Question

Condition of sex chromosomes in a male child of Down’s syndrome will be:
(a) XY
(b) XXY
(c) XX
(d) XO

Answer 1

Hint: It is an autosomal anomaly, characterized by autosomal trisomy. It is also called trisomy 21 syndrome. Down’s syndrome usually does not affect sex chromosomes.

Complete answer:
Down syndrome never affect any sex chromosome so a male child with down syndrome carries XY chromosome while a female child with down syndrome carries XX chromosome. Down's syndrome is formerly called mongolism or Mongoloid idiocy. It is an autosomal anomaly characterized by autosomal trisomy. In this case, an extra chromosome is added to the $21^{st}$ pair of autosomes. So, it is called trisomy 21 syndromes. The person will have 45 autosomes, instead of the normal 44, and the total number of chromosomes will be 47, instead of 46.

Trisomy 21 usually results from a primary autosomal nondisjunction in the male or female parent during gametogenesis. It causes a set of genes to exist in triplicate copies. This genotypic or karyotypic imbalance, in turn, causes corresponding phenotypic abnormalities. Nearly $5\%$ of the cases of Down’s syndrome may be mitotic in origin, occurring either in the gonad of one of the parents, or post-zygotically in the fetus. Occasionally, mongolism may result from translocation also. In that case, extra chromosome 21 gets attached to chromosome 14,15, or 22.

Additional Information:
- Down's syndrome is characterized by mild-moderate mental retardation or idiocy, stunted growth and short stature, round and puffed face, small nose and flat nasal bridge, prominent epicanthal fold etc.
- The victims will have a high risk of congenital heart defects and acute leukemia, and high susceptibility to respiratory infections and heart diseases.
- Males are usually impotent. But females may hear bear normal as well as mongoloid children at a later age, usually after 35 years of age.
So, the correct answer is '(a) XY'.

Note:
- An interesting aspect of Down’s syndrome is its incidence among the children of older mothers.
- Women in their forties have a much higher chance of bearing mongoloid children than still younger women.
- Down’s syndrome is a semi lethal condition. Nearly $25\%$ of the unfortunate victims die in the first year of life, $50\%$ before the fifth year, and $90\%$ before $40^{th}$ year.