Which chromosome is affected by Huntington's Disease and how is that chromosome affected? Which chromosome is affected by Cystic Fibrosis and how...

Genetic diseases are diseases that are caused by defects in the genome - mutations or deletions in the DNA of an individual. Both Huntington's disease and cystic fibrosis are genetic disorders. Usually, these diseases manifest due to defects - loss of function or gain of function - of proteins that arise from the affected DNA.

Huntington's Disease

Huntington's disease is a neurodegenerative genetic disorder. In particular, it is a trinucleotide repeat disorder, a family of disorders that are caused by a trinucleotide repeat exceeding a certain number or threshold or repeat. Trinucleotides are a set of three DNA bases or a codon, and each codon ultimately corresponds to a certain amino acid. For Huntington's diseases, the affected gene is the HTT gene which encodes for the protein huntingtin (Htt). HTT is located in the short arm of chromosome 4. The gene contains a repeated sequence of the CAG codon which encodes for glutamine, an amino acid. For this particular trinucleotide, the threshold is 36 - that is, when the codon is repeated 36 times (or in the protein there are 36 or less consecutive glumatine amino acids), then there is no phenotype for Huntington's disease. Higher than this, Huntington's diseases is observed. Genetically speaking then, HD is caused by an increase in the number of CAG repeats in the HTT gene. In terms of proteins, the increase in number of glutamine changes the properties of the protein affecting the rate of communication in neurons.

Cystic Fibrosis

Cystic fibrosis is another genetic disorder. It is caused by a defect in the Cystic Fibrosis Transmembrane Conductance Regulator or the CFTR protein. The protein is damaged and is misfolded. The most common mutation in the CTFR protein is the del508. Amino acid 508 is a phenylalanine, and a deletion of this key amino acid accounts for almost 70% of all cases. Its deletion causes the protein to mis-fold; thus, it is not able to carry out its function anymore. An amino acid deletion in a protein is caused by a deletion of three bases in the encoding DNA - note that since three bases are deleted, it is still in frame and the result is a deletion of one amino acid, rather than a shift in the reading frame. The gene encoding for this protein (CTFR gene) is found at the q32.1 locus of chromosome 7. 

--Short answer:

Both HD and CF are diseases that are caused by mutations in the genome. HD is caused by additional bases (additional CAG repeats in the HTT gene located in the short arm of chromosome 4). CF is caused by a deletion of three bases (deletion of bases encoding for a phenylalanine in position 508 in the CTFR gene in the q32.1 locus of chromosome 7).