Two families each have a child with Down syndrome. The empirically observed risk

Question

Two families each have a child with Down syndrome. The empirically observed risk of recurrence is 1 in 100. The first family, with two children older than the affected child, has been advised that they have a recurrence risk of 1 in 63, while the second family’s chances of having another child with Down syndrome is 1 in 6. This second family has also experienced two miscarriages and has no other children a)It has been determined in the first family that the mother is heterozygous for a gene for which she produces equal amounts of the normal and abnormal forms of the gene’s protein. The father is homozygous, producing only normal protein. The child affected with Down syndrome is shown to have twice as much abnormal as normal form of this protein. From this information, can it be determined in which parent non-disjunction occurred? Explain. b) The gene in question a. is on chromosome 21, very close to heterochromatic DNA of the centromere, thus reducing its crossing-over frequency. In which meiotic division did the error most likely occur? Explain.

Explanation / Answer

a). Down syndrome is a result of chromosomal abnormality. Patients with Down syndrome generally have three copies of chromosome 21, rather than the normal 2. So, they have a total of 47 chromosomes.

In this case, the father is producing normal proteins, whereas the mother is producing 50% normal and 50% defective proteins. So, the nondisjunctioning occurred in mother, as a result the production of abnormal protein has doubled in the child.

b). During meiosis, the homologous chromosomes align during the prophase. These chromosomes crossover at some point and rejoin to form recombinant chromatids, and move towards the opposite poles during anaphase.

Sometimes, error occurs in meiosis during the meiosis – I or II, i.e. during the separation of homologous chromosomes. These errors result in excess or less chromosomes in the daughter cell, and this is called “nondisjunctioning.”

In this case, the non-disjunctioning might have occurred during the meiosis-II because then only an extra copy of chromosome can be transferred to the gamete.

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