of the chromosome number 21 (trisomy of 21). This disorder was first described by Langdon Down (1866). The affected individual is short statured with small round head, furrowed tongue and partially open mouth (Figure 4.16). Palm is broad with characteristic palm crease. Physical, psychomotor and mental development is retarded.”