Cause of 'Cri-du-chat' syndrome is :

Question

Accepted Answer

The correct option is 3: Loss of half of the short arm of chromosome 5

Cri-du-chat syndrome, also known as 5p- syndrome or "cat cry syndrome," is a rare but well-known genetic disorder. The cause is definitively attributed to a chromosomal aberration involving the deletion of genetic material from the short arm—conventionally designated as the 'p' arm—of chromosome 5. This deletion is typically terminal, meaning it occurs at the end of the arm, and the loss of a critical region within the short arm (specifically band 5p15.2 and 5p15.3) is responsible for the syndrome's characteristic phenotype. The deletion leads to *haploinsufficiency*, where having only one functional copy of the genes in that region is insufficient for normal development. The most recognizable symptom, the high-pitched, monochromatic cry that sounds like a kitten, is caused by the deletion of a specific region and is what gives the syndrome its French name, which translates to "cry of the cat."

The incorrect option is 1: Fertilization of an XX egg by a normal Y bearing sperm.

This description of fertilization would result in a zygote with an **XXY** sex chromosome complement. This chromosomal configuration is the cause of a completely different condition known as **Klinefelter syndrome**, which affects males and is characterized by low testosterone, infertility, and often tall stature. It is a sex chromosome aneuploidy, not an autosomal deletion disorder, and therefore has no direct link to the Cri-du-chat syndrome which involves an autosome (chromosome 5). The egg is incorrectly depicted as having two X chromosomes (XX) instead of the normal single X, with the sperm contributing a Y chromosome, leading to the 47,XXY karyotype of Klinefelter syndrome.

The incorrect option is 2: Trisomy of chromosome 21.

This chromosomal anomaly, which involves having three copies of chromosome 21 instead of the usual two, is the genetic cause of **Down syndrome**. Trisomy is a type of aneuploidy, involving an extra whole chromosome, which is conceptually different from the deletion (loss of a segment) seen in Cri-du-chat syndrome. Moreover, the disorder affects a different autosome, chromosome 21, which results in a distinct pattern of physical features, developmental delays, and intellectual disability. While both are common chromosomal disorders, they are caused by different types of genetic errors on different chromosomes.

The incorrect option is 4: Loss of long arm of chromosome 5.

The long arm of a chromosome is designated as the 'q' arm. A deletion on the long arm of chromosome 5 (known as 5q- syndrome) would cause a genetic syndrome distinct from Cri-du-chat. While it is a deletion on the same chromosome, the long arm contains a different set of genes. For example, specific deletions on the long arm are often associated with myelodysplastic syndrome (MDS) in adults. Therefore, although the chromosomal change is similar (a deletion) and the chromosome number is the same (5), the resulting clinical condition is entirely different because the lost genetic material is unique to the q arm, not the p arm.