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What is A 35 years old lady has chromosomal translocation 21/21. The risk of down syndrome in the child is – and explain in detail?
Cvtogenetic aspects of own's syndrome -

The most common finding is trisomy 21.
Molecular studies have revealed that the additional number 21 chromosome is derived from the mother in 95% of Cases. (The recurrence risk in these families is approximately 1%.)
If a female with Down syndrome due to trisomy 21 conceives then there is a risk of 50% that the baby will also have trisomy 21.
Males with Down syndrome have rarely if ever reproduced.

Chromosome findings in Down syndrome

Trisomy 21, e.g. 47, XY, + 21                  -                  95%
Mosaicism, e.g. 46, XX, / 47, XX, + 21           -         2%
Robertsonian translocation,

e.g. 46 XX, - 15,+ t (15q21q)                           -               3%
Inheritence in Mosaic if own's syndrome

Children with mosaic Down syndrome are usually less severely affected than in the full-blown syndrome, and if only a small proportion of cells are trisomic then these individuals may lead normal lives.
In the event of reproduction, of these individuals there is a relatively high risk that the baby will have full trisomy 21 with the precise risk equalling the proportion of gametes which carry an additional number 21 chromosome.

Inheritance in Down's syndrome with tranclocation

When a child has Down syndrome as a result of an unbalanced Robertsonian translocation, there is a probability of around 25% that one of the parents will carry this in a balanced form. The remaining 75% of cases arise as de novo events and convey a low recurrence risk of approximately 1%
If, however, a parent is shown to be a carrier then there will be a significant risk that a future child will be affected, usually of the order of 2-5% for a carrier male and 10-15% for a carrier female.
In the very rare event that a parent carries a balanced 21q21e Robertsonian translocation, the risk of Down's   drome in liveborn offs rin• will be 100%.

​Risk of abnormal offspring for a carrier of a balanced translocation