Question

In humans Down syndrome is most of the time due to an error during meiosis, leading to trisomy of chromosome 21. There are however cases when Down syndrome “runs in family” and can be passed from parents to offspring. Propose a possible GENETIC explanation for familial Down syndrome. Also discuss how these rare cases can help identify the genes which, when in triple dose, lead to Down syndrome.

Answer 1

Down syndrome is a genetic disorder and the most common autosomal chromosome abnormality in humans, where extra genetic material from chromosome 21 is transferred to a newly formed embryo.

"Familial Down Syndrome"- it results from a translocation of part of Chromosome 21 onto Chromosome 14. Basically, part of 21 breaks off and attaches itself to 14, but it is a big enough chunk that it retains most of the genes and can function well enough to substitute for the whole chromosome.

To know which genes are causing the down symdrome, following tests can be done-

Cytogenetics Tests

• Karyotyping
• Fluorescence in situ hybridization (FISH)
• FISH-interphase
• FISH-metaphase

Molecular Genetics Tests

• Deletion/duplication analysis
• Targeted variant analysis
• Detection of homozygosity
• Mutation scanning of select exons