Question

Given below is a list of features observed in an individual with L type of syndrome. This chromosomal disorder is caused due to trisomy of the K chromosome.
i. Clenched fists with overlapping fingers
ii. Dysplastic or malformed ears
iii. Back part of the skull is prominent
​​​​​​​Identify L and K.

• A. L-Edwards syndrome; K-${18}^{th}$ chromosome
• B. L-Down’s syndrome; K-${21}^{st}$ chromosome
• C. L-Klinefelter’s syndrome; K-Sex chromosome
• D. L-Turner's syndrome; K-Sex chromosome

Answer 1

The correct option is A L-Edwards syndrome; K-${18}^{th}$ chromosome

The list of features mentioned in the question are seen in individuals affected with Edwards syndrome.

It is a chromosomal disorder which is caused due to the trisomy of the ${18}^{th}$ chromosome.

Hence, L is Edwards syndrome; K is ${18}^{th}$chromosome.

Trisomy is characterised by the presence of three copies of a particular chromosome in an individual instead of two. This disorder arises due to an error during cell division.

Gametes are formed through meiosis (reduction division).
During meiosis, the homologous chromosomes and sister chromatids separate and move to opposite poles during meiosis I and meiosis II respectively. This process is called disjunction. As a result of this, the gametes produced will have a single copy of each chromosome.

But failure of separation of particular homologous chromosomes or sister chromatids (nondisjunction) results in gametes with either both copies of a particular chromosome or with no copies of that particular chromosome.

If one of the gametes fusing to form the zygote has two copies of the ${18}^{th}$ chromosome (due to nondisjunction) instead of one, then the resulting zygote will have three copies of the ${18}^{th}$ chromosome. This is called trisomy of the ${18}^{th}$ chromosome and it results in Edwards syndrome.

Clenched fists with overlapping fingers, dysplastic or malformed ears and back part of the skull is prominent in individuals affected with this disorder.

If one of the gametes fusing to form the zygote has two copies of the ${21}^{st}$ chromosome (due to nondisjunction) instead of one, then the resulting zygote will have three copies of the ${21}^{st}$ chromosome. This is called trisomy of the ${21}^{st}$ chromosome and it results in Down’s syndrome.

The affected individuals have a short stature with rounded head, furrowed tongue, wide palm with distinct palm crease and retarded physical, mental and psychomotor development.

If the female gamete (egg) without X chromosome (sex chromosome) fuses with the sperm carrying X chromosome, it results in Turner’s syndrome. The affected females are sterile with rudimentary ovaries. The absence of one chromosome is called monosomy. In this case it is one of the X chromosomes which is absent.

Klinefelter’s syndrome is a chromosomal disorder characterised by the presence of an extra X chromosome in the human males giving rise to XXY condition. The affected individuals display overall masculine development; however, the feminine characters are also displayed.